Magnetic and magneto-optical studies in Co-Tb/Pt multilayers

Magnetic and magnetooptical properties of TbFeCo amorphous films

S. Takayama, T. Niihara, K. Kaneko, Y. Sugita, and M. Ojima

Citation: Journal of Applied Physics 61, 2610 (1987); doi: 10.1063/1.337889 View online: http://dx.doi.org/10.1063/1.337889

View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/61/7?ver=pdfcov Published by the AIP Publishing

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Magnetic and magneto .. opticai properties of TIl-Fe-Co amorphous films

S. Takayama, T. Niihara, K. Kaneko, Y. Suglta, and M. Ojima Central Research Laboratory, Hitachi Limited, Kokubunji, Tokyo, Japan

(Received 10 September 1986; accepted for publication 3 December 1986)

Characteristics of Th-Fe-Co amorphous films have been systematically investigated in order to optimize compositions suitable for magneto-optical recording medium with high readout signaL In order to optimize aHoy composition, Curie temperature T c and compensation temperature T comp are summarized as functions of both Th and Co contents. Crystallization temperature Tx of Tb-Fe-Co amorphous films has been also measured to be about 400 ·C and tends to increase with replacing Fe by either Co or Th. To evaluate Th-Fe-Co films as a magneto-optical recording medium, carrier C and modulation noise N mod (defined by the difference in noise before and after writing) are measured as a function of external field Hex. It is found that written bits of large N mod show an irregular shape, while those of small N mod are of regular one. To reduce N_mod and obtain high readout C IN, films with lower satnration magnetization Ms (that is, low demagnetization field) at just below writing temperature are preferred.

!. INTRODUCTION

The study of some rare-earth-transition metal (RETM) films has been directed to application of magneto-optical recording (erasable optical recording). Among these films, Tb-Fe-Co ferrimagnetic amorphous films are found to ex- hibit superior magneto-optical characteristics, leading to high readout carrier to noise ratio (C / N) of 50-57 dB (at 1 MHz with bandwidth of 30 kHz).l-4

In this work, we have extensively investigated charac- teristics of the Tb-Fe-Co amorphous alloy system in order to optimize compositions suitable for magneto-optical record- ing media,

II. eXPERIMENT

Tb-Fe-Co amorphous films were prepared on glass sub- strates of 10 mm in diameter by rfbias sputtering or rf mag- netron nonbias sputtering to provide an easy axis perpendic- ular to a film plane. These film thicknesses were 0.1-3 ""m.

The cathode targets were of a Fe disk with 110 mm in diame- ter, on which Th and Co chips of 5 mm² were placed to obtain a desired composition. Prior to sputtering, a vacuum chamber was evacuated to less than 9 X 10-⁷ Torr, Total gas pressure during sputtering was 5 mTorr. Uniaxial anisotro- py constant Ku and saturation magnetization .TId. were mea- sured using a torque magnetometer and a vibrating sample magnetometer, respectively. Both measurements were per- formed in the field strength of 12-14 kOe, Curie temperature

Tc was measured by using a magnetic balance. Kerr hystere- sis loops were obtained through the glass substrate by using a He-Ne laser (A = 633 nrn) in a field up to 5 kOe. The Kerr rotation angle Gk was measured at the state of remanent magnetization, Crystallization temperature T" was mea- sured by the change of resistivity with temperature using a four-probe method. Film compositions were determined by inductively coupled plasma spectroscopy (ICP) within an accuracy of ± 1 at, %.

To evaluate magneto-optical recording characteristics, quadrilayer structure media were made on a 120-mm-diam

glass disk pregrooved by ultra violet light curing resin (UV resin). The groove pitch is 1.6 p.m. The optical configuration has been reported elsewhere,² The observation of magnetic domains was carried out by a polarized optical microscope.

