CA2159233C - Method, apparatus and medium for magneto-optical recording - Google Patents
Method, apparatus and medium for magneto-optical recordingInfo
- Publication number
- CA2159233C CA2159233C CA002159233A CA2159233A CA2159233C CA 2159233 C CA2159233 C CA 2159233C CA 002159233 A CA002159233 A CA 002159233A CA 2159233 A CA2159233 A CA 2159233A CA 2159233 C CA2159233 C CA 2159233C
- Authority
- CA
- Canada
- Prior art keywords
- layer
- medium
- layers
- temperature
- rich
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
Abstract
The object of the invention is to eliminate by using a multi-layered film initializing field in magneto-optical recording capable of direct overwriting, and to decrease the restrictions on the compositions of materials for media. The medium consists of two exchange-coupled RETM
amorphous layers (a memory layer a reference layer) whose Curie temperatures arealmost the same and only one of which is RE-rich without a compensation temperature, the two layers being laminated directly or indirectly with an intermediate layer that allows exchange-coupling to be interposed. Before recording is carried out, the reference layer is magnetized in one direction. Pulses of energy are emitted from the memory layer side so that the temperature of the reference layer remains below its Curie temperature when one of the bit data is recorded but reaches its Curie temperature when the other of the bit data is recorded.
amorphous layers (a memory layer a reference layer) whose Curie temperatures arealmost the same and only one of which is RE-rich without a compensation temperature, the two layers being laminated directly or indirectly with an intermediate layer that allows exchange-coupling to be interposed. Before recording is carried out, the reference layer is magnetized in one direction. Pulses of energy are emitted from the memory layer side so that the temperature of the reference layer remains below its Curie temperature when one of the bit data is recorded but reaches its Curie temperature when the other of the bit data is recorded.
Description
_ METHOD, APPA}~ATUS, AND MEDIUM FOR
MAGNETO-OPTICAI I? F,CORDING
This invention relates to a method, apparatus, and medium for magneto-optical 5 recording capable of direct overwriting.
In magneto-optical recording, various methods for direct overwriting have been proposed to improve the data rate. They include a typical example of a light modulation method using a double-layered film, which is di.sclosecl in IA Published Unexamined Patent Application (PUPA) G2-17594~.
"The contents of this apl~lication are also rer~orted by Matsumoto et al. in "Direct Overwrite by Light Power Modulation on Magneto-Optical Double-Layered Media," Digest of 53rd Seminar, the Magnetics Society of lapan (1927), p. g7."
A recording medium used in this method has a recording layer consisting of two layers, a memory layer and a reference layer, which are exchange-coupled. Overwriting is performed by utilizing the difference in temperature clependence of the coercive forces of the two layers.
The general subject matter related to the present invention, together with the invention itself, will be more readily understood fronn the folk~wing description taken in conjunction with the appended drawings wherein:
Figure l illustrates the temperature dependence o f the coercive force of prior art magneto-optical recording media capal~lc Or direct overwtiting;
Figure 2 is an explanatory d iagram showing the principle of a prior art magneto-optical recording method capable of direct overwriting;
Figure 3 is a diagram i]luslrating the teml-eralute clependcnce of the coercive force of magneto-optical recording media capa~le of direcl overwriting according to the invention;
Figure 4 is a diagram illustrating the relatiotlship between the composition andmagnetic properties of TbFe films;
Figure 5 is a diagram illustrating the temr~eratllre del~endence of the coercive force of a first example of magneto-optical recording media;
Figure 6 is an explanatory view of a magneto-o,r~ical recording process using the medium shown in Figure S;
Figure 7 is an explanatory cliagram of a maglleto-optical recording process using the mediurn shown in Figure S;
Figure 8 is a diagram illustrating the temperat-lte depen(lcllcc of the coercive force of 5 a second example of magneto-optical recor(ling media;
Figure 9 is an explanatory diagl3m Or a magnel,o-optical recol(ling process using the medium shown in Figure ~;
Figure 10 is an explanatory diagtam Or a magneto-optical recording process using the medium shown in F~igure ~;
Figure 11 is a diagram illustrating the telllperat-ll-e der)endence Or the coercive force of a third example of magneto-optical recordillg media;
~igure 12 is an explanat(!ry diagram of a magnelo-or)tical rccor(ling process using t,he med;um shown in Figure I l;
Figure 13 is an explanatory diagl am or a magneto-ol-tical recording process using the medium shown in Figure I l;
Figure 14 is an explanatory diagram or a wr-iting process for domains of variable length;
Figure 15 is a schematic (liagram illustl-aling the compositioll of a magneto-optical recording apparatus according t,o the invenlioll;
Figure 16 is a schematic diagram illusl,rating t,he composition of a medium used in an experiment; and Figure 17 is a drawn copy ora ph(!tl!lllicroglclpll Ora me(lium sulface after overwriting.
Figure 1 shows the magl1etic properlies, an(l Figure 2 sllow.s the overwriting process.
As shown in Figure l, the c(!ml-(!siti(!ns Of the two layers are adjusted so that the coercive force of the reference layet (Hr2) is smaller thall thclt oi' the memory layer (Hrl) at room temperature (Tambl), and t,he Curie temperatllre of the refelence layer (Tc2) is higher than that of the memory layer (Tcl). As showll in Figute 2, One ~-f the characteristics of this method is that an initializing field, as well as a hias field for recor(ling, is applied before data is recorded on the memory layer. The directiolls of l;he hias fiel(l and the initializing field are anti-parallel. The magnitude of the bias l'iel(l Hb is set al suc11 a small value as to maintain the magnetization of the reference layer unrevetcsed in the L ptOCCSS, which will be referred to later. On the other hancl, the magnit-lde of the initializing fiekl Hini is set at a value larger S than Hr2 but smaller than Hrl. As a res~llt, only the maglleliz.ltion of the reference layer is oriented parallel to llini (downwarcl in the figure). The data recorcled ;n the memory layer is not affected by Hini.
For recording, the H proccss Or l process is performe(l, depending on the bit data to be recorded. In the L process, a low-powel laser ~eam in the rOrm of pulses is emitted so that the temperature of the memory layer TmL hecomec7 Tcl ~ Tm~ < Tc2. At this time, the magnetization of the rererence layel- is 11(!t reverse(l. Thererore, the magnetization of the memory layer is oriente~l in a directioll ~Ietermined by the exchange-coupling with the reference layer during the cooling process. Tlle term "exchange-coupling" here means a phenomenon such that the subllelwork ma~nelizations of RE and TM atoms are aligned to IS those of similar, respectively, evcn in cliffelent layel-s. ~hererore, depending on the eompositions of the two layers, the exchange-cour,ling exerted by one layer during the cooling of the other layer may result in these l~lyers having T-arallel or anti-parallel direct;ons of magnet;zation. Figure 2 shows the CaSC in whicll 1I1C (litcctions of magnetization of the two layers become parallel as a result of excllal1ge-coupling.
In the 11 process, a high-power lascl bcam in lhc ~orm Or pul<,es ;s emitted, with the result that the temperature of the memoly layel Tml--l bccl)mcs Tc2 < TmH. Consequently, during the cooling process, the magllelization Of llle relelellce layet first coincides with the direction of the bias field (upwar(l in the figure). Th:ll is, Ihc dilection of magnetizalion of the reference layer is reverscd. When ~he temrel~al~lle Or lhc recor(ling layer (lecr~ases, the magnetization of the memory layer is orienle(l in a direclioll delermined by the exchange-coupling with the reference layel-. Since thc ~litccl;oll of m~glletizatioll of the reference layer has been reversed from that in thc I r)lOCCSS, thc dilcction Or magnetization of the memory layer is also reversed from that in the l ptOCC'~S.
As described above, the metho(l of.lA p~lp~ 2-17.~94~ needs an external field for 21 5~233 initializing the reference layer (initializing field) before recor(ling (by the L process or H
process), in addition to an cxternal fiekl applied (lurillg recotding (a bias field). This makes the apparatus complicated. The above meth(ld als(l involves tlle problcm that data recorded in the memory layer are lost owing to the influence of thc sttOng initializing field. Moreover, 5 this method also involves the prohlem that strict requil-emcnts fot the Curie temperatures and coercive forces of respective layers result in Iess flexibility in the selection of materials and necessitate accurate control of the compositions of materialx during the preparation of media.
Some methods of eliminating the initi<llizing field have bccn proposed. Among them, T. Fukami and his colleagues' "Novel direcl Ovelwriting technology for magneto-optical disks by exchange-coupled RE-TM quadrilayered films," .1. Appl. Pllys. 67(9), 1 May 1990 uses quadrilayered films as recording media and make~i tlle Curie lemperatures, coercive forces, and inter-layer exchange-coupling forces of respeclive la~ers differcl1t. In this method, howevcr, the number of layers of the medium is incleased to rour and these layers need to satisfy certain relative requirements witl1 respect to Curie temperatures, exchange-coupling 1~ forces, and so on. Therefore, this method not only t'ails to rcmove the restrictions on the compositions of materials, but rather incl-eases them. In or(ler to satisfy the requirements, highly accurate control of the composition of cacll la.ycl- is necessary, and hence the production cost of media becomes a problen1 aff'ectillg tlleil pr.lctical use. F~ulther, the total thickness of four layers amounts to a value oF the Or(lCI' Or 2(()() angslloms. Tllis results in lower writing 20 efficiency and hence requires higher laser ener~y.
