CA1326547C - Thermomagnetic recording method - Google Patents

Abstract

(1) ABSTRACT OF THE DISCLOSURE

The present invention adopts as the basic structure of its thermomagnetic recording medium an arrangement consisting of a first and a second magnetic thin film having perpendicular anisotropy and a third magnetic thin film having in-plane magnetic anisotropy or small perpendicular magnetic anisotropy interposed therebetween, formed into a laminated structure by being magnetically coupled to the adjoining films in turn, modulates and switches, in accordance with information to be recorded, a first heating condition and a second heating condition, with the medium applied with a predetermined external magnetic field Hex in the direction perpendicular to the plane of the film, the first condition being that for raising temperature of the medium to a first temperature T1 which is virtually above the Curie temperature TCl of the first magnetic thin film and not causing reversal of the magnetic moment in the second magnetic thin film and the second condition being that for raising temperature of the same to a second temperature T2 which is virtually above the Curie temperature TCl and sufficient to cause reversal of the magnetic moment in the second magnetic thin film, (2) to thereby form an information bit (magnetic domains) in the first magnetic thin film, and adapts during the course the medium is cooled from the heated states such that two states established by the different relationships between the directions of magnetization of the first and second magnetic thin films are finally formed whereby the recording of the information is performed, and thus the present invention brings about conditions enabling alteration of recorded information to other information, that is, enabling so-called overwriting. Especially, by virtue of the third magnetic thin film, the present invention achieves simplification of the apparatus for applying the magnetic field.

Description

~326~47 ~AC~GROUND OF THE INVENTION
The present invention relate~ to a thermomagnetic recording method such a~, for example, a thermomagnetic recording method using irradi~tion of a la~er beam.
In the method to record information by thermomagnetic recording in ~ recording medium, from which information is reproduced by reading ln-formation bits (magnetic domaiD~) formed thereon by virtue of magneto~optical interaction, the recording medium having a magnetic thin film formed of a vertically magnetizable film i8 ~ubjected in advance to initialization, i.e., to a treatment to orient the magnetization in the ~edium into one direction perpendicular to the plane of the film, and thereafter, magnetic domain~ having vertical magnetization in the reverse direction to the initial magnetization are formed by heating the medium locally by irradiation of a laser be~m or the like, and thereby, the informa-tion is recorded thereon aR a binarized information bits.
In ~uch a thermom&gnetic recording method, when sltering recorded information, a process must, prior to the ~lteration, be performed to erase the recorded information ~the proces~ corre~ponding to the ~, , ~ , .-' ~ ., ` '.:

~32~7 above described initialization), so that a certain time i8 taken to perform the erasing proce~, and therefore, recording at a high tran~mission ra-te canno-t be achieved A~ countermeasures against that, there have been proposed vsrious real-time recording method~ in which overwrit.ing ia made po~sible and thereby the period of time for performing such an iDdependent era~ing process can be eliminated. Among such thermomagnetic recording methods executing th~
overwrite, hopeful ones, for example, are that applies modulated external magnetic field to the medium and that uses two hcads, an erasing head as well as a recording head. In the method using modulsted external magnetic field, the recording i9 performed a~ disclosed, for example, in Japanese Laid-open Patent Publication No.
60-48806 by applying ~ magnetic field with the polarity corresponding to the state of an input digital ~ignal current to a recording medium, which is provided thereon with an amorphous ~errimagnetic thin film haviDg an axis of easy magnetization perpendicular to the film plane, at its region irradiated by a temperature raising beam.
When it is attempeed to achieve a high speed recording at a high information transmissioD rate by the above described external magnetic field modulation `:~

~32~7 method~ an electromagnet operating at the rate, for example, on the order of one MHz becomes necessary, and a problem arises that it is difficult to ~abricate such an electromagnet, and even if it is fabricated, consumed power and heat generated thereby become huge, and therefore, it cannot be put to practical use. Meanwhile, the two-head method requires an extra head and the two heads must be located apart, and therefore, such a problem occurs that a hea~y-load drive system is required and the system becomes uneconomical and unsuitable for mass production.

The present applicant earlier proposed thermomagnetic recording methods intended to solve these problems in IJnited States Patent No. 4,955,007 which issued on Septernber 4, 1990. The thermomagnetic methods proposed in these applications are such that use a thermomagnetic recording medium provided with a :first and a second laminated structure of rare earth-transition metal magnetic th;n films and switch and modulate, in accordance with information to be recorded, for e~ample, of "O" and "1", a first heating condition to heat the ... . :

, ,, ., ~ . .

1 3 ~ 7 medium to a first temperature Tl which i~ virtually above the Curie temperature TCl of the first magnetic thin film and not rever~ing the qub-latice magnetization in the second magnetic thin -film and a second heating condition to heat the ~ame to a second temperature T2 which i3 above the temperature TCl and sufficient to reverse the ~ub-lattice magnetization in the second magnetic thin f.ilm, wi-th the medium applied with a required first external magnetic field, ~o that, in the cooling stage, the direction of ~ub-lattice magnetizQti~n in the first magnetic thin film i9 brought into agreement with the direction of the sub-lattice magnetization in the second magnetic thin film by virtue ~-of -Eir~t and second exchange coupling force$ whereby recorded bits ~magnetic domains), for example, of "O"
and "1" are formed in the first magnetic thin film, and the sub-lattice magnetization in the second magnetic thin film is rever~ed by virtue o:f a second external magnetic field or by virtue of only the first external magnetic field at room temperature when the compo~ition of the ~econd magnetic thin film has been selected ~o as to have it~ compensation temperature between the ~econd temperature T2 and room te~perature, and thereby obtain the conditions to make overwriting po~sible.

, : :

~ 3 2 ~

Since, throughout the above processes, there is no need of performing a special process (taking a time) for erasing, a high transmission rate can be attained, and thereby, the probIems involved in the above described two-head system or the external magnetic field modulation system can be solved.

The thennomagnetic recording method according to United States Patent No.
4,955,007 which issued on September 4, 1990 will be described below. The recording of information, for example, of "0" and "1" in this recording method is performed, as shown in Figure 1 which schematically indicates the above descIibed magnetized states of the first and second magnetic thin fil~s 1 and 2 with small arrows relative to temperature T, by providing, at room temperature T,~, a state A with the directions of magnetization in both the magnetic thin films 1 and 2 oriented in one direction and a state B with the same oriented in the reverse directions to each other. And these records are obt~ined by application of the external magnetic field Ho~ to the medium and heating the same to the first and second temperatures Tl and T2 by laser beam irradiation. For example, a laser beam is first impinged on the position in the state A, with the intensity or time of irradiation of the laser beam - , ~: .

~32~7 modula-ted in accordance with the recor~ling signAl, so that the heating temperature T i8 brought to the first heating temperature Tl virtually above the Curie temperature TCl of the first magnetic thiD film 1 and causing DO reversal of magnetization in the ~econd magnetic thin film 2 under the influence of the required external magnetic field HeX~ By such heating, the first magnetic thin film 1 exhibits ~ state C where it loses its magnetization but, when the laminated film of the magnetic thin films 1 and 2, after the heating has been fini~hed, i8 cooled below the temperature TCl~
magnetization is produced in the first magnetic thin film 1. In thi~ case, ~ince it has been previously adapted such that the exchange coupling force with the second magnetic thin film 2 iB domiDant, the direction of magnetization in the first magnetic thin film 1 i8 oriented into the same direction a~ that of the second magnetic thin film 2. Namely, the state A i~ produced whereby information, for example, of a "O" is recorded.
Otherwise, the heating temperature T is brought to the second heating temperature T2 beyond the above described temperature Tl and sufficieDt to reverse the ~agnetization in the second magnetic thin fil~ 2 with the external magnetic ~ield HeX applied. ~y .

, ~

~ 3,~ 7 performing sucll heating, a ætate D in which the fir~t magnetic thin film 1 ha~ lo~t its magnetization and the second magnetic thin film Z ha~ reversed it~
magnetization iæ brought about. ~ut, when the heating i~ fini~hed and the laminated film of the magnetic thin films 1 and 2 are cooled below the temperature TCls the ~firæt magnetic thin film 1 is subjected to the exchange coupling force from the æecond magnetic thin film 2, whereby a ~tate E, i.e., a magnetized ~tate oppo~ite to the original, initialized state, i8 produced but by virtue oP Q subsidiary externH1 magnetic field dgub applied in the vicinity of room temperature TR, the direction of the ~econd magnetic thin film 2 is reversed, and thereby, a magnetized ~tate B with magnetic domain walls 3 formed between both the magnetic thin filmæ 1 snd 2, the state B being only different from the magnetized state A in that the magneti~ation in the first magnetic thin film 1 has been reversed, is brought about, and thus, recording of information, for example, of a "1" i~ achieved.
The recording of information oE "O" and "1" i9 achieved by obtainir,g the state A and state B as deæcribed above. In thi~ ca~e, the light-inten~ity-modulated overwriting iæ applicable to both the ~tate A

"; ; ~ - . , .; ,.

~ ~26~3~7 .
, and the st~te B. More particularly, by having any position of those in the state A and the state B heated to the temperature T] or T2 past the state C, by virtue of ~elected temperatures Tl and T2 as described above, the overwrite of the state A or the state B
corresponding to the information "0" or "1" can be achieved no matter whether the original state WBS the ~tate A or the state B.
In the magnetic recording medium of the described structure, ths sur-face between the m~gnetic thin films 1 and 2 forming the laminated film is under the influence o~ e~change energy, whereby the magnetic domain walls 3 are formed in the first state B. The domain wall energy ~ w i~ e~pressed as ~ w --. 2(y AlKl + 'J ~2K2 ) .. . (1) (Al and A2, Bl and K2 are exchange constants and perpendicular magnetic anisotropic constants of the first and second magnetic thin film~ 1 and Z.) As the conditiona required for achieving the - overwrite, the conditioD under which transition ~rom the state B to the state A does not take pl~ce at room temperature (- 20C to 60C) i9 given by HCl > HWl = ~ w/2~Slhl ... (2 Also~ the condition under which transition , , 132~7 from the state B to the state E does not take place i~
given by HC2 ? HW2 = ~ w/2MS2h2 ~3) Further, in the ~tate E, in order that the ~agnetization in the fir~t msgnetic thin film 1 i~ not reversed by the subsidiary external m~gnetic field H8Ub~
the following condition must be sati~fied:
HCl ~ HWl > Hsub -- (41) where the sign +/- on the left-hand side become~ ~ign "~" when the Pirst magnetic thin film 1 is a rare earth metal rich film and the second magnetic thin fil~ 2 iB a transition metal rich film, whereas it becomes ~ign "-"
when both the first and the second magnetic thin film~ 1 and 2 are transmi~sion metal rich.
Beside3, in order that the tran~ition from the state E to the state B takes place, the condition HC2 ~ HW2 ~ ~sub -- (42 mu~t be satisfied.
Further, where the heated temperature i8 in the vicinity of the Curie tempersture T~l of the fir~t magnetic thin film 1, in orde~ that the transition from the state C to the state A -tal~es place, that i3, the direction of magnetization in the first magnetic thin film 1 is brought into agreement with the direction of . . .. . .

13~6~7 the magnetization isl the ~econd magnetic thin film Z, the condition HWl > Hcl + Hex must be satis~ied. Beside~, in order that trQnsition ~-from the state B to -the state E does not take place, the cvndition HC2 - Hw2 > HeX -- (6) must be satisfied.
In the above expressions, HWl and HW2 are quantities defined by the expressions t2) and (3), and HCl and HC2~ MSl and MS2~ and hl ~nd h2 respectively are coercive forces, saturation magnetizations, and thicknes~es of the first and ~econd m~gnetic thin fil~s.
: As apparent from these, in order to 6ati~fy the expressions (2) and (3), it is preferred that the domain wall energy ~ w at room temperature i~ as ~mall as possible, bllt, when asRuming that ~ . 4 X 106 erg/cm3, h = 2 x 10-6 erg~cm, we obtain w - 3.6 erg/cm2.
Meanwhile, actual mea~urementq on -the hy~teresis loop of the two-layer film give ~ w = 3 t~ 6 ; . erg/cm2. Now, as~uming that ~ w = 5 erg~cm2 and ~ing HCMS . 0.45 x 106 erg~cm2 and HeX = 2 kOe~ we obtain h2 = llo0R, H~2 = 4kOe, and HW2 ~ 2kOe as approximate .. . .
. -: ,: .

~326~7 values to sati~fy the condition of the expression ~6) ~t room temperature T~, i. e., to sati3fy the condition H~z - HW2 ~ 2kOe. Thus, a problem i9 posed that the thickness h2 of the second magnetic thin film 2 becomes large and the subsidiary extern~l magnetic field H~lb become~ large from the expression (42)-OBJECTS OF THE INYE~TION
It is an ob~ect of the present invention toprovide an improved thermomagnetic recording method capable o$ real-time overwriting.
It i~ another object of the pre~ent invention to provide a thermomagnetic recording method in which recorded bit i8 gtabili~ed.
It i~ a further object of the present invention to provide a thermomagnetic recording method in which external ma~netic fiald applied to a thermomagnetic recording medium to initialize the sa~e i8 reduced.
It is a still further object of the present invention to provide a thermumagnetie recording method in which higher Kerr rotation sngle i8 obtained upon playback.

~2~7 BRIEF DESCRIPTION OF THE DRAWINGS .
FIG. 1 i~ a schematic diagram showing states of magnetization in a thermomagnetic recording ~edium used in a thermomagnetic recordiDg method previou~ly applied by the present applicant;
FIG~. 2, 7, 8, 21, 22, 28, and 34 are each n sectional view of q thermomagnetic recordlng medium u~ed in the present invention;
FIGs. 3, 24, and 26 are each a schematic diagram showing states of magnetizHtion in a thermomagnetic recording medium u~ed in a thermomagnetic recording method of the present invention;
FIGs. 4, 5, 9, and 10 are each a graph showing dependence of characteristic~ of a thermomagnetic recording medium of the present invention on the thickness of a third magnetic film;
FIa~. 6, 11, and 13 are each a graph ~howing dependence of magnetic domain wMll energy ~ w in a thermomagnetic recording medi~m of the present invention on the thickness of a third magnetic film;
FIG. 12 is a graph showing a magnetization-temperature characteristic of an example of a third magne-tic film;
FIG. 14 is a graph showing temperature , , ' ' ' ' :

132~5~7 dependence of ~aturation magnetization;
FIG. 15 is a graph showing temperature dependence of effective perpendicular ani~otropy coDstant;
FIG. 16 i~ a drawing ~howing Kerr loop~
changing ~ith temperature in one example of a third magnetic thin ~ilm;
FIG. 17 is an explanatory drawing of a state of magneti~ation in a reference example;
FIG~. 18, 23, and 25 are temperature characteristic~ of magnetic domain wall energy and coercive force energy;
FIG. 19 is a schematic diagra- of a thermomagnetic recording medium of a reference example;
FIG. 20 iQ an explanatory drawing Qf a m~gnetized state in the above;
FIG. 27 iB a dependence on external magnetic field HeX o~ C/N at the time of overwriting;
FIG. 29 is temperature characteristic~ o~
coercive force in component ~ilms of a second magnetic thin film for use in the present in~ention;
: FIGs. 30 and 31 are graph~ of dependence on external magnetic field HeX of C/N;
FIG. 32 i~ a schematic diagram ~howing - , .
' .

~32~ 7 patterns of recorded domains; and FIG. 33 iq temperature characteri~tics o-f coercive force in component films of a seeolld magnetic thin film used in a reference example.

DESCRIPTION OF THE P~EFER~ED EMBODIMENTS
In the present invention, it is adapted 3uch that the above described domain wall energy ~ W at room temperature is made ~mall and the temper~ture characteristic of ~ W ~atisfying the above expre~sion t5) i9 improved to thereby reduce the film thickness of the second magnetic thin film Z and lower the subsidiary external magDetic field Hgub~
More pQrticularly, in the present invention, a thermomagnetic recording medium 10 as shown in FIG. 2 i9 prepared. The thermomagnetic recording medium 10 i9 provided thereon witll a laminated film 14 made up of first and ~econd magnetic thin films 11 and 12 h~ving perpendicular magnetic anisotropy with a third magnetic thin film 13 having an in-plane maneti~ anisotropy or a small amount of perpendicular magnetic anisotropy ~andwiched therebetween, -these films being magnetically coupled and laminated in turn to the adjoining one. The third magnetic thin film 13 is preferred, even if it has ', '. :

;~ , .

~326~7 perpendicular magnetic an;sotropy, to have ~ufficiently small perpendicular magnetic ani~otropy as against the perpendicular magnetic ani80tropy of the first ~nd second magnetic thin films 11 and 129 as low as, for example, 1 x lV6 erg~cm3 in perpendicular magnetic ani~otropy con~tant.
! In the present invention, recording of information i8 performed on the recording medium 10 a~
shown in FI~. 3, in the same way a~ de~cribed w~th reference to FIG. 1, by hea-ting the laminated film with la3er beam irradistion up to the first and second temperatures Tl and T2. More p~rticularly, e first heating condition to rai~e the temperature to a first temperature Tl virtually above the Curie temperature TC
of the first magnetic thin film 11 and causing no reversal of the magnetic ~oment in the ~econd msgDetic thin film 12 and a second heating cond1tion to rai~e the temperature to a second temperature T2 above the Curie temperature TCl of the first magnetic thin film 11 and sufficient to reverse the magnetic moment in the ~econd magnetic thin film 12 are modulated in accordance with the information signals to be recorded and the hested positions on the medium are cooled down 90 that the above described ~tate A and state B are obtained there.

..

1321~7 According to the present invention a~
described above, the recording of information is achleved by bringing about certain states of magnet.ization in the first and ~econd magnetic thin fil=s 11 and 12. However, by having the third magnetic thin film 13 sandwiched in-between the two films, the domain wall energy ~ W between the fir~t and second magnetic thin films 11 and 12 can be controlled and it is thereby made easier to satisfy the above mentioned expre~ions (2), (3) an~ (~2) More particularly, according to the present invention, the stQtes A and B are brought about ViR the states ~ - E as ~hown in FI~. 3, in the same way as described with FIG. 1. That is, thc recording of information by the ~tate A in which the fir~t and ~econd magnetic thin films 11 and lZ are magnetized in the same direction and the ~tate B in which they are magnetized in the reverse direction~ is performed, and at this time, by virtue of existence of the third magnetic thin film 13, the state of formation of the interface domain walls is ~tabilized, whereby the margin in designing the characteristic~ of the magnetic thin films are expanded, the domain wall energy i~ lowered, and the ~ubsidiary external magnetic field required for : .

the transition from the ~tate E to the state B can be decreased.
The thermomagnet:ic recording medium 10 u~ed in the pre~ent invention i~ formed, a~ shown in FIG. 2, of a light traDsmitting suostrate 15 of a glas9 plate, an acrylic plate, or the like provided witll a laminated film 14 deposited on one surface thereof, via a transparent dielectric film 16 serving as a protecting film or an interference film, by, for example, continuous ~puttering of the first magnetic thin film 11, the third magnetic thin film 13, and the second magnetic thin film lZ in turn, the laminated film being covered with a protection film 17 of a nonmagnetic ~etallic film or a dielectric film. In the thermomagnetic recording medium 10, however, the dielectric film 16 and the protecting film 17 may be omitted.

Embodiment 1 A laminated film 14 is formed of a first magnetic thin film 11 of a rare earth rich film, for example, of Tb~Feo.g~Coo.os) with a thickne~s hl = 600 R
and MSl = 60 emu/cc, a third magnetic thin film 13 of Feo 95Coo.o5 with Ms3 = 1600 emu/cc, and a second .

132~5L~7 magnetic thin film 12 of a transition metal rich film of Tb(Feo.gsCoo.os) with a thickness h2 = 600 R and MS2 =
200 emu/cc, laminated in turn to the adjoin~ng film by continuous sputtering. Here, it is preferred that the third magnetic thin film 13 has a strong in-plane anisotropic property, and i-ts thickness is arranged to ~be thin when the in-plane anisotropic property (k3 < 0) is strong and to be thicls when it is weak ~uch that I K3h3l becomes virtually equal to Klhl, K2h2. The dependence o~ HW2 ~ ~ W2/2Ms2h2 obtained from a Faraday hystere~is loop of the laminated fil~ 14 structured as described above on the thickness h3 of the third magnetic thin film 13 i~ shown in FIG. 4.
Referring to the figure, curve 31 (O ), curve 32 (O ) and curve 33 (- ) t respectively, are result~
from actual measurements of (HCl ~ HWl)- (HcZ + HW2)~
end (HC2 - HW2)~ while curve 34 (A ) and curve 35 (~ ) are re~ults calculated from the measurement results.
Further, FIG. 5 and FIG. 6 show dependence obtained by computer simulation on the thickness h3 of the third magnetic thin film 13, i. e., referring to FIG. 5, curve 42 indicates dependence of (HC2 + HU2)~ curve 4~ that of (HC2 ~ ~W2)- curve 44 that of H~z, and curve 45 that o-f HW2 on h3, while curve 50 in FIG. 6 indicates dependence ~:

132~7 of ~ W o~ h3. In this c~se, -the thicknesse~ hl aDd h2 of the fir~t and ~econd m~gnetic thin films 1 and 2 are arranged -to be 600 ~ and charac-teristic value~ of the first to third magnetic thin films 1 - 3 are arranged to be the values as shown in Table 1.

1st Magnetic Znd Magnetic 3rd Magnetic Thin Fil~ Thin Film Thin F.il~
__________________________________________________ A 0.3 x 10-60.3 x 10-6 2 x 10-6 (erg/cm)(erg/cm) (erg/cm) K 6 x 106 4 x 106 -20 x 106 (ergjcm3)(erg/cm3) (erg/cm3) MS ~40 180 180D
(emu/cm3)(emu/cm3) (emu/cm3 Table 1 Here, the minu~ sign of Ms indicates that the first magnetic thin ~ilm 11 i8 a rare earth rich film.
The results of computer ~imulation shown in FIG. 5 and FIG. 6 are in goGd agreement with the re0ult~
o* actual measurement in FIG. 4, and it i~ known th~t ~ W and hence HW2 can be controlled by the third magnetic thin film h3. If the thickness of the third magnetic thin film i~ selected to be about h3 - 15R
whereby HW2 i~ minimized, since Hwz is small at room temperature, it becomes easier to satisfy the expression .

' .

~26~7 (42) If the temperature i3 raised a-fter making Hgub small, the operating point goes o~f the minimum point and ~ W becomes relatively larger and it become~ easier to ~atisfy the expre66ion (5~.
If the composition i8 ~elected to corre~pond to the position where Hw~ snd hence ~ ~ exhibits a I trough at room temperature ~point a) in the characteristic of FIG. 4, then, since the temperature characteristic6 of the perpendiculHr ani60tropy, magnetization, and others with the increase in the temperature differ with the first to third magnetic thin film6 11 - 13, it can be expected that ~ W devi~tes from the minimum point relative to the film thicknes~ h3 and moves to the point b or point c in FIG. 4 and the increa~e of ~ W, or at least decrea~e of ~ W- with increase in the temperature become6 gentle. Thus, at the temperature T . TCl - ~ clo~e to TCl~ it becomes ea~ier to attain ~Wl ~ HCl ~ ~ex- `

Rmbodiment 2 ID thi~ embodiment, to avoid that.the expression (5) becomes unsatisfiable by lowering f ~ W
st room temperature, the first magnetic thin film 11 i8 formed, as ~hown in FIG. 7, of two layers of magnetic ~, ~2~7 thin films, first and second component films 111 and llz. In thi~ csse, the first component film 111 i8 made of a magnetic thin film, for example, of TbFe magnetic film who~e Curie temperature TCll iB 130Cand the second component film 112, the layer lying thereunder, is made of Tb(Feo 95Coo.o5) whoBe Curie tempersture TCl2 i8 approximately 160C. Further, the third magnetic thin film 13 i9 made, for example, of FeCo, while the second magnetic thin film 12 iB made of a magnetic thin film of GdTbFeCo whose Curie temperature Tcz i~ nbout Z2QC.
According to the described ~tructure, the abov~
expre~sion (5) will be superseded by ~ W/2MSl hl2 = HWl > HCl + Next ... (5a) Namely! in this ca~e, the film thickness o~ the fir~t layer Rpparently become~ thinner from hl to hl2, and thus, the expre3sion (5a) becomes easier to satisfy than the expres~ion (5)..