Therefore, an objcct oI' tlle invcntion i~ lo plovi(lc a methocl and apparatus for magneto-optical recording ca,nahlc of (litect OVCI'~'t'i~,illg, ~lsing a multi-layeled film that does not need an initializing field an~l thal tlCVCt' ca~lses Cl'l'OIlC~US crasulc of recorded data.
Another object of the inventioll is to provi(le a metllo~l all<l apparat~ls for magneto-25 optical recording capable Or dircct OVCI'Wi'itillg thal allcviatc~ the rcstrictions on the compositions of matcrials for media.
Still another object of the invenlion is to pro\/i(le a me(lium for use in the above-mentioned magneto-optical recorcling.
In both processes for recording bit data, the metho(l of IA PUPA 62-175948 utilized 2 1 5~233 exchange-coupling to orient the magnetizati(ln Or the memoly layel in a direction determined by the exchange-eoupling with the referencc layer. Tllcrefot-c, the magnetization of the reference layel has to be rever~ecl in the recorcling proce~s for one of the bit (lata. This is why the method needs an external fiel(l for initiali%ing the reference layer (an initializing field) S before recording.
In order to overcome the problem, lhe prcsetlt invention uxcs, in.stead of a medium as shown in Figure 1, a novel me(lium consisting of two exchange-couple(l rare earth-transition metal amorphous (RE-TM) layers whose Curic temperatures arc almost the same and only one of which is RE-rich without a competlsation felnperatute, the two layers being laminated directly or indirectly with an intertne(liafe layer that allows exchange-coupling to be interposed. Before recording, one of the two layers is magnetized in onc dircction beforehand.
Either one of the magnetized RE-TM layers is use(l as the refetence layer, and the other is used as the memory layer. Magnclizatioll Or the rererel1ce layer has to bc carried out only once, before all data writing ptOcesses. In contrast, .IA PUPA ~2-175948 magnetizes the IS reference layer in a desired writing area every time data writing is carried out, that is, before each emission of a laser pulse. Note, ~hcrcfore, that the magnetization of the referencc layer performed beforehand in the present in\~entiol1 is nol the same as tlle "initialization" referred to in JA PUPA 62-1 7594Y,.
Writing of data is carricd out by:
~a) moving the mcclium relalive ~o an energy .'iOlllCC ill a bias field, with the referencc laycr being farther rr'tn Ihc CllClgy SOUICC fllall thc mcmoly layer;
(b) emitting to the medium wllen recol(lin~ one ol' llle l~it (lata a pulse of energy such that the temperature or the memoly layer beconles necll Or above ils Curie temperature while that of the referencc laycr relllain~ l~elow il~ Curie femr~el atut-c; ancf (c) emitting to the me~liulll whell ICCOr(lillg ~hc o thcr of the bit data a pulse of energy such that the tempeJatules Or lhe two layer~i become near or above their Curie temperatures.
ln step (b), the directioll of maglletization of the memory layer is detcrmined by the exchange-coupling with the reference layel . In StCp (C), thc dilcc~ion Or magnetization of the 21 Sq233 memory layer is determine~l by the ditection Of lhe bias fiekl. On the other hancl, the direction of magnetization of the reletence lcayer maintains lhe direction of its original magnetization regardless of whethel ster) (b) or (c) is carriecl out. Since the direction of magnetization of the reference layer is nOt revetse(l, the me(lium does not need an initializing S field, which was needed in JA PUPA (!2-17594~.
Figure 3 shows two types Of temperalure del-elldence Of the coercive force of double-layered films usecl in the present invenliom A Curie ~empetLItute is a temperature at which the coercive force becomes zero. A coml-ensation temr~elatute is a temperature at which the eoercive force diverges. In either case, ~hc following ~W(? re~luirements must be satisfied:
(1) The Curie temperatures or ~he two layers at-e almo~t lhe ~ame; and (2) only one of the layers is R~-rich witlloul a coml~ensation temperature.
For e~ample, when a typical magneto-o[-lical recording ma~eti31, TbFe film, is used for both layers, since Curie temperatules do not substantially vary witll the ratio of Tb and Fe, 15 requirement (I) is automatically xatisfied. In this respect, consi~lering that tlle method of JA
PUPA 62-175948 controls the combinationx an(l compoxitionx of three to four kinds of elements in order to distinguish lhe Cutic tCmt-CratUlC.~ ol' two layers, it is easier to prepare media according to the presenl inventiorl. Re~luiremell~ (2) can re.l(lily be satisfied in the case of a TbFe film by merely setting the composilioll r.ltio of Th al a value above 26% for one 20 of the layers and below 26~/n for the other layer (see Fi~ute 4).
Thus, the invention largely remove!i resllicliolls On the materials used for media. In the subsequent description~ a me(lium in Ille l/-r m ol' a latnimltiotl consisting of (I) an RE-rich layer without a compensatioll teml-er.~ re an(:l (2) a T M-t ich laycr is callcd A-type, wllereas a medium in the form of a lamina~ion con~ tirlg of (I) al1 RE-rich layer without a 25 compensation temperature an(l (2) a RE-rich l~lyer witll a comr)ensatioll temperature is called B-type. In both typcs, either of the two layers m.ly he usc(l as the memory layer. In the present invention, the layer facing the laset soulce hehaves as the memory layer.
Between the memory layer and the reference layer may be interposed an intermediate layer whose thickness does not prevent exchange-courtling t~etween them. Insertion of a layer 2 1 5~233 such as Tb Ol GdFeCo in order ~o a(ljust the stlellgtll of exchclnge-coupling is a known techn;que. See, for example, K. Aratani et al.~ "Overwriting On a magneto-optical clisk with magnetic triple layers by mean~ or the light inten~ity m(!~lulatioll method," Proc. SPIE 1078, 265 ( 1 989) .
S The recording method used in the pre~ent in~elllion is exl~lained below. Before bit data is written, the magnetization of the layer lhclt will function a~; the reference layer is oriented in one directiom As will be understoo(l from the ~ubsequenl; descliption, overwriting is possible in whatever state the memory layer is magnetize(l berorehand. Thus, magnetization may be oriented in one direction heforehan(l througllout the entil-ety of the medium, including the reference layer. Therefore, prelimillary maglletization or the rcfcr-encc layer can be carried out by magnetizing it uniformly in a sufriciently ~trong fiekl before sllipment. If the medium is not magnetized before shil-ment, rnagnetization may be carried out by using a magneto-optical recording apparatus referled to later.
After magnetization of the referellce layel~ complete, hil data is written on the medium in a bias field. It is prererablc to use shorl r~ulses (of a few n~noseconcls) for writing one of the bit data and long pulses (lens of nanosccon(ls Or more) for writing the other of the bit data. As disclosed by T. Oht~uki et al. in "Direct overwlile by short pulses on double-layered MO media," Conference Dige~l of Topic.ll Meeting on Optical Data Storage, 172 (1990), when short pulses wil:h a dlllalioll of tlle order Of olle nallosecond are emitte(l to the magneto-optical recording medium from the memory layel side, mal ked temperature gradients are produccd in the two layers, an<l hcll(;c only lhC Icmpclcltul~c of the memory layer rises above the Curie temperature, wllile the reI'erellce layel renlclills at a tempcrature low enough to maintain the magnetization. 1~ Ccl~iC~ dUlillg tllC plOCC~!i (')I'COOlillg the memory layer, a strong exchange-coupling ahove 1()()() (Oe~ is exerte(l on ~he mellloly layer by the refelence ]ayer. Therefore, if the magnitude of lhe bias lickl i~ sct lowcr lhan that of the exchaMge-coupling, the direction of magnctization or the mernoly !ayer i~ (Ietcrmined by the exchange-coupling with the reference layer. It is surficienl lo apply a f'iel(l of the order of 500 (Oe) by using, for example, a permanent magnet. In the ple~ent method, ~he process executed while the reference layer is maintained at a low teml eralur-e bel(lu il~ Curie ~emperature is called .~
the L process.
In contrast, when long pulses wilh a dulatioll ol' lens of nanoseconds or more are emitted, both layers are heate(l al~ove their Curie tempelatures and marked temperature gradients are not induced in the clirection of the Ihickness of the medium. When the heating is finished and the medium has cooled to the Curie temperatute, at which magnetization of the memory ]ayer is determined, magnetization of eacl1 of ~he two layers is determined aecording to the direction of the bias fickl, bccausc the exchange-coupling between the two layers is sufficiently small (if the Curie teml-er.lt~lles of the two ]ayers are absolutely equal, the strength is 0 (Oe)). In the presell~ melh(~(l, the ptOCCSS that inclu(le~ a step in which the reference layer is heated to a higll ternperature ncar or abovc its Curie temperature is called the H process.
The step in the H process in wllich the direcliol1 or magnetization of the memory layer is determined is now cxplained in greater (lelail. Ir lhere is a dirfcl-ence between the Curie temperatures of the two layers, the direclion of magnelizalion is first determined by the bias field for the layer whose Curie temperature is highel (TcH layer). Therefore, at the instant when the medium has further col-led to the Curie temperature of the other layer (TcL layer), exchange-coupling witll the TcH layer .3S wcll as the bias fiekl is exerte(J on the TcL layer.
At this time, the larger the diflelence l etweel1 the Curie teml~er.ltules of the two layers, the greater the strength of the exchangc-couplillg. 1ll thc plCSCIlt invention, however, it is requ;red that the direction of magnetization of the TcL layer slloul(l rollow the direct;on of the bias field. Therefore, the dif'ference hetweell tl~e Cul ie Iemrerat-ll-e~ Or the two layers must be so srnall that the strength of the exchange-coul-lillg belweell tlle l~! layers does not prevent magnetization of the memory layer by the hias l'icl(l. Ai cxplaillc(l abovc, it is easy to produce such a double-]ayered film in which lhe Curie Ieml-eratules al-e surficiently close.