Embodiment 3 On a tran~parent gla~ ~ub~trate 15 provided with gulde groove~ by the well-known ~o-called 2P ~ethod (Photo Polymerization) i~ deposited a transpHrent dielectric film 16 made of Si3N4 as shown in FIG. 8.
Over the sa~e, fir~t and second component films 111 and ... .. .

132~7 112 constituting a first magnetic thin film 11, a third magnetic thin film 13, and first and seconcl component films 121 and 122 consti-tuting a ~econd magnetic thin film 12 are deposited in turn. Gomposition snd characteristics of these magnstic thin ~ilms are shown in Table 2.
I
Magnetic Compo- Curie Compen- Magne- Film Thin Film sition Point sation tiza- Thick-Temp. tion ne~s (C) (C) (emu/cc) (~) _______________________________________ . ______________ 1st Ma~netic Thin Fil~l 1st C. F. 111 TbFe 140 120 50 300 2nd C. F. 112 TbFeCo 152 120 60 220 3rd Magnetic Thin Film 13 GdFeCo 240 - 400 100 2nd Magnetic Thin Film 1st C. F. 121 GdTbFeCo Z40 185 12S 500 2nd C. F. 122 GdFeCo 250 180 lZ5 400 Table 2 With the thermom~gnetic recording medium 10 of the described structure, it i~ considered that domain walls are formed in the vicinity of the third magne-tic thin film 13 at room tempsrMture. By pro~iding the third mngnetic thin film 13, the interface domain wall energy ~ W on the interface between the second component .. . .

~326~7 film 112 of the fir~t magnetic thin film 11 and the first componellt film 121 of the second mngnetic thin film 12 became 1.5 erg/cm2, ~nd the subsidiary external magnetic field N8Ub neces~ary for rever~iDg the magnetiz.ation in the third mHgnetic thin film 13 and the first component film 121 of the second magnetic thin film 12 at room temperature became 2.5 kOe. The magnetic anisotropy constant K3 = - 1.0 x 106 erg/cm3 indicRtes ita in-plane ani~otropy. In the case without the use of the third magnetic thin ~ilm 13, ~ W become~

2.8 erg/cm2 and the condition for enablin~ the overwrite cannot be ~ati~fied unle~s the first component film 12 of the second magnetic thin fil~ 12 i9 made as thick a~
looQ R. Be~ides, even if the film is ~elected to be ~o thick, the ~ub~idiary exter.nal magnetic field H~Ub i~
required to be as high as 3.5 kOe, from which it i~
known that reduction in the ~ubsidiary external magnetic field can be attained by the embodiment 3.
The recording characteri~tic~ of the di~ A
provided by u~ing the thermomagnetic recording medium formed according to the embodiment 3 were evaluated and these characteri~tic~ are shown in Table 3. The mea~urement result~ are that obtained from the record made at a linear ~peed of lOm/~ec, and C/N indicate~ the ,, . , ~ . .
.'.' ! ~ .' ' . ; ~ ' '~` ~ ' ~, '; '. . . ' , , ~
. . .

132 f~7 value at the time of overwriting. In Table 3, there are also shown a disk B, in which a third magnetic thin film 13 a~ shown in the above Table 2 is not u~ed and a second magnetic thin -film formed of the material of the first component film l2l in a single layer having a thickness of loOOR i~ used, and a disk C, in which the first magnetic thin ~ilm is not formed of the fir~t and second component films lll and ll2, but formed of a single layer having a thickDes~ of 550 R made of the material of the first componeDt film lll.

Di3k A Di~k B Disk C
Optimum Recording Power (mw)9.3 10.8 g,1 C/N (f = 2 MHz) tdb)56.U 55.2 53.2 External Magnetic Field 300 - 300 - 300 -for Optimum Recording (Oe) 850 860 350 Subsidiary Magnetic Field HSub (kOe) 2.6 3.5 2.5 Ambient Temp. for Preserving Record tC)< 80C~ 55C < 70C
(under Zero Magnetic Field) Table 3 .

: A~ apparent from Table 3, the subsidiary external magnetic field H~Ub can be lowered by providing the third magnetic thin film 13 and the ambient , .. ,:....

~ , 1326~7 temperaturc for preserving the record can be rai~ed.
Further, it i~ known that C/N can be lmproved, with the sub~idiary external magnetic field H~Ub and the recording power kept constant, by forming the first magnetic thin film 11 into a two-l.ayer ~tructure, the layer~ thereof having different Curie point~. Namely, C/N is improved due to the fact that the conditional expre~sion (5a) i8 co~pletely satisfied and the range of the ambient temperature for stabilized re~ervation of the record can be expanded due to the fact that the abov~ de~cribed expressions (2) and (3) are sati3fied more easily.

Embodiment ~
Over a transparent substrate 15 of a glass ~ubgtrate, Q fir~t magnetic thin film 11 of a rare eqrth ri~h Tb(Feo.95Coo.o5) film having a thickne~ of hl -600 ~ and saturation magnetization MSl = 60 emu/cm3, third magnetic thin film 13 of a ~imilarly rare earth rich Tb~Feo.gsCoo.os) film having Yaturation magnetization Ms3 = 200 emu/cm3 , and a second magne-tic thin film 1~ of a transition metal rich Tb(Feo.gsCoo.os) film having a thickness of h2 ~ 600 ~ and saturation magnetization Msz = 200 emutcm3 are depo~ited in turn by . ~ .. . . . .
~ . :; : . . ..
.
, ~32~7 sputtering and the thu~ prepared thermomagnetic recording medium 10 was used. Results of meaqurement of dependence in thi~ csse of Hwz = ~ W/2Ms2h2 on the thickDess h3 of the third magnetic thin film 13 sre shown in FIG. 9. In the figure, curve 81 (O ), curve 82 ), snd curve 83 (~ ) ~re results of actual measurement of (HCl ~ HWl)~ (Hc2 ~ Hw2), and (HC2 ~
HW2)~ respectively, and curve 84 (~ ) and curve 86 (~ ) Hre results cqlculsted from the measurement results. FIG. 10 and FI~. 11 show depenclence on the thickness h3 of the third magnetic thin film 13 obtained by computer simulation, namely, in FIG. 10, curve 92 shows dependence on h3 of (~CZ + H~2), curve 93 shows that of ~HC2 - HW2)~ curve 94 shows that of HC2~ and curve 95 shows that of HW2. The computer ~imulation was carried out with a messured value vf K3 = - 1.0 x 106 er~tcm3. In FIG. 11, curve~ 100 and 101 show dependence of a W on h3 when the snisotropy constant K3 of the third magnetic thin film 13 wss set to K3 = 0.2 x 10-6 (emujcm3) snd K3 = - 1 x 10-6 ~emu/cm3), re~pectively.
In this case1 the thicknesses hl ~nd h2 Of the first and second magnetic thin films 11 and 12 were set to 600 a and characteristics of the first to third magnetic thin films 11 - 13 were set to be a~ shown in Table ~.

'' ~326~7 Fir~t Magnetic Second Magnetic Third Magnetic Thin Film 11 Thin Film 12 Thin Film 13 ______________________________________~________________ 0.3 x ~0-~ 0.3 ~ 10-60.06 x 1o-6 (erg/cm) terg/cm)(erg~cm) K6 x 10-6 4 x 10-~0.2 x 106 (erg~cm3) (erg/cm3) (erg/cm3)- 1 x 106 (erg/cm3) ~S- 40 (emu/c~3) 180 (emu~cm3) - 200 (emu/c~3) HC 18 (kOe) 4 ~kOe)0.2 (kOe) Table 4 According to the above re~ult~, it i~ known that ~ W and hence Hwz can be controlled even i:F the third magnetic thin film 13 is that having weak perpendicular anisotropy.
Further, a~ the third magnetic thin film 1~, that having large satur~tion magnetization Ms at room temperature and having small Ms at the temperature T in the vicinity of TCl~ namely, such a magnetic thin film having a compensation temperature characteristic near there a~ ~ho~n in FIG. 12 ~ay be used. Since K = Ku -2~ MB2, ~ W - 4Yr~, where Ku i~ a uniaxial anisotropic con~tant, ~ ~ becomes small when M5 is large, and ~ ~ become~ large when Ms is small.
Then, it become~ po~sible to provide a recording ~edium of which the temperature characteri~tic .
' . :
,...
~' ; ~ , .,' ~32~

of ~ W i~ small Rt room temperature and it becomes relatively larger at the temperature T in the vicinlty of TC 1 In the presen-t invention, between a first magnetic thin film 11 and a second magnetic thin film 12 is interposed a -third magnet:ic thin film 13 having in-~plane ani~otropy or weak perpendicular anisotropy, 60that stabilization of magnetic domain wall~ iB achieved.
Thereby, 3tabilized and po~itive recording, recording with high C/N, can be achieved.
By achieved reduction of the magnetic domain wall energy ~ W at room temperature, reductioo in the sub~idiary external magnetic field ~sub and hence simplification of the apparatus is achieved.
Further, by reduction of ~ W- the range within which the above described expressions (3) and (6) are satisfied can be expanded. Further, when the first magnetic thin film is formed into a two-layer ~tructure as described above, the conditional expression (5) iB
~uperseded by the expression (5a), and thereby, the range within which the condition i8 ~atiBfied i~
expanded and allowance for the design can be enlarged.
While in the aforementioned example, the ; lowering of the subsidiary magnetic field has been ~, . .

132~7 achieved by i.mproving charaoteristic~ a-t room temperature, that is, by stabilizing magnetic domain walls and decrea~ing domain wall energy at room temperature, an example which add~ the above method a function to pro~ide ~ufficient domain wall energy 1n the vicinity of the Curie temperature TCl of the fir~t magnetic thin film, so that the proces~ for the magnetization in the transition metal of the first magnatic thin film 1 to be aligned with the magnetization in the -tran~ition metal of the second magnetic thin film 2 described in FIG. 1, that is, the transition ~rom the s-tate C to the state A, or the trQnsition from the state D to the state ~, may be positively performed will be described.
In the following example, a thermomagnetic recording medium 10 a9 shown in FIG. 2 i~ u~ed, but the third magnetic thin film 13 is formed of a magnetic thin film of a r~re earth rich metallic film, effective magnetic anisotropic constant K of which exhibits a temperature charecteristic being convex upward or linear, and the ~aturation magnetization Ms of which at room temperature is 0 to 450 emu/cm3.
Recording of information i~ performed on the recording medium lU as ~hown in FI~. 3, in the ~ame way "
. : .
:

~ 3 ~ 7 as de~cribed with reference to FIG. 1, by heating the laminated film with la~er beam irradiation up to the fir~t and second temperatures Tl and T2. More particularly, a first heating condition to raise the temperature to a firs-t temperature Tl virtually above the Curie temperature TCl of the first ~agnetic thin film 11 and causing no reversal of the magnetic moment in the second magnetic thin film 12 and a second heating condition to raise the temperature to a ~econd temperature T2 above the Curie temperature TCl o~ the first magnetic thin film 11 and ~ufficient to reverse the magnetic moment in the second magne-tic thin film 12 are modulated in accordance with the information ~ignals to be recorded and the heated positions on the ~edium are cooled down BO that records by magnetization are obtained there.
With de~cribed arrangement, the recording of iDformation i~ achieved by bringing about certain states of magnetization in the fir~t and second magnetic thin films 11 and 12. However, by having the third magnetic thin film 13 sandwiched in-between the two films, the domain wall energy ~ W between the first and second magnetic thin films 11 and 12 can be controlled and it is -thereby made easier to satisfy the above mentioned .