Presented below is a detaile(l exl lancltioll Of how ovelwritillg is carried out when the aforementioned requ;rements are met. First, with ICI'ClCllCC to Figures 5 to 7, an overwriting process using an A-type melliull1 is exl lained. An A-tyl~e double-layered film is stable at room temperature in a state in which the (lirections of magnetizatiotl al-e oriented anti-parallel to each other by exchange-coupling. It is assume(l here thclt the memory layer 10 is TM-rich and 21 592~3 '.._ that the reference layer 12 is RE-rich witlloLIt a compcnsation tcmpcrature. It is also assumed that the magnetization of the rcfcrellcc laycr 12 has bccll oricntcd upward l~eforelland and that the direction Or the bias ficld is UPWL1td.
When short pulscs arc cmitte~l, thc mcmory laycr l() is hcatc(l to a tempcrature TmL
5 above its Curie temperature, but lhc rcrcrcncc la~cr 12 is hcatc~l only to a temperature TrL
below its Curie temperature (scc Figurc 5). As a rcsult, immc<liatcly aftcr the heating, only the reference layer 12 in the hcatc(J arca maintains thc magnctization set bcfore the heating (see (A) in Figure 6). Whcn thc mcmol-y laycr I() has coolcd, thc magnetization is oriented downward by exchange-couTpling ~T~7ilh lhc rcfercncc laycl 12 (scc (B) in Figure 6).
When long pulscs arc emiltc(l, hotll thc mcmory laycr 10 and thc reference layer 12 are heated to temperatures ~TmH an(l TrH) al~OVC thcir Curic tclnr~clatures (see Figure 5). As a result, both layers in thc hcatcd arca losc thcil magnctizations sct bcforc the heating (see (A) in Figure 7). When the mcdi-lm has coolccl, thc magnctizatioll of both layers is oriented upward by thc bias field ((B) in Figurc 7). ThJougllout thc l proccss (Figure 6) and the H
15 process (Figure 7), the magnctization Or thc rcfcrencc laycr 12 is maintained upward as in the original state. Even though thc mem(lry laycr l () is RE-rich an(l thc rcfcJcnce layer 12 is TM-rich, overwriting is also carried out througll thc salllc stel~s.
Next, with rcferencc to l~igurcs X to 1~), an cxalnl-lc Of an ovcrwJiting process using a B-type mcdium is cxplaine(l. Thc magtlc~iz~tioll diJcctioll ol' ~1 layer Wit]l a compensation 20 temperature is changed at the con1pcnsatioll tcnlTpcralulc. I hcrerore, a B-type double-laycred film is stable at room tempclat:ule in a statc in ~7hicll lllc (lircctions of magnetization of tbe two layers are oriented parallel ~o c~ch OlhCI by CXChallgC-COUr)lillg. Whcn lhe tempcrature of a layer with a compensatioll tcmr)claturc cxcccds IllC comrcnsation tcmpcraturc, thc film l~ecomes stahle in a statc in w]licll thc (litccti(llls of nlagllctizatioll of thc two layers are anti-25 parallel. It is assumcd herc thal thc mcmoly laycr 2() has a compcnsation tempcraturc andthat the reference layer 22 docs nol. 1~ is als0 assulllc(l thal thc magl1ctization of the refcrcnce layer 22 has been orientcd up~Tard bcrorchan(l all(l thal tnc dircction of the b;as field is upward.
When short pulses arc cmittc(l, thc mcmory laycr 2() is hcalc(l to thc tcmperature TmL
JA9-90-528B lO
above its Curie temperature, but the rcfcrcncc laycr 22 is healcd only to thc temperaturc TrL
below its Curie temperature (scc Figurc ~) Thcrcrolc, immc(liatcly aftcr the heating, only the refercnce layer 22 maintains thc magnctization sct bcf(llc thc hcrltillg in the heated area (see (A) in Figure 9) When thc mcmory layer 2~) is coolc~ clow Illc Curie tcmperature, the 5 magnetization is first orientcd downwar(l by thc cxch3ngc-cou~ lg with the reference layer 22 (see (B) in Figure 9) Whcll thc memoly laycr 2() i~ fultllcl c(loled bclow its compcnsation temperature, the magnetizalion i~ changc~l urwal(l, an(l thc mc(liulll is stabilized in this state at room temperaturc When long pulses are emittc<l, holh thc mcmoly laycr 2() and the refcrence layer 22 are 10 heated to temperatures (TmH an(l Trl 1) abo~c thcir Curic tclllpcratures (sce Figure 2) As a result, both layers in the hcatcd arca losc ~hcil- magllctizalioll sct bcfore thc heating (see (A) in Figure 10) When the mcdium starts lo co(ll dowll, tllC magllclization of both layers is first oriented upward by the b;as ficlcl ((B) itl Fi'igurc 1()). WI1CI1 tlle me(lium has cooled furthcr and the temperature of thc mcmory laycr 2() dccrca~cs hclow ils compensation tempcrature, 15 only the magnetization of thc mcmory laycr 2() is rc~ersc(l (lowllward ((C) in Figure 10) Throughout the L process (Figure 9) an(l thc T I r~roccss (Figurc 10), the dircction of maglletization of the refercncc layel~ 22 rcmains ur~ward as in thc initial state Next, ~ith reference to Figules 11 lo 13, anotllcl cxalnplc of an ovcrwriting proccss using a B-type medium is cxplainc(l ll- is assumc<l hcre lllat the memory layer 30 docs not 20 have a compensation temperatul-c bul tha l I llc rcrcrcllcc laycr 32 (locs It is also a~sumed tllat the rnagnetization of the re~rcncc laycr 32 has bccn Oricntc~l ul~w,lr(l bcl'orehand and that the bias field is oriented downwal(l When short pulscs arc Clllil~C~]~ lllc mclllory laycr 3() is hcatc(l to a temperature TmL
above its Curie temperaturc, but Ihc rercl-encc laycl 32 i~ hclltc(l ollly to a temperaturc TrL
25 below its Curie temperaturc (scc Figurc ~ r thc r~cak ~cml cralure TrL is lower than the compcnsation temperature immcdiatcly aflcr lhc hcating, Ollly lllc rcfcrcnce layer 32 maintains the magnetization set bcforc the hcating in the hcatc(l arca (scc (A) in Figurc 12) Thercfore, when the memory layer 30 is coolcd bclow ils Curic tcmr?eratulc, lhe magnetization is oricnted upward by cxchangc-coupling with thc rcfcrcncc laycl 32 (scc (B) in Figure 12) In another ., ,, ~
JA9-gO-528B 1 1 case where the peak temperatllre Trl of tlle Ieferellce Iayer 32 exceeds the compensation temperature, the direction of magnetizalion of lhe reference layer 32 immediately after the heating is reversed downward ((A') in FigUlC 12) When the medium is coolecl, themagnetization of the reference layer 32 is again re~else(l upwal d I lowever, regardless of the 5 direction of the magnetization of the rerelence layer 32, downward ((B') in Figure 12) or upward ((C') in Figure 12), thl-oughout the process, tlle fin(ll slate when the peak temperature TrL exceeds tlle compensation temperatule ((C') hl Figut-e 12) is the same as that of when it does not exceed the compensation temperatu r e ((B) hl l~igure 12), because exchange-coupling is exerted by the reference ]ayer on the mem(ll y layer ~() to orienl the magnetization upward When long pulses are emilted, hoth the memory layer 3() and the reference layer 32 are heated to temperatures (TmH and Trl 1) above ~heir res~eclive Cul ie temperatures (see Figure 11) Therefore, both layers in lhc heale(l area lose theil- magnetization set before the heating (see (A) in Figure 13) Whell tlle tnedium starts to cool d own, the magnctization of both layers is first oriented downward by the bias fiekl ((B) in Figure 13) ~hen the medium is cooled 15 further and the temperature of the reference layer ~2 decreases below the compensation temperature, only the magnetization of the reference layer 32 is revcrsed upward ((C) in Figure 13) Inthefinalstateoftllel plOCCS.'i(~ig~llC 12)all(1tlleHpl-ocess(Figure 13),the magnetization of the refercnce laycr 32 remaills ul-wald as in the initial stateFig~1re 14 is a schemalic ~liagram of the la~ier powel all(l the shape of the clomains 20 formed upon exccution of overwri~irlg In or(ler ~o recor(l data of variable length, a continuous series of short pulses is use(l for one ol' tlle bil (lala, and long pulses different ;n duration are used for the olhcr ol' ~he l-il dalcl. Tlle rorme(l ~lomains are shaped like arrow feathers, in the same wa~ as tllose rorme(l b~ the fiel(l mo(lulati-lr1 overwrite metho(l Therefore, compatibility of data i~ expec~e(l betweell the ligl1l modula~ioll overwrite method 25 and the field modulation overwrite methocl Figure 15 shows a schematic view illustl-ating the compositioll of a magneto-optical reeording apparatus accorcling to tl~e invention Thi~ al-r)aratus includes a means 42, which may be a rotating means, for moving a recor(ling medi-lm 4(), a means 44 ror generating a bias field, a laser source 46, and a means r esponsive lo the bil data lo be recor~ed for modulating . _ ~
the duration and power of laser pulses. Between lhe laser source 46 and the reeording medium 40 may be inlerposed a known optical systeln. The means for generating a bias field may be an electromagnet or a perrnanent magnet. A llermanellt magnet is more advantageous from the viewpoint of pOWCt consulnption ancl heat generation. If the laser source 4G consists 5 of a semiconduetor diode, the means 4~ modulates the time width and strength of the current pulses to be supplied to the semic(!llduclot (liode, ;n ICSr)(!l1SC to the hit data to be recorded.