1326~7 expresslonA (5) and t6).
More par-ticularly, in the present ex~mple, the states A and B are brought about via the atate~ A - ~ a~
shown in FIG. 3, in the ~ame way a~ de~cribed with FI~.l 1. That is, the recording of information by the ~tate A
in which the fir~t aDd second magne-tic thin films 11 and 12 are magnetized in the ~ame direction and the state B in which they are magnetized in the reverae directions iB performed, and by virtue of exl~te~ce o~
the third magnetic thin ~ilm 13 at this time, the state of formation of the interface domain wall~ can be ~tabilized in the vicinity of room temperature, whereby the margin in de3igning the ch~racteri~tics of the magnetic thin films are expanded, the domain wall energy i~ lowered, and the ~ubsidiary external magnetic field required for the tran~ition from the ~tate E to the state B can be decreased.
Further, it i~ adapted ~uch that ~ufficient domain wall energy iB provided in the vicinity of the Curie temperature TCl~ i.e., at a high temperature, so that the proce~a in which the magnetization in the transition Detal of the first magnetic thin film 11 i~
aligned with the magnetization in the ~econd magnetic thin film 12, namely, the tral~sition from the stQte C to :. , , ' ";

; . ~ , , ~L32~7 the ~tste A, or the tran~ition from the ~tate D to the state E in FIG. 3, are performed accurately.
The third magnetic thin ~ilm 13 can be selected to be a thin film, for example, of composition of GdFeCo group who~e s~turation magnetization Ms at room temper~ture is such that 0 S Ms ~ 450 emu/cm3, or to be concrete, it i~ selected to be of composition of GdxtFel-ycoy)l-x~ where 0.25 ~ x ~ 0.40, 0 ~ y ~ 1.0 (x, y being atomic ratio). In thi~ ca~e, other rare e~rth element~ such as Dy, Tb, Nd may be added to GdFeCo used a~ the basic composition.
First, relation~hip between the effective magnetic anisotropic constant K of the tllird m~gnetic thin film and the domain wall energy ~ W will be described, In FIG. 13 ~re plotted, b~ ~ and 0 , mea~urement re3ults of relation~hip oÇ the domain wall energy ~ W of each o~ Feo 95Co0.05 and Gd(FeO.95C0.06)-the effective magnetic anisotropic con=tant K of the former tK = - 1.8 x 107 erg~cm3~ being relatively larger than that of the latter (K ~ - 1.0 x 105 erg/cm3~, again~t the film thicknes~ h3 of the intermediate film.
A~ apparent from cDmperison o~ the thu~ obtained curves 131 Hnd 132, the l~rger the ln-plane anisotropy) the , . ' . ~ '': ' smaller the do~ain wall energy ~ W and the greater the de~cent thereof with increa~e in the film thickness h3.
From this, lt follows that the domain wall energy ~ W-when a film has a large amount of i~-plane anisotropy at room temperature and ha~ a small amount of in-plane ani30tropy or an amount of perpendicular anisotropy at high temperature~ (in the vicinity of the Curie temperature TCl)~ become~ ~mall at room temperature and become~ large in the vicinity of TCl~ Here, it i9 ideal th~k the in-plane ani~otropy 1~ large at room temperature a~ de~cribecl above, but even if a film ha~ a perpendicular anisotropy at room temperature, the domain wall energy ~ W cHn be kept low if the perpendicular ani~otropy i~ of a small value.
The effective magnetic anisotropic con~tant K
i that determined by N ~ ~U - 2~ MS2 ... ~7 (KU is the uniaxial ani~otropic cons-tant~, and the temperature characteri~tic i~ dependent on the temperature char~cteri~tic~ of Ku and Ms, of which the temperature characteri~tic of Ku i~ monotone decrea~ing.
In Fig. 14 are shown mea~urement re~ult~ of the temperature characteri~tic of the saturation magnetiz~tion Ms with the use of a vibrating ~ample ; ' ' , ' ' ' ' , ' "~: `-.

, ~326~7 magnetometer (VSM). Referring to the .Figure, the curve plotted by ~ is the measurement result of ¦ Ms~ of n :.
rare earth rich fil.m (hereinafter to be called ~'~E rich film") of the composition f GdO.38(FeO.95C0.05)0.62 and the curve plotted by ~ i~ th~t of a tranHition metal rich film ~hereinafter to be called "TM r;ch film") of the co~position of GdO.22(FeO.95C0.06)0.78-From the curves 141 and 142 representing the measurement results, it i~ apparent, in the case of the TM rich curve 14Z, that the value Ms at the sa~e level a5 at room temperature is obtained at temperatures up to the vicinity of the Curie temperature Tc. From this it follow~ that a composition having l.arge in-plane ani~otropy at room temperature will have large in-plsne anisotropy even at high temperature~, i.e., in the vicinity of the Curie temperature TCl of the first magnetic thin film 11. Meanwhile, in the case of the RE
rich curve 141, the value I MSl decrease~ with increa~e in the temperature, aDd hence, e~en if a compo~ition ha3 in-plane anisotropy at room temperature, it will hhve sufficiently small amount of in-plane anisotropy or perpendicular anisotropy in the vicinity of TCl.
Further, FIG. 15 ~hows measurement re~ults of dependence on temperature of the effective magnetic ~ ~ 2 6~ P~

anisotropic con3tant K of Gd(Feo gsCoo Ob) obt~ined with the u~e of a magnetic torque meter. ~eferring to the figure, curve 151 (~ ) represents Gdo.38(Feo.95Coo 05)0 62 whose Ms at room temperature i~
approximately 440 emu/c~3, curve 152 (~ ) represents Gdo.32(Feo.95Coo.06)0.6g whose Ms at roo~ temperature i~
approximately 280 emu/cm3, curve 153 (O ~ repre~ent~

o.28(Feo,s5Coo.06)0~72 whose Ms at roo~ temperature is 100 emu/cm3, and curve 154 (~ ) represents GdO.22(Feo.95Co0.o6~o.78 whose MS at room temperature i~
lOO emu/cm3. Here, values of N plotted by ~ are 10 time~ the value~ indicated along the axis of ordinate of FIG. 15. As apparent from the temperature characteristics lSl - 154 o~ K obtaiDed from the ~ea~urement re~ults, the ~E rich film represented by the curve 152 indicate~ the most preferable characteri~tics exhibiting in-plane magnetic ani~otropy at room temperature but exhibiting perpendicular magnetic anisotropy in the vicinity of the Curie temperature.
The curve 151 exhibit~ sufficiently great in-plane magnetic anisotropy at room temperature and exhibit~
small in-plane magnetic ani~otropy in the vlcinity of the Curie temperature, which is al~o a pre~erable characteristic. Further, the curve 153 exhioit~

. ~ '` ' , .
:

13265~7 perpendicular magne-tic ani~o-tropy at room temperature but it iB of a small amount and exhibits smaller perpendicular magnetic anisotropy in -the vicinity of the Curie temper~ture, but thi~ composition may sometimes be u~ed if the characteristic at room temperature in que~tion is compensated for by ~election of materials and thicknesses of the first and second magnetic thin films 11 and 12, or the like. As to the curve 154, however, this curve shows a characteristic not only exhibiting perpendicular magnetic ani~otropy at room temperature but also exhibiting in-plane magnetic ani~otropy in the vicinity of the Curie temperat~re, a characteri~tic contrary to that desired.
In FIG. 16 are ~ho~n measurement results of the Kerr loop (angle of Kerr rotation 9 - magnetic field H curve) for the the magnetic thin film ~howing the characteristic of the curve 152 in FIG. 15 at various temperatures.
In view of the aboYe described meQsurement results, de~ired composition of the third magnetic thin film 13, for example, in Gdx(Fel_yCo)l_x i~ gi~en by 0.25 ~ x 5 0.40, a ~ y ~ l.o, end the value Ms is de~ired to be 0 ~ M$ ~ 450 emuJcm3.

: ' 1326~7 Embodi~ent 5 A disk was made of a polycarbonate substrate with a fir~t magnetic thin film 11 of TbFeCo having a thickness of 400 ~, a third ma~netic thi~ ~ilm 13 of Gd(Fe~.g~Coo.os) having a thickne~s of 160 R and saturation magnetizstion Ms = 280 emu/cm3 at room temperature, and a ~econd magnetic thin film 12 of (GdTb)(FeCo) having a thickness of 650 R depo~ited thereon by sputtering. ~ith the use of this disk, i.e., thermomagnetic recording medium, thermomagnetic recording with a semicQnductor laser beam in the manner as describsed with reference to FIG. 3, and reading the record with a similar laser beam by virtue of the Kerr effect were carried out.
At thi~ time, the power PL for writing, for example, a "0" by obtaining the ~tate A via the state C
was set to 3.6 mW, the power PH for writing, for example, a "1" by obtaining the state B via the state D
was set to 11 mW, and the power P~e~d fvr reading was set to 1. 5 mW. Further, at thi3 time9 the external ~agnetic field HeX was set to 400 Oe, the subsidiary external magnetic field HSub to approximately 3.6 kOe, the linear speed to approximately 10 m/~ec, and the bit length to approximate~y 2.5 ~ m.

:, i32~7 Under the above described condition~, a 2 MHz signal was recorded over a record of a 3 MHz ~ignal previously mRde. A~ the result, the level of the previous 3 MHz signal was lowered.yirtually to that of noises and the overwriting was attained with the CJN
being ~pproximately 47 dB. At this time, the sub~idiary external magnetic field H~Ub required by the medium wa9 aB low a~ just above 3.5 kOe. Further, the total thickness of the laminated film of the first to third magnetic thlo ~ilms 11 to 13 could be made as small a~
1200 8.

Reference ~xample 1 A recording medium Wa9 ~ade fir~t depositing Q
dielectric film of Si3N4 on a polycarbonate sub~trste and then depositing thereon a first magnetic thln film 11 of TbFeCo with a thickness of 400 ~, a third ma~net~c thin film 13 of an RF rich film (Ms - 600 emuJcm3) of Gd(Feo.gsCoo.os) with a thicknes~ of 60 R, and a second magnetic thin film 12 of (Gdo.~Tbo.2)(Feo.gcoo.2~ with a thickness of 650 ~ in turn. First, a 3 MHz signal wa~
recorded therein and then a 3.5 MHz signal was overwritten. Relationships between the signal leve~s and the recording magnetic field (external magnetic .

,; ,:

132~7 field HeX~ used at that time are shown in Table ~.

Signal Le~vels ___________________________________ Hex 3.6 MHz 3.0 MHz (Oe) ~dB) (dB) 100 29.4 6.5 20~ ~3.2 11.2 3~0 2~.2 15.2 400 24.9 17.3 Table 5 Although it i9 desired at thia time that the 3,5 ~Hz signQl becomes large snd the 3 MHz ~i~nal become~ ~m~ll, this medium has not exhibited good valùes for either signal.
Further, the 3.0 MHz 3ignal has increased with increase in the Field HeX~ As the rea~on for it, the following con~ideration may be made. In FIG. 17 are shown the t~tal ~agnetizstion and the magnetization in the tran~ition ~etal of the first and ~econd magnetic thin films 11 and 12 in the vicinlty of the Curie temperature T~l of the first msgnetic thin film snd the e~ternaI magnetic fleld ~recording field) HeX
respectively indisated by white arrow~ and blsck arrow~
drawn within the magnetic thin films 11 and 12 and by a white arrow drawn at the right-hand ~ide of the magnetic .

~326~7 thin films 11 and 12. Since the recording field i9 applied at this time in the direction preventlng the magnetization in the first magDetic thin film 11 from reversing, the reversal in the fir~t magnetic thiD film 11 doe~ not take place unle~ th~ exchange force at thi~
temperature i9 sufficient, namely, the above described expre3~ion (5), HWl > HCl + HeX~ is 3atisfied. Under these condition~, the conditional expre~ion (5) is satisfied ~ore easily when HeX is low, and thereby, the 3 MHz signal is decrea~ed, but when HeX i3 high, i-t is increased. When such an extreme R8 rich film iB used a~
the intermediate magnetic thin film, i.e., the third magnetic thin film 13, a sufficient exchangs -Eorce i8 not ohtained in the vicinity of the Curie temperature TCl of the first magnetic thin film 11, 90 that ~uch an arrangement becomes un3uitable for light-intensity-modulated overwriting.
Accordingly, the freedom of selection of characteristic~ iB increased by the arrangement o~ the third magnetic thin film 13 interposed between the first and ~econd ma~netic thiD films 11 and lZ. E~pecially, since it i~ arranged such that the effect to decrea~e the domain energy in the vicinity of room temperature and increase it in the vicinity of the Curie point can 132~P~

be obtained, ~tabilization of magnetic domain ~all~ at room temperature i~ nchieved and thereby reduction vf the sub~idiary external magnetic field ~9ub a~d hence simplific~tion of the apparatu~ can be attained. By ma~ing high domain wall energy obtainable in.the vicinity of the Curie polnt, it i~ made pos~ible to perform writing into an area while destroying the information previously recorded there, i.e., overwriting, accurately.
Even with the three-l~yer structure of the fir~t to third magnetic thin film~ 11 ta 13, it i8 at lea~t required i~ order to ac~ieve ligbt-intensity-modulated ~verwriting by subjecting the medium to temperatures Tl ~nd Tz that the following expre~sion~
(8) snd ~9) ~re satisfied. More particul~rly, in order that trsnsfer of a magnetized state of the second magnetic thin film lZ to the first magnetic thin f;lm 11J that ist transition from the ~tate C to the state A, or frvm the state D to the ~tage E, take~ place at a temperature just below the Curie temperature TCl of the first magnetic thin fil~ 11, i.e.; a high tenperature below and in the vicinity of T~1, it is required that the following expres~ion corresponding to the above described expression (6) . .~
~, ~32~547 ~ wa > 2MSlhlHCl (8) i8 satisfied, and, on the other hand, it i8 required, in order that the recorded domain~ of the first msgnetic thin film 11 are preserved at room temperQture, or at the time of reproduction, that the fol]Lowing eXpres~ion i9 sati~fied ¦ ~ wa ~ 2MSlhlHCl (9) where ~ wa i8 the domain wHll energy existing between the first and second maglletic thin ~ilms 11 and 12, namely, virtually at the po~ition where the third magnetic thin film 13 is present and corre~ponds to the above described a w In these conditlonal expression~
(8) and ~9), the external magnetic field i8 neglected because it i3 as low, for e~ample, as 200 to 300 Oe a~
Qtainst the coercive force Hal which i~ 1 kOe to 2 ~Oe.
Thus, it i~ required that the relative magnitude of a wa to 2MSlhlNcl at room temperature is reversed at the high temperature ju~t below the Curie temperature TCl of the firat magnetic thin film 11.
That is, as ~hown in FIG. 18, in which the relation~hip between temperature chara~terigtics f ~HCl = 2MSlhlHCl Qnd Ewa = ~ w~ ~re plotted by Q ~olid line curve 231 and a broken line cu~ve 232, respectively, the relative magnitude between theae energy item3 ia required to be ~3 ~ 32~i7 reversed at a ~pecifi.ed temperature Tl. In thi~ case, the temperature Tl at which ~ wa and 2MSlhlHcl become equal is virtually the temperature at which the magnetization in the first magnetic thin film 11 i~
oriented in the same direction a~ the magnetization in the second magnetic thin film 12, namely, the erasing temperature.
In practical use, however, mere satisfaction of the expressions (8) and (9)is not a sufficient condit:ion. It i8 further desired that the quantity ~ w~

- 2MSlhlHCl is a8 great a3 po~sible at -the temperature right below the Curie temperature TCl of the first magnetic thin film 11, and the greater the quantity 2MSlhlHCl ~ ~ wa is, the more steadily the recorded bits or magnetic domains can be preserved. While it i9 reguired that the temperature Tl is controlled to be steady at the time of mass production, since, practically, a large difference between RWa and EHCl cannot be obtained from the above described three-layer structure, the temperature Tl suffers a great change when EWa or R~Cl varies.
Imagining now that a reproducing layer having a large Kerr rotation angle ~ K for enhancing reproduction output as described above is -to be provided ~4 ~ .

~ .
......
_ .. . _ ., ~ . .

~2~7 for a thermomagnetic recording medium of the above described three-layer structure, the reproducing layer, i.e., the reproducing layer Z22 formed o-f a vertically magnet.izable film having a high Curie temperature TC
hence a large Kerr rotation angle ~ k. will be deposited on a substrate 15, as schematically shown in FIG. 19, and further, the fir~t magnetic thin ~ilm 11, the third magnetic thin film 13, and the second magnetic thin film 12 as described ;n FIG. 3 will be deposited thereon one after another, and thereby the medium will be constructed. With ~uch an arrangemen-t, when -the previously recorded state is uch, a~ silown in FIG. 20 indicating the directions of the spin in the transition metal, for example iron Fe, by arrow3 in the respective magDe-tic film~, that the directions of the spin in the reproducing layer 222 and the second magnetic thin film I2 are reveree, and then, if the medium i6 subjected to the temperature Tl for performing overwriting, it sometimes occurs that the direction of magnetization in the fir~t magnetic thin film 11 becomes unstable by the effects of the spin from both the reproducing layer 222 and the second magnetic -thin film 12, and thereby, the phenomenon sf the magnetization ln the first magnetic thin film 11 to comply with that of the second magnetic ~326~7 thin film 12, i.e., transfer of the latter to tlle former, becomes difficult to occur, and thus, the state a as described, for example, in FIG. 3 becomes difficult to be smoothly a-ttained. In order that the trans-~er is smoothly performed, the following condition i8 required to hold 5 WQ > 2MSLhl~ICl ~ 2MSRhRHCP- .,. (10) where MSR~ hR, and HCR are saturation magneti~a-tion, film thickness, and coercive force of the reproducing layer 222. And in this case, it is assumed that the interface domain wall energy between the reproducing layer 222 and the first magnetic thin *ilm 11 is sufficiently larger than 2MSRhRHcR. By providing the reproducing layer 22Z as de~cribed above, the right-hand side of expression (10) becomea larger, and -there-fore, as a means to make it easier to satisfy expression (10), the film thic~ness h~ of the reproducing layer 222 should be made thin. Then, there arises a problem that the reproducing layer 222 becomes less effective in performing its function as the reproducing layer. In order not to sacrifice the thickness of the reproducing layer, it becomes necessary to increase a wa Then, it contradicts with the provision of the third magnetic thin film 13 having in-plane magnetic anisotropy or 4~

. .

13265~7 small perpendicular magnetic ani~otropy at room temperature.
Therefore, it becomes Decessary, while making use of the -three-l~yer ~truc-ture of the first and second magnetic thin films and the third magne-tic thin film interposed therebetween as the basic structure, to provide the means to make sure th~t the above exprQ~sion~ (8) and (9) are satis~ied thereby both at the temperature right below the Curie temperature TCl of the first magnetic thin film 11 and at room temperature, and further, to have the above de~cribed erasing temperature Tl steadily set up.
It is further required to overcome the problem of the instability occurring uhen a reproducing layer, i.e., a magneto-optical reproducing medium having a large Kerr rotation angle, is provided for the above described three-layer basic ~tructure.
To meet the aforesaid necessity i8 used a thermomagnetic recording medium Sl which as shown in a schematic sectional view of FIG. 21 includes a laminated film consisting of a first magDetic thin film 11 formed of a first component film 111 a~d a ~econd component film 112, each thereof having perpendicular magnetic anisotropy, a second magnetic thin film 12 having , : : -: ~
:

~ 3 ~ 7 perpendicular magnetic ani~otropy, and a third magnetic thin film 13 having in-plane magne-tic anisotropy or small perpendicular magnetic ani~otropy in-terpoaed between the first component film 111 of -the first magnetic thin film 11 and the second magnetic thin film 12, formed into a laminated structure being magnetically coupled to the adjoining films in turn. The Curie temperature TCl2 of the second component film 112 is set to be higher than the Curie temperature TCll o~ the first component film 111 of the first magnetic thin film 11. A first heating condition for heating the medium to a temperature T1 whicb is iD the vicinity of the Curie te~perature TCll, not causing reversal of the magnetic moment in the second magnetic thin film 12, and sufficient to change the magnetic moment in the second component film llz of the first magnetic thin film 11 in compllance with the magnetic moment in the second magnet:ic thin film 12 and a second heating condition for heatiDg the same to a temperature T2 which is above the Curie -temperature TCl2 and sufficient to cau~e rever~al of the magnetic moment in the second magnetic thin film 12 are modulated in accvrdance with an information signal to be recordedj whereby, while the medium is cooled from the heated states, record magnetization ia ,:
, ~ .
. "
~:. ,. ;

1 3 ~ 7 formed also in the first component film 111 in compliance with the magnetization in the second componeDt film llz of the first magnetic thin film 11.
Further, to meet the afore~aid requirement is used a thermomagnetic recording medium S2 which as shown in a schematic sectional view of FIG. 22 includes a laminated film consisting of a first magnetic thin film 11 formed of a first component film 111 and a secon(l component film 112, each thereof having perpendicular magnetic anisotropy, a second magnetic thin film 12 having perpendicular magnetic anisotropy, and a third magnetic thin film 13 having in-plane magnetic anisotropy or small perpendicular magnetic anisotropy interposed between the fir~t component film 111 of the fir~t magnetic thin film 11 and the second magnetic thin film lZ, formed into a laminated structure being magnetically coupled to the adjoining films in turn, further having a magneto-optical reproducing thin film 18 disposed in the front of the first component film 11 of the fir~t magnetic thin film 11 magnetically coupled thereto. The Curie temperature TC]2 oP the second component film 112 i8 set to be higher than the ~urie temperature TCll of the fir~t co~ponent film 111 of the first magnetic thin film 11, and further, the m~gneto-132~7 optical reproducing film 18 is adapted to sati~fy 2MSRhRHCR ~ 2MSllhllHCll ~ ~ wa ~ 2MSlZhl2HClZ
... (11)(wheré MSR~ MSll and Msl2; hR, hll~ and hl2; HCRl HCll~
and HClz are ~aturation magnetization, film thickDess, and coercive force of the magneto-optical reproducing thin film 18, first, and ~econd component films 111 and 112, respectively, and ~ wa i8 domain wall energy between the second componeat film 112 and the second magnetic thin film 12) and having a larger Kerr rotation angle ~ K than the first component film 111. A fir~t heating condition for heating the medium to a temperature Tl which is in the vicinity of the Curie temperature TCll~ not.causing reversal of the magnetic moment in the second magnetic thin ~ 12, and suEficient to change the magnetic moment i~ the ~econd component film 112 o~ the first magnetic thin film 11 in compliance with the magnetic moment in the second magnetic thin film 12 and a ~econd heating condition for heating the same to a temperature T2 which i9 above the Curie temperature TCl2 and sufficient to cause reversal of the magnetic ~oment in the second magnetic thin film 12 are modulated in accDrdance with an informatioD
~ignal to be recorded, whereby, while the medium i~

. .
:
~ .