If the apparatus shown ;n F;igure 15 is use(l to emit a continuous series of laser beams in a bias field and to heat both layers oF the me(liulll 4() above theh- Curie temperatures, the magneti~ation ean be oriented in one clileclioll lhrougl1out Ille me(lium, including the reference 10 layer, before the bit data are recordecl.
Figure 16 shows the constructioll OF a me<lium usecl in an exT-eriment on overwriting.
On a glass substrate 50 were clepositecl, by SpUttClillg, a RE-ricll reference layer 52 that is 1200 angstroms thick and made of Tb31 Fe(.9, an in~erme~liate layer 54 that is 6 angstroms thiek and made of Tb, a TM-rich memory layer 5(~ that is ~()() allgstroms thick and made of Tb20.5F;e79.5, and a protective layer 58 that is 7()() angs~loms thick and made of SiN. The thieknesses of the respeetive layets were estimated from their slluttering times. The memory layer 5~ and the reference layeI 52 of lhe llrepale~l medium were oriente(l in one dircetion beforehand .
The diameter of lhe focu~ecl laseI sro~ C(I in ~l1C expetiment was ().47 mierolls (full-20 width at half-maximum), an~l thc lascr bCall1 was emitte-l fIonl Ihe side of the proteetive layer 58 of the medium. The strengtll of the bias l'ickl \~as 4~() (Oc), and the numerical aperture of the objeetive lens of the optical ~iyslem ~vas ().95. The 1 Illocess (wtiting) and H proeess (erasure) of overwriting arc as sllowl1 in ~iglltC~i 7 .Illd X~ reslleclively. In the L proeess, the duration of the laser pulse was 3.g ns, an~l lhe pOWCI was 14..~ InW In Ihe H proeess, the 25 duration of the laser pulse was 5() ns, and the powel was 4.5 mW.
In the foregoing eonditions, ~lomaills oF val iable Ienglh \~ere written and erased by the wr;ting proeess (L proeess) and by the erasing rlrOees~i (H ploeess), after both of whieh the medium was moved by 0.3 mierolls. More sl~ecifically, clomains of variable length were first written (WlitC' 1), a part of the donnains was erase(l, ancl subseq-lently writing was again _.
performed in the erased area ~wlite 2). I~igUtC 17 ix a dlawn copy of a photograph of the written domains observed witll a polal~izillg micloscope from the side of the memory layer 56.
It is clear from the figure that overwliting W,lS cert,~ ly cl(!lle in accordance with the H
process and the L process.
Next, the power range that permi~ writing WclS CXalllillC(I ~y varying the power while maintaining the durat;on of the laser pulsc rO, the writing process (L process) at 3.8 ns. The successful range was from lO mW to 37 mW. ~ ux, in the L plOCCSS, ir short pulses of the order of I ns are emitted, the powel rallge pelmitling the memory layer to be heated while maintaining the reference layer 52 at a low tempel-ature ix extendcd, and the margin is l O increased, accordingly.
In the same con~litions, an CXr~CI itllCIl~ was car r ied oul to examine the power range that permits writing, by varying the p ower while maintaining lhe duration of the laser pul~e for the writing process (L procesx) at 5() llX. As a result, Wl itillg was achieved in the range from l.5 mW to 3.1 mW. The reasollci for the xuccesx of \~riting appear to be that a 50 ns pulse l S can induce rather small temperature gradients along the thickne~s ~ecause of the intermediate layer 54, and that the Curic temperatures of the actually prepale(l reference layer 52 and memory layer 56 do not strictly coinci(le. Tllus, even when lllc laser pulse in the 1 proces~
in the prescnt invcntion is the same long pulxe ax lhat in Ihc 11 ptOCCSS, it is possihle to write one of the bit (lata. However, the lasel power range Illclt pelmitx writing becomes narrower than for a short pulse.
The invention makes posxil~le ditect overwlitillg witllolll atl initializing field. In this way, erroneous erasure of recot-(led datLl ix avoided.
Further, the invention largely l ClllOVC~ thc l CXt I iCtiOIl~ 011 ~llC compositions of materials for magneto-optical recording media l'or direct over~ ing. As a result, it allows more flexibility in the selection of materials an(l lowel- accllracy in the control of matcrial compositions than the prior art.
MAGNETO-OPTICAI I? F,CORDING
This invention relates to a method, apparatus, and medium for magneto-optical 5 recording capable of direct overwriting.
In magneto-optical recording, various methods for direct overwriting have been proposed to improve the data rate. They include a typical example of a light modulation method using a double-layered film, which is di.sclosecl in IA Published Unexamined Patent Application (PUPA) G2-17594~.
"The contents of this apl~lication are also rer~orted by Matsumoto et al. in "Direct Overwrite by Light Power Modulation on Magneto-Optical Double-Layered Media," Digest of 53rd Seminar, the Magnetics Society of lapan (1927), p. g7."
A recording medium used in this method has a recording layer consisting of two layers, a memory layer and a reference layer, which are exchange-coupled. Overwriting is performed by utilizing the difference in temperature clependence of the coercive forces of the two layers.
The general subject matter related to the present invention, together with the invention itself, will be more readily understood fronn the folk~wing description taken in conjunction with the appended drawings wherein:
Figure l illustrates the temperature dependence o f the coercive force of prior art magneto-optical recording media capal~lc Or direct overwtiting;
Figure 2 is an explanatory d iagram showing the principle of a prior art magneto-optical recording method capable of direct overwriting;
Figure 3 is a diagram i]luslrating the teml-eralute clependcnce of the coercive force of magneto-optical recording media capa~le of direcl overwriting according to the invention;
Figure 4 is a diagram illustrating the relatiotlship between the composition andmagnetic properties of TbFe films;
Figure 5 is a diagram illustrating the temr~eratllre del~endence of the coercive force of a first example of magneto-optical recording media;
Figure 6 is an explanatory view of a magneto-o,r~ical recording process using the medium shown in Figure S;
Figure 7 is an explanatory cliagram of a maglleto-optical recording process using the mediurn shown in Figure S;
Figure 8 is a diagram illustrating the temperat-lte depen(lcllcc of the coercive force of 5 a second example of magneto-optical recor(ling media;
Figure 9 is an explanatory diagl3m Or a magnel,o-optical recol(ling process using the medium shown in Figure ~;
Figure 10 is an explanatory diagtam Or a magneto-optical recording process using the medium shown in F~igure ~;
Figure 11 is a diagram illustrating the telllperat-ll-e der)endence Or the coercive force of a third example of magneto-optical recordillg media;
~igure 12 is an explanat(!ry diagram of a magnelo-or)tical rccor(ling process using t,he med;um shown in Figure I l;
Figure 13 is an explanatory diagl am or a magneto-ol-tical recording process using the medium shown in Figure I l;
Figure 14 is an explanatory diagram or a wr-iting process for domains of variable length;
Figure 15 is a schematic (liagram illustl-aling the compositioll of a magneto-optical recording apparatus according t,o the invenlioll;
Figure 16 is a schematic diagram illusl,rating t,he composition of a medium used in an experiment; and Figure 17 is a drawn copy ora ph(!tl!lllicroglclpll Ora me(lium sulface after overwriting.
Figure 1 shows the magl1etic properlies, an(l Figure 2 sllow.s the overwriting process.
As shown in Figure l, the c(!ml-(!siti(!ns Of the two layers are adjusted so that the coercive force of the reference layet (Hr2) is smaller thall thclt oi' the memory layer (Hrl) at room temperature (Tambl), and t,he Curie temperatllre of the refelence layer (Tc2) is higher than that of the memory layer (Tcl). As showll in Figute 2, One ~-f the characteristics of this method is that an initializing field, as well as a hias field for recor(ling, is applied before data is recorded on the memory layer. The directiolls of l;he hias fiel(l and the initializing field are anti-parallel. The magnitude of the bias l'iel(l Hb is set al suc11 a small value as to maintain the magnetization of the reference layer unrevetcsed in the L ptOCCSS, which will be referred to later. On the other hancl, the magnit-lde of the initializing fiekl Hini is set at a value larger S than Hr2 but smaller than Hrl. As a res~llt, only the maglleliz.ltion of the reference layer is oriented parallel to llini (downwarcl in the figure). The data recorcled ;n the memory layer is not affected by Hini.
For recording, the H proccss Or l process is performe(l, depending on the bit data to be recorded. In the L process, a low-powel laser ~eam in the rOrm of pulses is emitted so that the temperature of the memory layer TmL hecomec7 Tcl ~ Tm~ < Tc2. At this time, the magnetization of the rererence layel- is 11(!t reverse(l. Thererore, the magnetization of the memory layer is oriente~l in a directioll ~Ietermined by the exchange-coupling with the reference layer during the cooling process. Tlle term "exchange-coupling" here means a phenomenon such that the subllelwork ma~nelizations of RE and TM atoms are aligned to IS those of similar, respectively, evcn in cliffelent layel-s. ~hererore, depending on the eompositions of the two layers, the exchange-cour,ling exerted by one layer during the cooling of the other layer may result in these l~lyers having T-arallel or anti-parallel direct;ons of magnet;zation. Figure 2 shows the CaSC in whicll 1I1C (litcctions of magnetization of the two layers become parallel as a result of excllal1ge-coupling.
In the 11 process, a high-power lascl bcam in lhc ~orm Or pul<,es ;s emitted, with the result that the temperature of the memoly layel Tml--l bccl)mcs Tc2 < TmH. Consequently, during the cooling process, the magllelization Of llle relelellce layet first coincides with the direction of the bias field (upwar(l in the figure). Th:ll is, Ihc dilection of magnetizalion of the reference layer is reverscd. When ~he temrel~al~lle Or lhc recor(ling layer (lecr~ases, the magnetization of the memory layer is orienle(l in a direclioll delermined by the exchange-coupling with the reference layel-. Since thc ~litccl;oll of m~glletizatioll of the reference layer has been reversed from that in thc I r)lOCCSS, thc dilcction Or magnetization of the memory layer is also reversed from that in the l ptOCC'~S.
As described above, the metho(l of.lA p~lp~ 2-17.~94~ needs an external field for 21 5~233 initializing the reference layer (initializing field) before recor(ling (by the L process or H
process), in addition to an cxternal fiekl applied (lurillg recotding (a bias field). This makes the apparatus complicated. The above meth(ld als(l involves tlle problcm that data recorded in the memory layer are lost owing to the influence of thc sttOng initializing field. Moreover, 5 this method also involves the prohlem that strict requil-emcnts fot the Curie temperatures and coercive forces of respective layers result in Iess flexibility in the selection of materials and necessitate accurate control of the compositions of materialx during the preparation of media.
Some methods of eliminating the initi<llizing field have bccn proposed. Among them, T. Fukami and his colleagues' "Novel direcl Ovelwriting technology for magneto-optical disks by exchange-coupled RE-TM quadrilayered films," .1. Appl. Pllys. 67(9), 1 May 1990 uses quadrilayered films as recording media and make~i tlle Curie lemperatures, coercive forces, and inter-layer exchange-coupling forces of respeclive la~ers differcl1t. In this method, howevcr, the number of layers of the medium is incleased to rour and these layers need to satisfy certain relative requirements witl1 respect to Curie temperatures, exchange-coupling 1~ forces, and so on. Therefore, this method not only t'ails to rcmove the restrictions on the compositions of materials, but rather incl-eases them. In or(ler to satisfy the requirements, highly accurate control of the composition of cacll la.ycl- is necessary, and hence the production cost of media becomes a problen1 aff'ectillg tlleil pr.lctical use. F~ulther, the total thickness of four layers amounts to a value oF the Or(lCI' Or 2(()() angslloms. Tllis results in lower writing 20 efficiency and hence requires higher laser ener~y.
Therefore, an objcct oI' tlle invcntion i~ lo plovi(lc a methocl and apparatus for magneto-optical recording ca,nahlc of (litect OVCI'~'t'i~,illg, ~lsing a multi-layeled film that does not need an initializing field an~l thal tlCVCt' ca~lses Cl'l'OIlC~US crasulc of recorded data.
Another object of the inventioll is to provi(le a metllo~l all<l apparat~ls for magneto-25 optical recording capable Or dircct OVCI'Wi'itillg thal allcviatc~ the rcstrictions on the compositions of matcrials for media.
Still another object of the invenlion is to pro\/i(le a me(lium for use in the above-mentioned magneto-optical recorcling.
In both processes for recording bit data, the metho(l of IA PUPA 62-175948 utilized 2 1 5~233 exchange-coupling to orient the magnetizati(ln Or the memoly layel in a direction determined by the exchange-eoupling with the referencc layer. Tllcrefot-c, the magnetization of the reference layel has to be rever~ecl in the recorcling proce~s for one of the bit (lata. This is why the method needs an external fiel(l for initiali%ing the reference layer (an initializing field) S before recording.
In order to overcome the problem, lhe prcsetlt invention uxcs, in.stead of a medium as shown in Figure 1, a novel me(lium consisting of two exchange-couple(l rare earth-transition metal amorphous (RE-TM) layers whose Curic temperatures arc almost the same and only one of which is RE-rich without a competlsation felnperatute, the two layers being laminated directly or indirectly with an intertne(liafe layer that allows exchange-coupling to be interposed. Before recording, one of the two layers is magnetized in onc dircction beforehand.
Either one of the magnetized RE-TM layers is use(l as the refetence layer, and the other is used as the memory layer. Magnclizatioll Or the rererel1ce layer has to bc carried out only once, before all data writing ptOcesses. In contrast, .IA PUPA ~2-175948 magnetizes the IS reference layer in a desired writing area every time data writing is carried out, that is, before each emission of a laser pulse. Note, ~hcrcfore, that the magnetization of the referencc layer performed beforehand in the present in\~entiol1 is nol the same as tlle "initialization" referred to in JA PUPA 62-1 7594Y,.
Writing of data is carricd out by:
~a) moving the mcclium relalive ~o an energy .'iOlllCC ill a bias field, with the referencc laycr being farther rr'tn Ihc CllClgy SOUICC fllall thc mcmoly layer;
(b) emitting to the medium wllen recol(lin~ one ol' llle l~it (lata a pulse of energy such that the temperature or the memoly layer beconles necll Or above ils Curie temperature while that of the referencc laycr relllain~ l~elow il~ Curie femr~el atut-c; ancf (c) emitting to the me~liulll whell ICCOr(lillg ~hc o thcr of the bit data a pulse of energy such that the tempeJatules Or lhe two layer~i become near or above their Curie temperatures.
ln step (b), the directioll of maglletization of the memory layer is detcrmined by the exchange-coupling with the reference layel . In StCp (C), thc dilcc~ion Or magnetization of the 21 Sq233 memory layer is determine~l by the ditection Of lhe bias fiekl. On the other hancl, the direction of magnetization of the reletence lcayer maintains lhe direction of its original magnetization regardless of whethel ster) (b) or (c) is carriecl out. Since the direction of magnetization of the reference layer is nOt revetse(l, the me(lium does not need an initializing S field, which was needed in JA PUPA (!2-17594~.
Figure 3 shows two types Of temperalure del-elldence Of the coercive force of double-layered films usecl in the present invenliom A Curie ~empetLItute is a temperature at which the coercive force becomes zero. A coml-ensation temr~elatute is a temperature at which the eoercive force diverges. In either case, ~hc following ~W(? re~luirements must be satisfied:
(1) The Curie temperatures or ~he two layers at-e almo~t lhe ~ame; and (2) only one of the layers is R~-rich witlloul a coml~ensation temperature.
For e~ample, when a typical magneto-o[-lical recording ma~eti31, TbFe film, is used for both layers, since Curie temperatules do not substantially vary witll the ratio of Tb and Fe, 15 requirement (I) is automatically xatisfied. In this respect, consi~lering that tlle method of JA
PUPA 62-175948 controls the combinationx an(l compoxitionx of three to four kinds of elements in order to distinguish lhe Cutic tCmt-CratUlC.~ ol' two layers, it is easier to prepare media according to the presenl inventiorl. Re~luiremell~ (2) can re.l(lily be satisfied in the case of a TbFe film by merely setting the composilioll r.ltio of Th al a value above 26% for one 20 of the layers and below 26~/n for the other layer (see Fi~ute 4).
Thus, the invention largely remove!i resllicliolls On the materials used for media. In the subsequent description~ a me(lium in Ille l/-r m ol' a latnimltiotl consisting of (I) an RE-rich layer without a compensatioll teml-er.~ re an(:l (2) a T M-t ich laycr is callcd A-type, wllereas a medium in the form of a lamina~ion con~ tirlg of (I) al1 RE-rich layer without a 25 compensation temperature an(l (2) a RE-rich l~lyer witll a comr)ensatioll temperature is called B-type. In both typcs, either of the two layers m.ly he usc(l as the memory layer. In the present invention, the layer facing the laset soulce hehaves as the memory layer.
Between the memory layer and the reference layer may be interposed an intermediate layer whose thickness does not prevent exchange-courtling t~etween them. Insertion of a layer 2 1 5~233 such as Tb Ol GdFeCo in order ~o a(ljust the stlellgtll of exchclnge-coupling is a known techn;que. See, for example, K. Aratani et al.~ "Overwriting On a magneto-optical clisk with magnetic triple layers by mean~ or the light inten~ity m(!~lulatioll method," Proc. SPIE 1078, 265 ( 1 989) .
S The recording method used in the pre~ent in~elllion is exl~lained below. Before bit data is written, the magnetization of the layer lhclt will function a~; the reference layer is oriented in one directiom As will be understoo(l from the ~ubsequenl; descliption, overwriting is possible in whatever state the memory layer is magnetize(l berorehand. Thus, magnetization may be oriented in one direction heforehan(l througllout the entil-ety of the medium, including the reference layer. Therefore, prelimillary maglletization or the rcfcr-encc layer can be carried out by magnetizing it uniformly in a sufriciently ~trong fiekl before sllipment. If the medium is not magnetized before shil-ment, rnagnetization may be carried out by using a magneto-optical recording apparatus referled to later.