132~7 cooled from -the heated ~-tate~, record magnetization is formed also in the first component film 111 and magneto-optical reproducing thin film 18 in compliance with the magnetiza-tion in the second component film 112 of the first magnetic thin -film 11.
In both the above described structur~, the Curie temperatures TCll and TCl2 o~ the fir~t and second component films 111 and 112 of the first magnetic thin film.l~ and the Curie t~mperatures TC2 Hnd Tc3 of the second Hnd third magnetic thin films 12 and 13 are selected to be TCll ~ Tcl2 < TC3- TC2-The above described first example of FIG. 21is characterized in that the first magnetic thin film 11 is formed of the Eirst and ~econd component fil~s 111 and llz, and tha Curie temperatures TCll and T~12 of the component films 111 and 112 are selected -to be such that the Curie temperature TClz of the second component film 112 i~ higher than the other, i-e-, TCll < TC12 According to this example, the effective coercive force energy of the first magnetic thin film 12 of a two~layer structure i9 given, as ~hown, for example, in FIG. 23, by the sum total of tbe energy of the first and second component films 111 and 112 having different Curie temperatures, i-e., ~um total of EHCll (= 2MSllhllHCll) . , . .~ .,~ , 1326~7 represented by the curve 20411 and EHCl2 (=
2MS12hl2HC12) represented by the curve Z412, that i~, it i6 represented by the curve 241 which has an inflection point in the vicinity of the Curie temperature TCll and shows a ~teeper temperature characteristic on the side lower than TGll toward room temperature. Me~nwhile, Ew~ has a linear temperature characteri~tic as shown by the curve 242, and hence, the difference therebetween becomes large at the temperature lower than the temperature Tl, whereby the recorded information bits, i.e., magnetic domains, can be steadily retained, and ~ur-ther, the temperature Tl where both the characteristic curves 242 and 241 inter~ect can be prevented from greatly varying even when some variatiOIlS are made in Ewa, EHcll~ and EHC12 in the manufacturing proce~ of the thermomagnetic recording media. Further, at the high temperature in the vicinity of Tl, the characteristic of the fir~t magnetic thin film 11 depends only on the characteristic of the second component film 112, so that the effec-tive thicknes~ of the first magnetic thin film 11 i~ reduced to the small thickness hl2 only of the ~econd component film llz, and therefore, expression (8) can also be satisfied.
Further, according to the example de~cribed in , ~326~

FIG. 22, the structure u~ed therein is provided with a magneto-optical reproducing thin film 18 having a large Kerr rota~tion angle ~ K added to the above de~cribed structure and adapted to sati~fy the above de~cribed expre~sion (ll). Hence, in overwriting, the first magnetic thin film ll i8 prevented ~rom becoming `un~table affected by the direction of the magneti~ation in the magneto-optical reproducing thin film 18 having a high Curie temperature TCl and the first component film lll is ensured to -Porm recorded magnetization therein in compliance with the ~econd component film ].12 in the vicinity of the Curie temperature TCll of the first component fil~
A ther~omagnetic recording medium Sl u~ed here is provided, as shown in FIG. 21, by depositing1 in turn, first and second component films lll and ll2 constitutio a first magnetic thin film ll, a third m~gnetic thin film 13, and a secoDd magDetic thin film 12, through a dielectric film 16 serving as a protecting film or interference film, over one side Df a light transmitting ~ubstrate 15 made of a glass plate, acrylic plate, or the like.
The first and second component films lll and ll2 of the first magnetic thin film ll are rare earth-', '; ; -, . . -:;
.~ ;

: `

transition metal thin films made of a material having rather grea-t perpendicular magnetic anisotropy Ku, such as TbFeCo. Both the component films l:Ll and 112 may be made of either a rare earth rich film or a tran3ition metal rich film but the following conditions must be sati~fied. That is, the condition wa > 2MSlzhl2Hcl2 + 2MS12hl2Hex -- (12) must be satisfied at the temperature right below the Curie temperature TClz of the second component ~ilm 112, and the condition ~ wb ~ 2MSllhll~C11 ~ 2MSllhllHex -- (13) must be satisfied at the temperature right below TC
(where ~ wb i~ the domain wall energy density on the interface between the first component film 111 and the second ~omponent film 11~, and He~ i~ the external magnetic field, i.e., the external recording mognetic field).
A thermomagnetic recordin~ medium S2 is provided with the above described structure of the thermomagnetic recording medium Sl and additionally a magneto-optical reproducing thin film 1~ as ~hown in FI~. 22. More particularly, also in the thermomagnetic recording medium S2, a light transmitting substrate 15 made of a gla~s plate, acrylic plate, or the like 1 3 2 6 ~ Dr 7 is used as shown in FIG. 22, and a magneto-optical reproducing thin film 18, -first and second component films 111 and 112 constitu-ting a first magnetic thin film 11, a tllird magnetic thin film 13, and a second magnetic thin film 12, are deposited, in turn, throu~h 8 dielectric film 16 serving as a protecting film or interference film, over one side of -the substrate.
The depa~iting of the ~ilms 16, 111, 112, 13, and 12, or the films 16, 18, 111, 112, 13, and 12 of the thermomagnetic recording media Sl aDd Sz are each achieved by making laminating sputtering in a successive or simultaneous manner -through the use, for example, of a magnetron type ~puttering apparatus performing, for example, multiple-source spu~tering, namely, sputtering from multiple-source targets.
The third magnetic thin film 13 of each of the thermomagnetic recording media Sl and S2 is de3ired to have in-plane magnetic anisotropy or lower perpendicular magnetic anisotropy than tha-t o-f the ~irst and second magnetic thin films 11 and 12, as low as, for example, 1 X 106 erg/cm3 at room temperature and, in addition, be made oE a rare eHrth rich metallic film having the temperature characteri3tic of its effecti~e magnetic anisotropy const~nt K being convex upwHrd or linear and " . . , ~

1326~7 the saturation magnetization Ms at room temperature being 0 to 450 emu~cm3.
In the media Sl and Sz, the ~econd magnetic thin ~ilm 12 can be formed of GdTbFeCo having great perpendicular magnetic anisotropy.
The thermomagnetic recording medium Sl will ~irst be described mentioning an embodiment of it.

Embodiment 6 A thermomagnetic recording medium Sl of the structure as shown in FIG. 21 including the magnetic thin films 111, 112, 13, and 12 having the compo~itions, magnetic characteristics, and film thiCkne~BeB Q8 shown in Table 6 below W8~ prepared.

Thin Compo- Magneti- Coer- Curie Film Film Sition zation cive Temp. Thick-Force ness (emu/cc~ (kOe) (C) (R) ____________________________________________ (111) TbFeCo 30 15 170 250 (112) TbFeCo 20 23 210 Z50 (13)GdFeCo 400 - 235 150 (12~GdTbFeCo180 3.2 350 580 Table 6 The manner of operation~ when thermomagnetic recording iB made with the above de~cribed thermomagnetic recording medium Sl will be described , .

:, .
, with reference to the drawing of FIG. 24 showing magnetized s-tates. In FIG. 24, the directions of the spin of the transition metal Fe in the films 111, llz, 13, and 12 are indicated by arrows. In this case, the directions of the external magnetic field HeX snd the subsidi~ry external magnetic field H~Ub differ wi-th the composition of the second magnetic thin film 12, but the illustrated case is where a transition metal rich f:ilm is u~ed for it. The first temperature Tl i~ selected, for example, to be rigllt below the Curie temperature TCll of the first component film 111, and the second temperature T2 is selected to be above the Curie temperature TCl2 of the second component film 112. Also in this case, in the same manner as described with reference to FIG. 1 and FIG. 3, information is recorded by the states A and B, that is, by the state A wherein the first and the second magnetic thin films 11 and 12 are magnetized oriented in the same direction and the state B wherein the same are magnetiæed oriented in the rever~e directions. In this case, once the medium has been heated at the first temperature Tl (the erasing temperature Tl in FIG. Z3) by irradietion, for example, of a la~er beam, the direction of the spin in the second component film 112 in the process of the medium cooled - , . ..
.`' ~ ' ' ~ , ' ~ ~2g~7 from that temperature is brought to the state C wherein it is in agreemeDt with that of -the second magnetic thin film 12, no matter whether the previous ~tate wa~ A or B, according to the above descri,bed expression ~13) and the intersection of the curves 241 and 242 in FIG. 239 and in the process cooled dowD to right below the Cu:rie ~temperature TCll of the first component film 111, the direction of the spin in the first component film 111 i9 brought into agreement with that of the second component film 112 by the arrangement made BO that expression (13) is satisfied. Thus, no matter whether tlle state i~ A or B, overwriting of the state A i~ achieved by bringing the medium to the first temperature Tl. By heating the medium to the second temperature T2 above the Curie temperature TC2 of the second magnetic thin film 12, namely, above the fir~t and second Curie temperatures TCll and TC12- gimilarly by irradiation, for example, of a laser beam, the direction of the spin in the ~econd magnetic thin ~ilm 12 is rever~ed by virtue of the external magnetic field trecording field) HeX~ and in the subsequent cooling ~tsge, the state E i9 brought about wherein the direetions of the spin in the first and second component films 111 and 112 of the first magnetic thin film 11 are in agreement with the : :. .: , . , . . :. . . ~

i326~7 direction of the Bpin in the second magnetic thin film 12 according to the conditional expressions (12) and (13). And in the state cooled down to room temperature, the state E is changed by virtue of the subsidiary external magnetic field HBUb to the state B wherein the direction of the spin in the second magnetic thin film 12 is reversed. In order that this tran~ition takes place, the subsidiary external magnetic field H~Ub is selected to satisfy the following condition.
Hgub ~IC2 ~ ~ wa / 2MS2h2- ... (14) In the present embodiment, referring to FIG. Z3 showing temperature characteristios of the coercive force energy in the fir~t magnetic thin film 11 and the domain wall energy between the first and the second magnetic thin filcs, the domain wall energy is kept sufficiently small in the vicinity of room temperature as indicated by the curve 242, and therefore, the sub3idiary external magnetic field HSub in expression (14) can be made sufficiently small.
Further, in order to stabilize the magnetized state of the first and second component films 111 ~nd 1l2 of the first magnetic thin film 11 in the state B, the following condition must be sati~fied.

1 3 2 ~ 7 MsllhllHcll -~ Msl2hl2Hcl2 ~ wa ____________-_-~~~~~-~~~- > Hgub + ----------~-------~
MSllhll ~ Msl2hlz 2(MSllhll ~ MS12hl2) .... (15) Reference E~ample 2 ~ ther~ol~agnetic recording medium of a three-layer structure of the ~tructure shown in FIG. 21 but the fir0t magnetic thin film therein is formed of a single film was used. Compositions, magnetic characteri3tics, and film thicknesses of the constituent fil~s in this case are shown in Table 7 below.

Thin Compo- Magneti- Coer- Curie Film Film SitioD zation cive Temp. Thick-Force ness (emu/cc) ~kOe) (C) (~) (11)TbFeCo 30 15 170 500 (13)GdFeCo 400 - 235 125 (12)GdTbFeCo 180 ~.2 350 580 Table 7 Temperature characteristics of the domaiD wall energy Rwa (= ~ wa) between the fir~t and second magnetic t~in film~ 11 and 12 and the coercive ~orce energy of the first magnetic thin film 11 in this case are shown by the curves 251 and 252~ respectively, in FIG. 25.
V~riations of the era~ing temperature Tl for 1326~

the embodinnent 6 and the reference ex~mple 2 will be considered referrin~ to FIG. 23 and FIG. 2~. Variations of the domain wall energy EWa with charlge in the film thickness h3 of the third magnetic thiD film 13 or the like are such, at the temperature 20C, that Ew~ . 2 erg/cm2 when the film thickness h3 = 125 ~ and that EWa _ 1.6 erg~cm2 when the film thicknes~ h3 = 150 R. It is found i~ the case oE FIG. 25 for the reference example 2 that ~ wa = 2 erg/cm2 and the tempera-ture at which EHCl = RWa .is obt~ined is around 130 C, while in the case of FlG. 23 for the embodiment 6, ~ wa = 1.6 erg/cm2 and the temperature Tl . 165C. Since 0.2 erg/cm2 or so of error for ~ wa generally occurs in the manufacture, it is now as~umed that it has become ~ wa =
1.8 erg/cm2 (at 20C). Then, from FIG. 25, Tl becomes Tl . 148C for the three-layer fil~ of the reference example 2, while it becsmes Tl . 162C for the embodiment 6 of FIG. 23. Hence, while 18C of variation in the temperature Tl i8 produced in -the three-layer film again~t a change of 0.2 erg/cm2 in ~ wa, that for the four-layer film of the embodiment 6 of the present invention is kept to a variation a~ small as 3C.
Thu~, by forming the first magnetic thin film of two layers having different Curie point~, it becomes - ~ . .
. . , ~ .
. . . -i ~ ~

~32~7 pos~ibl0, at the time of mass production, to reduce the varia-tions in -the temperature Tl against changes in ~wa or ENC-With the above described structure, it ispre~erred that TCll - TCl2 i~ lO - 70C. Thi~ is because, if it is less than 10C, the effect as described in FIG. 23 is not obtained so much, and, if it exceeds 70C, i.e., i~ TCl2 becomes too high, it becomes necessary to raise the second temperature T2 and hence to have large recording power.

Embodiment 7 Thermomagnetic recording ~edin S2 of the structure as shown in FIG. 22 including the magnetic thin fllms 18, lll, llz, 13, ~nd 12 having the compositions, magnetic characteristics, and film thicknesses as shown in Table 8 below were prepared.

Thin Compo- Magneti- Coer- Curie Film Film Sition zation cive Temp. Thick-- Force ness (e~u/cc) (kOe)- (C) (~) ________________ ___________________________ (18)GdFeCo 30 0 4 400hR
(11l)TbFeCo 30 15 170250 (1l2)TbFeCo 20 23 210250 (13)GdFeCo 400 - 235150 (12)GdTbFeCo 180 3.2 360580 Table 8 ' :
, ., 132~47 A -thermomagnetic recording medium S2A was prepared by settiDg the thicknes~ hR o.f the magneto-optical reproducing thin film 18 to 75 R, and a thermomagnetic recording med.ium S2B was prepared by setting the thickness hR of the magne-to-optical reproducing thin film 18 to 150 R. These media S2A and S2g were structured so as to satisfy the above described expression (11). The manner of operations for thermomagnetic recording u~ing these thermomagnetic recording media S2A and S2g is shvwn in FIG. 26.
Referring to FIG. Z6, the directions o~ the spin in the transition metal Fe are shown by arrow~ drawn in each of the films 18, 111, 112, 13, and 12. ~190 in thi~ case, the first temper~ture Tl was selected to be the Curie teDperature TCll of the first component film 111 and the second temperature Tz wa~ ~elected to be above the Curie temperature TClz of the second component film 112. A1BO
in thi~ case, the same a9 described in FIG. 1 or FIG. 3, recording of information is made by the states A and B, namely, by the state A wherein the first and the second magnetic thin films 11 and 12 are magnetized in the same.
direction and the state B wherein they are magnetized in the reverse directions. In either of the states, the reproducing thin film 18 is magneti~ed in the same . " . , . i , , ; , "
. ~ .. . . .

~3~6~

direction a~ the first magnetic thin film ll. In this case, if the medium is irradiated, for example, by a laser beam and heated up, for example, to th~
-temperature Tl right below the Curie temperature TCll of the. first component film lll, the direc-tion o~ the second component film ll2 is brought into agreement with that of the second magnetic thin film 12 according to the characteristics shown in FIG. 23. Since, ~t this time, the Curie temperature TCR of the magneto-optical reproducing thin film 18 is high, either a state C~ or a state C~ is brought about depending on whether the previous state was the state A or the state B. However, as the medium is cooled toward room temperature TR, even if there has been produced the state CB, it i9 ensured to be changed to the state A during the cooling stage because conditions satisfying expression (ll) have been provided, or more particularly, the ~um total of the coercive force energy of the magneto-optical reproducing thin film l8 and the coercive force energy of the first component film lll has been selected to be smaller than the ~um total of the domain wall energy ~ wa between the second component film ll2 and the second magnetic thin film 12 and the coercive force energy of the second component film ll2. Thus, by subjecting the medium to ' ' ~ :'.

~2~7 the first temperature Tl, recording of information in the state A is achieved. Similarly, by heating the medium with irradiation, for exnmple, of a laser beam up to the second temperature Tz above the Curie temperature TC2 of the second magnetic thin film 12, i~e., ~bove the ~irst and second Curie temperatures TCll and TClz~ the :direction of -the SpiD in the second magnetic thin film 12 is reversed under the influence of the external magnetic field (recordit3g field) HeX~ and in the cooling stage of the medium, the state E iB brought about wherein the directions of both the fir~t and the second component films 111 and 112 of the first magnetic thin film 11 are in agreement with the direction of the second magnetic thin film 12 according to the ~bove de~cribed expressions (12) and (13). When the medium i~
cooled down to room temperature, the state E is changed by the influence of the ~ub~idiary external magnetic field H8Ub to the state B wherein the spin of the second magnetic thin film 12 is reversed. To effect this, the ~ubsidiary external magnetic fi.eld HSub has been selected to satisfy the above described expression (14), and further, the subsidiary external magnetic field HSub can be made sufficiently ~mall the same as described in the embodiment 6.

, . . . ,:
, ' ' ~32~7 Reference Example 3 A thermomagnetic recording medium with a reproducing layer 222 add.itionally laminated to the three-layer structure described in FIG. l9 wa~ used and thermomagnetic recording media having thin film~ 222, 11, 13, and 12 of the compositions, ~agnetic characteristic~, and film thicknesse~ a~ ~hown in Table 9 below were prepared.

Thin Compo- Magneti- Coer- Curle Film Film Sition zation cive Temp. Thick-Force Dess (emu/cc) (kOe) (C) (R) _____________________.._____ ________.________ (222) GdFeCo 30 0 4400 hR
(ll)TbFeCo 30 15 170500 (13)GdFeCo400 - 235125 (12)GdTbFeCo180 3.2 350580 Table 9 Here, media ScA and SCB in which the reproducing layer~ 222 were of different film thicknesses, i.e., hR = 75 R and hR = 150 ~, respectively, were prepared.

.
Measurement results of C/N obtained at the times of overwriting made on the media S2A and S2g as well as Sc~ and ScB of the embodiment r and reference example 3 with the external magnetic field HeX~ strength .:
.
. :. ~
, 1326$~7 of W}liCh was varied1 applied actually are shown in FIG.
27. Referring to the figure, the curve3 271, 272, 273, and 274 represent measuremellt results for the media S2A, S2B. SCA- and Scg. ~s apparent from comparison of the~e, a ~ignificant improvement in C/N is achieved in the embodimeDt 7 of the present invention shown by the curves 271 and 272 a~ against the reference example 3 shown by the curves 273 and 274. The recording conditioas at this time were such that the relative linear speed of the la~er beam to the medium was 11.3 m/s, the recording frequency was 5 MHz, the numerical aperture of the objective lens N.A. = 0.53, and the wave1ellth of the la~er beam was 780 nm.
As described above, an improvement in eliminating noise at the time of overwriting i3 achieved in this invention.
Thus, ~he first tempersture Tl can be set not to vary so much, i.e., stabilized operation of the device can be achieved, and while C/N (S/N) can be improved, reduction of t}le subsidiary external magnetic field can also be attaiDed by reduction of the domain wall energY ~ w (~ a~-Further, by the provision of the magneto-optical reproducing thin film 18 having a large Kerr ~ ' , ' ' . ' .

.. .
-: ". ~
, ' ~ ~ ' - ' ' 1326~7 rotation angle ~ K, enhancement of the reproduced output can be achie~ed, and further, by the provision of the magneto-optical reproducing thin film 18 having a large Kerr rotation angle ~ K. i.e., a high Curie temperature TClj improvement. for stabilized operation and reduced noise can be achieved.
The second magnetic thln film 12 i~ assigned the role to determine the state of recorded magnetic domains and the role to determine the magnitude of the initializing magnetic field (subsidiary external magnetic field). Therefore, when a material having a rather low coercive force H~2 at room temperature i~
u3ed for the second magnetic thin film lZ in order to lower the initializing magnetic field, the state of the recorded magnetic domains ~form, state of magnetization) is disturbed. Hence, a problem is posed that recording noise is increased and it becomes impossible to keep S/N
(C/N) sufficiently high.
Then, to achieve both decrease in the initializing magnetic field ~nd decrease in the noise, which are conditions conflicting with each other, a thermomagnetic recording mediu~ S, as shown in FIG. 2~, i3 used in the present invention, which is formed of, at least, first and second magnetic thin films 11 and 12, 6~

~ , .

13 2 6 ~

each thereof having perpendicular magnetic aniso-tropy, laminated to each other, the second magnetic thin film 12 being formed by lamination through exchQnge coupling of its first and second component film~ 121 and 122.
When coercive forces at room temperature of the fir~t and second component film~ 121 and 122 are represented by HCzlR and HC22R~ respectively, aDd their Curie temperatures are represeoted by TC21 and TC22~
their relationships are adspted, as shown in cur~es 321 and 322 in FIG. 29, to be expre9ge~ as ~C21R >~lC22R and TC21 C TC22- Wlth the use of such a -thermomagnetic recording medium S3, the first heating condition to heat the medium to the first temperature Tl which i8 virtually in the vicinity of the Curie temperature TCl of the first magnetic thin film and not causing reversal of the magnetic moment o~ the second magnetic thin film 12 and the secand heating condition to heat the medium to the second temperature T2 which is above the Curie temperature TCl and sufficient to cauae reversal of the magnetic moment of the second magnetic thin film 12 are modulated according to the information signal to be recorded, and adapts ~uch that both of the magnetic moments in the second magnetic thin ~ilm 12 during the course the medium is cooled from the fir~t and second , , :

1~2~7 heated sta-tes are brought into the same state.
While the same manner of magnetization as that in the process shown in FIG. 3 is performed, decrea~e of the initializing magnetic field tsubsidiary external magnetic field) can be attained without inviting increase in recording noise and lowering of the S/N.
The thermomagnetic recording medium S3 is of similar structure as that described above, but the flrst and second component -~ilms 121 and 122 of the second magnetic thin film 12 are made of a material having small perpendicular magnetic anisotropy and a material having relatively great perpendicular magnetic anisotropy.

Embodiment 8 A thermomagnetic recording medium S3 having magnetic thin fi1ms 11~ 13, lZl, and 122 of compositions, magnetic characteristics, and film thicknesses as shown in Table 10 below and structured as shown in FIG. Z8 was prepared.

.. ~ , . . : ., :, ., : ' 132~5~7 Thin Compo- Magneti- Coer- Curie Magne-tic Film Film Sition zation cive Temp. Compensa- Thlck-Force tion Temp. ne~s emu/cc (kOe) (C) (C) (~) ______ ___________________________________ _____________ (11) TbFeCo 35 12 170 - 510 (13) GdFeCo 4QO - 236 - 125 (121) GdTb~eCo 1603.9 340 230 300 (lZ2) GdFeCo 150 0.4 400 200 Z~O

Table 10 !

At this time, the measured reverse magnetic field, i.e., coercive force Hc, of the first and second component films 121 and 122 in the exchange coupled two-layer state was 2.4 kOe. The specimen of the above structure will be called the specimen 1.

~bodiment 9 A magnetic recording medium was formed of the same constituents as those of the embodiment 8 only having the relative arrangement of the fir~t and second component films of the second magnetic thin film in FIG.
28 reversed. This will be called the specimen 2.

~eference Example 4 In the arrangement of FIG. 281 the second magnetic thin film 12 was ~ormed into a single-layer structure. A specimen was prepared with composition~, magnetic characteristics, and thicknesses of the films - . . . ..

, .;j ~ , ~ 3 ~

set to be as shown in Table 11 below.