After magnetization of the referellce layel~ complete, hil data is written on the medium in a bias field. It is prererablc to use shorl r~ulses (of a few n~noseconcls) for writing one of the bit data and long pulses (lens of nanosccon(ls Or more) for writing the other of the bit data. As disclosed by T. Oht~uki et al. in "Direct overwlile by short pulses on double-layered MO media," Conference Dige~l of Topic.ll Meeting on Optical Data Storage, 172 (1990), when short pulses wil:h a dlllalioll of tlle order Of olle nallosecond are emitte(l to the magneto-optical recording medium from the memory layel side, mal ked temperature gradients are produccd in the two layers, an<l hcll(;c only lhC Icmpclcltul~c of the memory layer rises above the Curie temperature, wllile the reI'erellce layel renlclills at a tempcrature low enough to maintain the magnetization. 1~ Ccl~iC~ dUlillg tllC plOCC~!i (')I'COOlillg the memory layer, a strong exchange-coupling ahove 1()()() (Oe~ is exerte(l on ~he mellloly layer by the refelence ]ayer. Therefore, if the magnitude of lhe bias lickl i~ sct lowcr lhan that of the exchaMge-coupling, the direction of magnctization or the mernoly !ayer i~ (Ietcrmined by the exchange-coupling with the reference layer. It is surficienl lo apply a f'iel(l of the order of 500 (Oe) by using, for example, a permanent magnet. In the ple~ent method, ~he process executed while the reference layer is maintained at a low teml eralur-e bel(lu il~ Curie ~emperature is called .~
the L process.
In contrast, when long pulses wilh a dulatioll ol' lens of nanoseconds or more are emitted, both layers are heate(l al~ove their Curie tempelatures and marked temperature gradients are not induced in the clirection of the Ihickness of the medium. When the heating is finished and the medium has cooled to the Curie temperatute, at which magnetization of the memory ]ayer is determined, magnetization of eacl1 of ~he two layers is determined aecording to the direction of the bias fickl, bccausc the exchange-coupling between the two layers is sufficiently small (if the Curie teml-er.lt~lles of the two ]ayers are absolutely equal, the strength is 0 (Oe)). In the presell~ melh(~(l, the ptOCCSS that inclu(le~ a step in which the reference layer is heated to a higll ternperature ncar or abovc its Curie temperature is called the H process.
The step in the H process in wllich the direcliol1 or magnetization of the memory layer is determined is now cxplained in greater (lelail. Ir lhere is a dirfcl-ence between the Curie temperatures of the two layers, the direclion of magnelizalion is first determined by the bias field for the layer whose Curie temperature is highel (TcH layer). Therefore, at the instant when the medium has further col-led to the Curie temperature of the other layer (TcL layer), exchange-coupling witll the TcH layer .3S wcll as the bias fiekl is exerte(J on the TcL layer.
At this time, the larger the diflelence l etweel1 the Curie teml~er.ltules of the two layers, the greater the strength of the exchangc-couplillg. 1ll thc plCSCIlt invention, however, it is requ;red that the direction of magnetization of the TcL layer slloul(l rollow the direct;on of the bias field. Therefore, the dif'ference hetweell tl~e Cul ie Iemrerat-ll-e~ Or the two layers must be so srnall that the strength of the exchange-coul-lillg belweell tlle l~! layers does not prevent magnetization of the memory layer by the hias l'icl(l. Ai cxplaillc(l abovc, it is easy to produce such a double-]ayered film in which lhe Curie Ieml-eratules al-e surficiently close.
Presented below is a detaile(l exl lancltioll Of how ovelwritillg is carried out when the aforementioned requ;rements are met. First, with ICI'ClCllCC to Figures 5 to 7, an overwriting process using an A-type melliull1 is exl lained. An A-tyl~e double-layered film is stable at room temperature in a state in which the (lirections of magnetizatiotl al-e oriented anti-parallel to each other by exchange-coupling. It is assume(l here thclt the memory layer 10 is TM-rich and 21 592~3 '.._ that the reference layer 12 is RE-rich witlloLIt a compcnsation tcmpcrature. It is also assumed that the magnetization of the rcfcrellcc laycr 12 has bccll oricntcd upward l~eforelland and that the direction Or the bias ficld is UPWL1td.
When short pulscs arc cmitte~l, thc mcmory laycr l() is hcatc(l to a tempcrature TmL
5 above its Curie temperature, but lhc rcrcrcncc la~cr 12 is hcatc~l only to a temperature TrL
below its Curie temperature (scc Figurc 5). As a rcsult, immc<liatcly aftcr the heating, only the reference layer 12 in the hcatc(J arca maintains thc magnctization set bcfore the heating (see (A) in Figure 6). Whcn thc mcmol-y laycr I() has coolcd, thc magnetization is oriented downward by exchange-couTpling ~T~7ilh lhc rcfercncc laycl 12 (scc (B) in Figure 6).
When long pulscs arc emiltc(l, hotll thc mcmory laycr 10 and thc reference layer 12 are heated to temperatures ~TmH an(l TrH) al~OVC thcir Curic tclnr~clatures (see Figure 5). As a result, both layers in thc hcatcd arca losc thcil magnctizations sct bcforc the heating (see (A) in Figure 7). When the mcdi-lm has coolccl, thc magnctizatioll of both layers is oriented upward by thc bias field ((B) in Figurc 7). ThJougllout thc l proccss (Figure 6) and the H
15 process (Figure 7), the magnctization Or thc rcfcrencc laycr 12 is maintained upward as in the original state. Even though thc mem(lry laycr l () is RE-rich an(l thc rcfcJcnce layer 12 is TM-rich, overwriting is also carried out througll thc salllc stel~s.
Next, with rcferencc to l~igurcs X to 1~), an cxalnl-lc Of an ovcrwJiting process using a B-type mcdium is cxplaine(l. Thc magtlc~iz~tioll diJcctioll ol' ~1 layer Wit]l a compensation 20 temperature is changed at the con1pcnsatioll tcnlTpcralulc. I hcrerore, a B-type double-laycred film is stable at room tempclat:ule in a statc in ~7hicll lllc (lircctions of magnetization of tbe two layers are oriented parallel ~o c~ch OlhCI by CXChallgC-COUr)lillg. Whcn lhe tempcrature of a layer with a compensatioll tcmr)claturc cxcccds IllC comrcnsation tcmpcraturc, thc film l~ecomes stahle in a statc in w]licll thc (litccti(llls of nlagllctizatioll of thc two layers are anti-25 parallel. It is assumcd herc thal thc mcmoly laycr 2() has a compcnsation tempcraturc andthat the reference layer 22 docs nol. 1~ is als0 assulllc(l thal thc magl1ctization of the refcrcnce layer 22 has been orientcd up~Tard bcrorchan(l all(l thal tnc dircction of the b;as field is upward.
When short pulses arc cmittc(l, thc mcmory laycr 2() is hcalc(l to thc tcmperature TmL
JA9-90-528B lO
above its Curie temperature, but the rcfcrcncc laycr 22 is healcd only to thc temperaturc TrL
below its Curie temperature (scc Figurc ~) Thcrcrolc, immc(liatcly aftcr the heating, only the refercnce layer 22 maintains thc magnctization sct bcf(llc thc hcrltillg in the heated area (see (A) in Figure 9) When thc mcmory layer 2~) is coolc~ clow Illc Curie tcmperature, the 5 magnetization is first orientcd downwar(l by thc cxch3ngc-cou~ lg with the reference layer 22 (see (B) in Figure 9) Whcll thc memoly laycr 2() i~ fultllcl c(loled bclow its compcnsation temperature, the magnetizalion i~ changc~l urwal(l, an(l thc mc(liulll is stabilized in this state at room temperaturc When long pulses are emittc<l, holh thc mcmoly laycr 2() and the refcrence layer 22 are 10 heated to temperatures (TmH an(l Trl 1) abo~c thcir Curic tclllpcratures (sce Figure 2) As a result, both layers in the hcatcd arca losc ~hcil- magllctizalioll sct bcfore thc heating (see (A) in Figure 10) When the mcdium starts lo co(ll dowll, tllC magllclization of both layers is first oriented upward by the b;as ficlcl ((B) itl Fi'igurc 1()). WI1CI1 tlle me(lium has cooled furthcr and the temperature of thc mcmory laycr 2() dccrca~cs hclow ils compensation tempcrature, 15 only the magnetization of thc mcmory laycr 2() is rc~ersc(l (lowllward ((C) in Figure 10) Throughout the L process (Figure 9) an(l thc T I r~roccss (Figurc 10), the dircction of maglletization of the refercncc layel~ 22 rcmains ur~ward as in thc initial state Next, ~ith reference to Figules 11 lo 13, anotllcl cxalnplc of an ovcrwriting proccss using a B-type medium is cxplainc(l ll- is assumc<l hcre lllat the memory layer 30 docs not 20 have a compensation temperatul-c bul tha l I llc rcrcrcllcc laycr 32 (locs It is also a~sumed tllat the rnagnetization of the re~rcncc laycr 32 has bccn Oricntc~l ul~w,lr(l bcl'orehand and that the bias field is oriented downwal(l When short pulscs arc Clllil~C~]~ lllc mclllory laycr 3() is hcatc(l to a temperature TmL
above its Curie temperaturc, but Ihc rercl-encc laycl 32 i~ hclltc(l ollly to a temperaturc TrL
25 below its Curie temperaturc (scc Figurc ~ r thc r~cak ~cml cralure TrL is lower than the compcnsation temperature immcdiatcly aflcr lhc hcating, Ollly lllc rcfcrcnce layer 32 maintains the magnetization set bcforc the hcating in the hcatc(l arca (scc (A) in Figurc 12) Thercfore, when the memory layer 30 is coolcd bclow ils Curic tcmr?eratulc, lhe magnetization is oricnted upward by cxchangc-coupling with thc rcfcrcncc laycl 32 (scc (B) in Figure 12) In another ., ,, ~