Thin Compo- Magneti- Coer- Curie Magnetic Film Film Sition ~ation cive Temp. Compensa- Thick-Force tion Temp. ness emu/cc ~kOe) (C) (C) (~) ________________________________________________________ (11) TbFeCo 35 12 170 - 520 (13) GdFeCo 400 235 125 (12) GdTbFeCo ~160 HC2 TC2 ~Z30 580 Table 11 ~ thermomag1letic recording medium was prepared in the above described arrangement with the coercive force Hcz of the second magnetic thin -film lZ set to 3.9 kOe and its Curie temperature TC2 set to 340 C, as the specimen 3. Another thermomagnetic recording medium was prepared in the same arrangement as above with the coercive force Hcz set -to 3.1 kOe Hnd its Curie temperature TC2 set to 3~0 C, as the specimen 4.
Another thermomagnetic recording medium was prepared in the same arrangement as above with the coercive force HC2 se-t -to Z.2 kOe and its Curie temperature ~C2 sst to 360 C. as the specimen 5.
The measurement results of C/N on the specimens 11 and 12 of the embodiments 8 and 9 according to the present invention are shown by curves 331 and 332 in FIG. 30. Further, ~imilar measurement re~ults on the , 1326~7 specimens 3 to 5 according to the reference example 3 are shown by curves 333 to 335 in FIG. 31. The measurement of FIG. 30 and FIG. 31 are per-formed under the conditions of the linear speed of the irradiating laser beam relative -to the medlum beiDg 10 m/s, the recording frequency being 6.5 MHz, the numerical aperture of the objective lens system N. A. = 0.53, and the wavelength of the laser beam being 780 nm. From the comparison o~ FIG. 30 with FIG. 31, it is apparent that an excellent thermomagnetic recording has been made CQUsing little lowering of C/N against changes in the recording magnetic field HeX-When considering the recording noise, it isknown that the noise is produced from unevenness of the shapes of the recorded magDetic domains and unevenness from bit to bit of the ~tate of subdivided structures of the recorded magnetic domains. If ideally recorded magnetic domains are to be shown, they may, for example become uni-form circles as in FIG. 32A. Against this, FIG. 5~ and FIG. 5C show the noi~e-producing unevenly shaped magnetic domains and subdivided magnetic domaiDs, respectively. A8 to how the recorded magnetic domains are produced in the formation of information bits, i.e., recording, it depends on various conditions such as the : . . .

,~

~L 3 2 ~

recording power, the coercive ~orce HCl thicknes3 h, magnetization Ms, and domain wall energy ~ B of the magnetic thiD films, and the external magnetic field ex-Generally, since the border line where HeX
becomes HeX = HC is unclear with a material having a low Hc value, the ~hapes of the recorded magnetic domains are frequently disturbed.
On the other hand, when HeX i8 :illgUffiCieDt Qr ~ B is not uniformly distributed, the recorded magnetic domains sometimes take the form of the subdivided magnetic domains 8S schematically shown in FIG. 32C.
Since there are present no subdivided ~agnetic domains in a material having stabilized magnetic domain~ being large in diameter, rmin ~ ~ B/~NSHC ~rmin = mini~um magnetlc domain radius), 9uch a phenomenon hardly occurs. For example, since the ~roduct MSHc is small in GdFeCo, rmin becomes large and the recorded domains a~
shown in FIG. 32C are hardly produced. However, since HC is small in GdFeCo, the recorded domains as ~hown in FIG. 32B are liable to appear, producing a great noi~e resulted therefro~. On the o-ther hand, ~ince TbFeCo has high Hc, the noise from the formation of the magnetic domains as shown in FIG. 32C is easily made while the .
:: .

1 3 ~ 7 noi~e from the formation of the magnetic domain~ a~
shown in FIG. 32B i8 hardly mnde. In conclu~ion, such a material is pre~erred as the material for the ~econd magnetic thiD film 12 for light-modulated overwriting that has relatively low Hc and e~hibiting low recording noise even if HeX is small. Namely; HeX is required to be small to have the earlier de~cribed expres6ion (5) ~atisfied easily.
Further, the second magnetic thin fil~l 12 will be considered. When the same is formed in a ~ingle layer, it is required to decrease HC2 for lowering H8Ub~
from which it necessarily follows that the reco~ded domains as shown in FIG. 32B are easily formed causing the noise. When forming the second magnetic thin film 12 into A two-layer ~tructure, two arrangement~ are pos~ible, one being that characteri~ed as sho~n in FIG.
29 and the other being that characterized by HC2lR >

HC22~ and TC21 > Tc22 as shown by curves 361 and 362 in FIG. 33. In the csqe where the film i9 ~elected tD be as characterized in FIG. ~3, the magnetic field for rever~ing magnetization of the second magnetic thin film 12 at room temperature T~ is given by the average of HC2l and HC22, and therefore, lowering of the aubsidiary external magnetic field H~Ub, i.e., the initializing , ~ , 1 3 2 ~ ~ ~r 7 magn~tic field of the second magnetic thin film 12 can be achieved. The state at the time of recording is virtually the 8ame as that when recording i~ perfor~ed on a single-layer film 12 of the first component film 121. Hence, the effective Hc at the time of recording on the secolld magnetic thin film 12 becomes large 80.
!that the formation of the magnetic domains as shown in FIG. 32B is suppressed and production of the resultant noise is suppressed. Accordingly, the noi~e resulting from the formation of the magnetic domains as shown in FIG. 32C comes into question. In contra~t, when the arrangement as described in FIG. 29 is employed as in the presen-t invention, since the shapes of the magnetic domaihs at the time of recording are determined by the first component film 121 having greater Hc, formation of the magnetic domains of FIG. 32B is suppressed and thereby production of the resultant noise is suppressed.
Further, at the recording, reversed ~agnetizatioD is first produced in the second component film 12~ having smaller Hc, so -that formation of the magnetic domains of FIG. 32C is suppressed and the magnetic domains producing little nolse as shown in FIG. 5A are produced on the second component film 122, and the~e are transferred onto the first component film 121. Thus, . .

: . .. .: ~: ...
. :: . ~ :

~326~7 all in all, formation of the magnetic domaills of FIC.
32B and FIG. 32C can be suppressed and effective reduction of noises can be achieved.
Further, in the thermomagnetic recording medium Sl de~cribed in FIG. 21, its second magnetic thin film 12 may be formed of first and second component films lZl and 122 as shown in FIG. 34. In ~uch an arrangement, the fir~t and second component films 12 and 122 may be formed of ~a~netic thin films both thereof having perpendicular maglletic anisotropy, temperature characteristics of the coercive forces ~1C2l and HC22 thereof being a~ shown by curves 321 and 322 in FIG. 29, namely, coercive forces at room temperature HC21R and HC22R o-f the coercive force~ HC2l and HC22 being set to be as HC2lR > HC22R and Curie temperatures TC2l and TC22 thereof being set to be a~ TC21 c TC22 and thereby, reduction of recording noise and improvement of reproduction C/N (S/N) cnn be achieved.
The arrangement of the second magnetic thin film 12 formed of a two-layer s-tructure may be applied to the medium S2 having a magneto-optical reproducing thin film 18 as described in FIG. 22, and in such ways, various ~odifications of the embodiment other than those described above can be made.

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Claims (4)

1. A thermomagnetic recording method using a thermomagnetic recording medium including a laminated film consisting of a first and a second magnetic thin film having perpendicular magnetic anisotropy and a third magnetic thin film having in-plane magnetic anisotropy or small perpendicular magnetic anisotropy interposed therebetween, laminated by being magnetically coupled to the adjoining films in turn, comprising the steps of:
modulating in accordance with an information signal to be recorded a first heating condition to heat said medium to a temperature T1 which is virtually above the Curie temperature TCl of said first magnetic thin film and not causing reversal of the magnetic moment in said second magnetic thin film and a second heating condition to heat the same to a temperature T2 which is above said temperature TCl and sufficient to cause reversal of the magnetic moment in said second magnetic thin film; and cooling the medium from the heated states so that record magnetization is formed in said thermomagnetic recording medium.

2. A thermomagnetic recording method using a thermomagnetic recording medium including a laminated film consisting of a first and a second magnetic thin film having perpendicular magnetic anisotropy and a third magnetic thin film interposed therebetween, laminated by being magnetically coupled to the adjoining films in turn, said third magnetic thin film being made of a rare earth rich metallic film, which has in-plane magnetic anisotropy or smaller perpendicular magnetic anisotropy than that of said first and second magnetic thin films at room temperature and has a temperature characteristic of the effective anisotropy constant K
being convex upward or linear, and having its saturation magnetization Ms being from 0 to 450 emu/cm3 at room temperature, comprising the steps of:
modulating in accordance with an information signal to be recorded a first heating condition to heat said medium to a temperature T1 which is virtually above the Curie temperature TCl of said first magnetic thin film and not causing reversal of the magnetic moment in said second magnetic thin film and a second heating condition to heat the same to a temperature T2 which is above said temperature TCl and sufficient to cause reversal of the magnetic moment in said second magnetic thin film; and cooling the medium from the heated states so that record magnetization is formed in said thermomagnetic recording medium.

3. A thermomagnetic recording method using a thermomagnetic recording medium including a laminated film consisting of a first magnetic thin film formed of a first component film and a second component film each thereof having perpendicular magnetic anisotropy, a second magnetic thin film having perpendicular magnetic anisotropy, and a third magnetic thin film having in-plane magnetic anisotropy or small perpendicular magnetic anisotropy interposed between said second component film of said first magnetic thin film and said second magnetic thin film, laminated by being magnetically coupled to the adjoining films in turn, the Curie temperature TCl2 of said second component film of said first magnetic thin film being higher than the Curie temperature TCl1 of said first component film thereof, comprising the steps of:
modulating in accordance with an information signal to be recorded a first heating condition for heating the medium to a temperature T1 which is in the vicinity of said Curie temperature TC11, not causing reversal of the magnetic moment in said second magnetic thin film, and sufficient to change the magnetic moment in said second component film of said first magnetic thin film in compliance with the magnetic moment in said second magnetic thin film and a second heating condition for heating the same to a temperature T2 which is above said Curie temperature TC12 and sufficient to cause reversal of the magnetic moment in said second magnetic thin film; and cooling the medium from the heated states so that record magnetization is formed, in compliance with the magnetization in said second component film of said first magnetic thin film, also in said first component film thereof.

4. A thermomagnetic recording method using a thermomagnetic recording medium including a laminated film having a first magnetic thin film formed of a first component film and a second component film, each thereof having perpendicular magnetic anisotropy, a second magnetic thin film having perpendicular magnetic anisotropy, and a third magnetic thin film having in-plane magnetic anisotropy or small perpendicular magnetic anisotropy interposed between said second component film of said first magnetic thin film and said second magnetic thin film magnetically coupled to the adjoining films in turn, and further having a magneto-optical reproducing thin film disposed in the front of said first component film of said first magnetic thin film magnetically coupled thereto, the Curie temperature TCl2 of said second component film of said first magnetic thin film being higher than the Curie temperature TCl1 of said first component film thereof, said magneto-optical reproducing film satisfying 2MSRhRHCR + 2MS11h11HCl1 < .sigma. wa +

2MS12h12HCl2 (where MSR, MS11 and MS12; hR, h11, and h12; HCR, HCl1, and HCl2 are saturation magnetization, film thickness, and coercive force of said magneto-optical reproducing thin film, first, and second component films, respectively, and .sigma. wa is domain wall energy between said second component film and said second magnetic thin film) and having a larger Kerr rotation angle than said first component film, comprising the steps of:
modulating in accordance with an information signal to be recorded a first heating condition for heating the medium to a temperature T1 which is in the vicinity of said Curie temperature TCl1, not causing reversal of the magnetic moment in said second magnetic thin film, and sufficient to change the magnetic moment in said second component film of said first magnetic thin film in compliance with the magnetic moment in said second magnetic thin film and a second heating condition for heating the same to a temperature T2 which is above said Curie temperature TCl2 and sufficient to cause reversal of the magnetic moment in said second magnetic thin film; and cooling the medium from the heated states so that record magnetization is formed, in compliance with the magnetization in said second component film of said first magnetic thin film, also in said first component film thereof and said magneto-optical reproducing thin film.

6. A thermomagnetic recording method using a thermomagnetic recording medium formed in a laminated structure of, at least, fist and second magnetic thin films each having perpendicular magnetic anisotropy, wherein said second magnetic thin film is formed of first and second component films laminated to each other by exchange coupling, said first and second component films having characteristics HC21R > HC22R and TC21 <

TC22, HC21R and HC22R representing coercive forces of said first and second component films at room temperature and TC21 and TC22 representing the Curie temperatures of the same, comprising the steps of:
modulating a first heating condition to heat the medium to a first temperature T1 being virtually in the vicinity of the Curie temperature TCl of said first magnetic thin film and not causing reversal of the magnetic moment in said second magnetic thin film and a second heating condition to heat the medium to a second temperature T2 being over said Curie temperature TCl and sufficient to cause reversal of the magnetic moment in said second magnetic thin film in accordance with an information signal to be recorded, and adapting in the course of the medium cooling down from the first and second heated states such that the magnetic moments within said second magnetic thin film are brought into the same state.