JA9-gO-528B 1 1 case where the peak temperatllre Trl of tlle Ieferellce Iayer 32 exceeds the compensation temperature, the direction of magnetizalion of lhe reference layer 32 immediately after the heating is reversed downward ((A') in FigUlC 12) When the medium is coolecl, themagnetization of the reference layer 32 is again re~else(l upwal d I lowever, regardless of the 5 direction of the magnetization of the rerelence layer 32, downward ((B') in Figure 12) or upward ((C') in Figure 12), thl-oughout the process, tlle fin(ll slate when the peak temperature TrL exceeds tlle compensation temperatule ((C') hl Figut-e 12) is the same as that of when it does not exceed the compensation temperatu r e ((B) hl l~igure 12), because exchange-coupling is exerted by the reference ]ayer on the mem(ll y layer ~() to orienl the magnetization upward When long pulses are emilted, hoth the memory layer 3() and the reference layer 32 are heated to temperatures (TmH and Trl 1) above ~heir res~eclive Cul ie temperatures (see Figure 11) Therefore, both layers in lhc heale(l area lose theil- magnetization set before the heating (see (A) in Figure 13) Whell tlle tnedium starts to cool d own, the magnctization of both layers is first oriented downward by the bias fiekl ((B) in Figure 13) ~hen the medium is cooled 15 further and the temperature of the reference layer ~2 decreases below the compensation temperature, only the magnetization of the reference layer 32 is revcrsed upward ((C) in Figure 13) Inthefinalstateoftllel plOCCS.'i(~ig~llC 12)all(1tlleHpl-ocess(Figure 13),the magnetization of the refercnce laycr 32 remaills ul-wald as in the initial stateFig~1re 14 is a schemalic ~liagram of the la~ier powel all(l the shape of the clomains 20 formed upon exccution of overwri~irlg In or(ler ~o recor(l data of variable length, a continuous series of short pulses is use(l for one ol' tlle bil (lala, and long pulses different ;n duration are used for the olhcr ol' ~he l-il dalcl. Tlle rorme(l ~lomains are shaped like arrow feathers, in the same wa~ as tllose rorme(l b~ the fiel(l mo(lulati-lr1 overwrite metho(l Therefore, compatibility of data i~ expec~e(l betweell the ligl1l modula~ioll overwrite method 25 and the field modulation overwrite methocl Figure 15 shows a schematic view illustl-ating the compositioll of a magneto-optical reeording apparatus accorcling to tl~e invention Thi~ al-r)aratus includes a means 42, which may be a rotating means, for moving a recor(ling medi-lm 4(), a means 44 ror generating a bias field, a laser source 46, and a means r esponsive lo the bil data lo be recor~ed for modulating . _ ~
the duration and power of laser pulses. Between lhe laser source 46 and the reeording medium 40 may be inlerposed a known optical systeln. The means for generating a bias field may be an electromagnet or a perrnanent magnet. A llermanellt magnet is more advantageous from the viewpoint of pOWCt consulnption ancl heat generation. If the laser source 4G consists 5 of a semiconduetor diode, the means 4~ modulates the time width and strength of the current pulses to be supplied to the semic(!llduclot (liode, ;n ICSr)(!l1SC to the hit data to be recorded.
If the apparatus shown ;n F;igure 15 is use(l to emit a continuous series of laser beams in a bias field and to heat both layers oF the me(liulll 4() above theh- Curie temperatures, the magneti~ation ean be oriented in one clileclioll lhrougl1out Ille me(lium, including the reference 10 layer, before the bit data are recordecl.
Figure 16 shows the constructioll OF a me<lium usecl in an exT-eriment on overwriting.
On a glass substrate 50 were clepositecl, by SpUttClillg, a RE-ricll reference layer 52 that is 1200 angstroms thick and made of Tb31 Fe(.9, an in~erme~liate layer 54 that is 6 angstroms thiek and made of Tb, a TM-rich memory layer 5(~ that is ~()() allgstroms thick and made of Tb20.5F;e79.5, and a protective layer 58 that is 7()() angs~loms thick and made of SiN. The thieknesses of the respeetive layets were estimated from their slluttering times. The memory layer 5~ and the reference layeI 52 of lhe llrepale~l medium were oriente(l in one dircetion beforehand .
The diameter of lhe focu~ecl laseI sro~ C(I in ~l1C expetiment was ().47 mierolls (full-20 width at half-maximum), an~l thc lascr bCall1 was emitte-l fIonl Ihe side of the proteetive layer 58 of the medium. The strengtll of the bias l'ickl \~as 4~() (Oc), and the numerical aperture of the objeetive lens of the optical ~iyslem ~vas ().95. The 1 Illocess (wtiting) and H proeess (erasure) of overwriting arc as sllowl1 in ~iglltC~i 7 .Illd X~ reslleclively. In the L proeess, the duration of the laser pulse was 3.g ns, an~l lhe pOWCI was 14..~ InW In Ihe H proeess, the 25 duration of the laser pulse was 5() ns, and the powel was 4.5 mW.
In the foregoing eonditions, ~lomaills oF val iable Ienglh \~ere written and erased by the wr;ting proeess (L proeess) and by the erasing rlrOees~i (H ploeess), after both of whieh the medium was moved by 0.3 mierolls. More sl~ecifically, clomains of variable length were first written (WlitC' 1), a part of the donnains was erase(l, ancl subseq-lently writing was again _.
performed in the erased area ~wlite 2). I~igUtC 17 ix a dlawn copy of a photograph of the written domains observed witll a polal~izillg micloscope from the side of the memory layer 56.
It is clear from the figure that overwliting W,lS cert,~ ly cl(!lle in accordance with the H
process and the L process.
Next, the power range that permi~ writing WclS CXalllillC(I ~y varying the power while maintaining the durat;on of the laser pulsc rO, the writing process (L process) at 3.8 ns. The successful range was from lO mW to 37 mW. ~ ux, in the L plOCCSS, ir short pulses of the order of I ns are emitted, the powel rallge pelmitling the memory layer to be heated while maintaining the reference layer 52 at a low tempel-ature ix extendcd, and the margin is l O increased, accordingly.
In the same con~litions, an CXr~CI itllCIl~ was car r ied oul to examine the power range that permits writing, by varying the p ower while maintaining lhe duration of the laser pul~e for the writing process (L procesx) at 5() llX. As a result, Wl itillg was achieved in the range from l.5 mW to 3.1 mW. The reasollci for the xuccesx of \~riting appear to be that a 50 ns pulse l S can induce rather small temperature gradients along the thickne~s ~ecause of the intermediate layer 54, and that the Curic temperatures of the actually prepale(l reference layer 52 and memory layer 56 do not strictly coinci(le. Tllus, even when lllc laser pulse in the 1 proces~
in the prescnt invcntion is the same long pulxe ax lhat in Ihc 11 ptOCCSS, it is possihle to write one of the bit (lata. However, the lasel power range Illclt pelmitx writing becomes narrower than for a short pulse.
The invention makes posxil~le ditect overwlitillg witllolll atl initializing field. In this way, erroneous erasure of recot-(led datLl ix avoided.
Further, the invention largely l ClllOVC~ thc l CXt I iCtiOIl~ 011 ~llC compositions of materials for magneto-optical recording media l'or direct over~ ing. As a result, it allows more flexibility in the selection of materials an(l lowel- accllracy in the control of matcrial compositions than the prior art.
Claims (48)
1. A magneto-optical apparatus for directly overwriting data on a magneto-optical medium comprised of two exchange coupled rare earth-transition metal (RE-TM) amorphous layers whose Curie temperatures are substantially the same and only of which is RE-rich without a compensation temperature, said two layers being laminated directly or indirectly with an intermediate layer that allows exchange-coupling to he interposed, comprising:
(a) means for magnetizing one or said two layers in one direction beforehand;
(b) an energy source capable of emitting a pulse of energy;
(c) means for generating a bias field;
(d) means for moving said medium relative to said energy source in said bias field with said one layer farther from the energy source than the other layer; and (e) means responsive to bit data signals for switching said pulse of energy emitted from said energy source between a first pulse which heats the medium such that the temperature of said one layer remains below its Curie temperature while that of said other layer becomes near or above its Curie temperature, and a second pulse which heats the medium such that the temperature of said two layers become near or above their Curie temperatures.
(a) means for magnetizing one or said two layers in one direction beforehand;
(b) an energy source capable of emitting a pulse of energy;
(c) means for generating a bias field;
(d) means for moving said medium relative to said energy source in said bias field with said one layer farther from the energy source than the other layer; and (e) means responsive to bit data signals for switching said pulse of energy emitted from said energy source between a first pulse which heats the medium such that the temperature of said one layer remains below its Curie temperature while that of said other layer becomes near or above its Curie temperature, and a second pulse which heats the medium such that the temperature of said two layers become near or above their Curie temperatures.
2. An apparatus according to claim 1, wherein said two layers are a TM-rich layer and a RE-rich layer without a compensation temperature.
3. An apparatus according to claim 1, wherein said two layers are a RE-rich layer with a compensation temperature and a RE-rich layer without a compensation temperature.
4. An apparatus according to claim 1, wherein said energy source is a laser source.
5. An apparatus according to claim 1, wherein said first pulse has such a short duration as to induce marked temperature gradients in said two layers and said second pulse has such a long duration as to not induce marked temperature gradients in said two layers.
6. A magneto-optical apparatus for directly overwriting data on a magneto-optcalmedium (i) said medium comprised of two exchange-coupled rare earth-transitions metal (RE-TM) amorphous layers whose Curie temperatures we substantially the same and only one of which is RE-rich without a compensation temperature said two layers being laminated directly or indirectly with an intermediate layer which allows exchange-coupling to be interposed, (ii) one of said two layers being magnetized in one direction beforehand comprising:
(a) an energy source capable of emitting a pulse of energy;
(b) means for generating a bias field;
(c) means for moving said medium relative to said energy source in said bias field with said one layer farther from the energy source than the other layer and (d) means responsive to bit data signal for switching said pulse of energy omitted from said energy source between a first pulse which heats the medium such that the temperature of said one layer remains below its Curie temperature while that of the other layer becomes near or above its Curie temperature and a second pulse which heats the medium such that the temperatures of said two layers become near or above their Curie temperature.
(a) an energy source capable of emitting a pulse of energy;
(b) means for generating a bias field;
(c) means for moving said medium relative to said energy source in said bias field with said one layer farther from the energy source than the other layer and (d) means responsive to bit data signal for switching said pulse of energy omitted from said energy source between a first pulse which heats the medium such that the temperature of said one layer remains below its Curie temperature while that of the other layer becomes near or above its Curie temperature and a second pulse which heats the medium such that the temperatures of said two layers become near or above their Curie temperature.
7. An apparatus according to claim 6, wherein said two layers are a TM-rich layer and a RE-rich layer without a compensation temperature.
8. An apparatus according to claim 6, wherein layers are a RE-rich layer with a compensation temperature and a RE-rich layer without a compensation temperature.
9. An apparatus according to claim 6, wherein said energy source is a laser source.
10. An apparatus according to claim 6, wherein said first pulse has such a short duration as to induce marked temperature gradients in said two layers, and said second pulse has such a long duration as not to induce marked temperature gradients in said two layers.
11. A magneto-optical recording medium comprising:
a first layer comprised of a rare earth-transition metal (RE-TM) material which is RE-rich and does not have a compensation temperature; and a second layer overlying the first layer, the second layer comprised of a RE-TM material, the material of the second layer having a Curie temperature substantially equal to the Curie temperature of the material of the first layer.
a first layer comprised of a rare earth-transition metal (RE-TM) material which is RE-rich and does not have a compensation temperature; and a second layer overlying the first layer, the second layer comprised of a RE-TM material, the material of the second layer having a Curie temperature substantially equal to the Curie temperature of the material of the first layer.
12. The medium of claim 11, wherein one of the first and second layers is magnetized initially in a first direction.
13. The medium of claim 11, wherein the second layer is a TM-rich layer.
14. The medium of claim 11, wherein the second layer is a RE-rich layer with a compensation temperature.
15. The medium of claim 11, further comprising a substrate underlying the first layer.
16. The medium of claim 11, further comprising an intermediate layer located between the first and second layers.
17. The medium of claim 16, wherein the intermediate layer is comprised of GdFeCo.
18. The medium of claim 16, wherein the intermediate layer is comprised of Tb.
19. The medium of claim 16, wherein the intermediate layer is substantially 6 Angstroms thick.
20. The medium of claim 11, further comprising a protective layer overlying the second layer.
21. The medium of claim 11, wherein the materials of the first and second layers are both comprised of Tb and Fe.
22. The medium of claim 21, wherein the first layer contains greater than 26% Tb and the second layer contains less than 26% Tb.
23. The medium of claim 21, wherein the first layer contains greater than 26% Tb and the second layer contains less than 20% Tb.
24. The medium of claim 11, wherein the first layer consists of Tb31Fe69.
25. The medium of claim 24, wherein the first layer is substantially 1200 Angstroms thick.
26. The medium of claim 11, wherein the second layer consists of Tb20.5Fe79.5.
27. The medium of claim 26, wherein the second layer is substantially 800 Angstroms thick.
28. The medium of claim 11, wherein the first and second layers are exchange-coupled.
29. The medium of claim 28, wherein the second layer may be magnetized by an external bias field generated by a magneto-optical recording apparatus.
30. The medium of claim 11, wherein the first and second layers are each comprised of amorphous materials.
31. A magneto-optical recording medium comprising:
a first layer comprised of amorphous rare earth-transistion (RE-TM) material which is RE-rich and does not have a compensation temperature;
a second layer overlying the first layer, the second layer comprised of a RE-TM material, the material of the second layer having a Curie temperature substantially equal to the Curie temperature of the first layer; and an intermediate layer between the first and second layers, the intermediate layer is of a thickness which does permits exchange-coupling between the first and second layers.
a first layer comprised of amorphous rare earth-transistion (RE-TM) material which is RE-rich and does not have a compensation temperature;
a second layer overlying the first layer, the second layer comprised of a RE-TM material, the material of the second layer having a Curie temperature substantially equal to the Curie temperature of the first layer; and an intermediate layer between the first and second layers, the intermediate layer is of a thickness which does permits exchange-coupling between the first and second layers.
32. The medium of claim 31, wherein one of the first and second layers is magnetized initially in a first direction.
33. The medium of claim 31, wherein the second layer is a TM-rich layer.
34. The medium of claim 31, wherein the second layer is a RE-rich layer with a compensation temperature.
35. The medium of claim 31, further comprising a substrate underlying the first layer.
36. The medium of claim 31, wherein the intermediate layer is comprised of GdFeCo.
37. The medium of claim 31, wherein the intermediate layer is comprised of Tb.
38. The medium of claim 31, wherein the intermediate layer is substantially 6 Angstroms thick.
39. The medium of claim 31, further comprising a protective layer overlying the second layer.
40. The medium of claim 31, wherein the materials of the first and second layers are both comprised of Tb and Fe.
41. The medium of claim 40, wherein the first layer contains greater than 26% Tb and the second layer contains less than 26% Tb.
42. The medium of claim 40, wherein the first layer contains greater than 26% Tb and the second layer contains less than 20% Tb.
43. The medium of claim 31, wherein the first layer consists Of Tb31Fe69.
44. The medium of claim 31, wherein the first layer is substantially 1200 Angstroms thick.
45. The medium of claim 31, wherein the second layer consists of Tb20.5Fe79.5.
46. The medium of claim 31, wherein the second layer is substantially 800 Angstroms thick.
47. The medium of claim 31, wherein the first and second layers are exchange-coupled.
48. The medium of claim 31, wherein the second layer may be magnetized by an external bias field generated by a magneto-optical recording apparatus in the presence of exchange-coupling between layers..
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP21472/91 | 1991-01-23 | ||
| JP3021472A JP2567996B2 (en) | 1991-01-23 | 1991-01-23 | Magneto-optical recording method and apparatus |
| CA002059179A CA2059179C (en) | 1991-01-23 | 1992-01-10 | Magneto-optic overwriting medium with rare earth-transition metal layers |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002059179A Division CA2059179C (en) | 1991-01-23 | 1992-01-10 | Magneto-optic overwriting medium with rare earth-transition metal layers |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2159233A1 CA2159233A1 (en) | 1992-07-24 |
| CA2159233C true CA2159233C (en) | 1998-11-03 |
Family
ID=25674922
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002159233A Expired - Fee Related CA2159233C (en) | 1991-01-23 | 1992-01-10 | Method, apparatus and medium for magneto-optical recording |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2159233C (en) |
-
1992
- 1992-01-10 CA CA002159233A patent/CA2159233C/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| CA2159233A1 (en) | 1992-07-24 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| CA2059179C (en) | Magneto-optic overwriting medium with rare earth-transition metal layers | |
| US4794560A (en) | Eraseable self biasing thermal magneto-optic medium | |
| US5707727A (en) | Magneto-optical recording medium | |
| JPS6280846A (en) | Magnetic memory medium | |
| US5087532A (en) | Direct-overwrite magneto-optic media | |
| US5337293A (en) | Magneto-optical recording method and recording medium for use in the same | |
| US5459701A (en) | Magneto-optical recording method | |
| CA2159233C (en) | Method, apparatus and medium for magneto-optical recording | |
| GB2077065A (en) | Magnetooptic memory medium | |
| US6894954B2 (en) | Domain wall-displacement type magneto-optical medium and reproducing method for the same | |
| US5187694A (en) | Magneto-optical recording medium comprising recording layer and thermal bias layer, and method for recording, erasing and overwriting on the medium | |
| KR100238692B1 (en) | Magneto-optical recording medium | |
| EP0440486B1 (en) | Magneto-optical recording medium | |
| KR930003034A (en) | Magneto-optical recording media | |
| JPS5724046A (en) | Photoelectric recording medium and its recorer and reproducer | |
| EP0431946B1 (en) | Method and apparatus for magneto-optical recording | |
| US5422865A (en) | Light modulation method for magneto-optical recording device | |
| JP2828993B2 (en) | Magneto-optical recording medium and information recording method using the same | |
| US6124048A (en) | Magneto-optical storage medium with double mask | |
| EP0487847A1 (en) | Magnetooptical recording medium | |
| EP0428946B1 (en) | Magneto-optic recording medium | |
| JP2538916B2 (en) | Magneto-optical recording medium | |
| JP3228964B2 (en) | Magneto-optical recording medium | |
| JPH0755692Y2 (en) | Magneto-optical recording medium | |
| EP0492359A1 (en) | Magneto-optical recording medium |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| MKLA | Lapsed |