In. RESULTS

A.. Characteristics of T~Fe~Co amorphous films I t is well known that on bias sputtering of binary rare- earth-transition metal films, rare-earth elements are prefer- entially resputtered so that content of rare-earth elements in films decreases with increase of bias voltage,⁵ This fact is also noticed in ternary 1O-Fe-Co amorphous films as shown in Fig. 1, The figure shows the content of each element in films, analyzed by the rcp method, as a function of an applied bias voltage during sputtering. All the films were prepared by using the same composite target of Th₃₂Fe_s7Co_ll in area %, Even in ternary alloy systems, rare-earth Th content de- creases with increase of bias voltage as reported in binary alloy systems, Note, however, that in comparing Fe and Co

70

_...,---0 GO _501-

0_0--

I

~ ^40f

~

300r:--.O___ Tb

u o _ o

-0----0

20 a -b - -a -;;:---a

t::. ___ A- - ~

10 Co

o~ ____ ^~~ ____ ^~~ ______ ^~~

o -50 -100 -150

Bias Voltage Vb{V)

FIG. 1. The change of film composition as a function of bias voltage in sputtering a Th'2Fe"COll (area %) composite target.

2610 J. Appl. Phys. 61 (7), 1 April HIS7 0021-6979/87/072610-07$02.40 @) 1967 American institute of Physics 2610 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

contents, the concentration of Co in films is nearly indepen- dent of applied bias voltage, whereas the Fe content varies greatly, increasing with the increase of an applied bias vol- tage. This may indicate that Fe atoms also play an important role in preferential resputtering of Th atoms in Tb-Fe-Co amorphous alloy systems.

Corresponding to the above compositional change, the characteristics of amorphous films thus made vary as shown in Fig, 2. Judging from the polarity of Kerr hysteresis loop, the compositions of amorphous films at Vb = 0 were on the rare-earth metal-rich side where the sublattice magnetiza- tion of rare-earth metal elements is larger than that of the transition metal elements at room temperature (hereafter called RE-rich side). As shown in Fig. 1, the Tb content decreases as one increases the applied bias voltage in sputter- ing, so that composition afthe amorphous films shifts to the transition-metal-rich side where sublattice magnetization of transition metal elements is dominant at room temperature (hereafter caned TM-rich side) ⁰ This is clearly noticed from the change of both saturation magnetization Ms and coerciv- ity He' In the figure, ¥, first decreases and reaches zero, indicating that the compensation composition exists at about Vb = - 60 V. After crossing the compensation com- position, the film composition enters into Ii TM-rich side where its saturation magnetization increases again with further increase of a bias voltage. In accord with the compo- sitional dependence of magnetic properties of binary rare- earth-transition metal amorphous films, the coercivity He rapidly increases around the compensation composition at

Vb = - 60 Vo It should be emphasized here that the squareness of both magnetization and Kerr hysteresis loops are dramatically improved by a negative bias voltage applied to substrate during sputtering. In fact, as seen in the figure, the perpendicular magnetic anisotropy Ku increases with increasing bias voltage. In the figure, apparent drop offs of Ku at Vb = - 50 to - 70 V are due to field strength insuffi- cient to saturate the magnetization of a film having a high coercivity. In accord with increase of Ku, the remanent Kerr rotation angle increases and reaches 036°; the same as the saturation field value. As can be seen from the results, even in Th-Fe-Co ternary aHoy systems, the effect of bias sputter-

FrG. 2. Bias voltage dependence of perpendicular magnetic anisotropy Ku, Kerr rotation angle Ok, saturation magnetization M, and coercivity lIe for Th-Fe-Co amorphous films sputtered from a ThJ2FeS7Coll (area %) com- posite target.

2611 J. Appl. Phys., Vol. 61, No.7. 1 April 1987

ing causes not only a change in composition but also induces a strong perpendicular magnetic anisotropy, resulting in in- creased squareness of the Kerr hysteresis loop. However, rf magnetron sputtering with no bias voltage can also induce a strong perpendicular magnetic anisotropy in Tb-Fe-Co amorphous films. Thus, the perpendicular magnetic anisot- ropy Ku of Th-Pe-Co films is significantly influenced not only by sputtering condition but also by sputtering method.

Such effect of process variables on Ku in amorphous films is not well understood and requires further attention. All the samples discussed hereafter were made either by a bias sput- tering at a fixed bias voltage of - 70 V or magnetron sput- tering with no bias voltage.

In addition to the measurement of magnetic and mag- neto-optical properties of amorphous Th-Fe-Co films, we also attempted to measure the crystallization temperature Tx by the temperature dependence of resistivityo A typical resistivity change with temperature is shown in Fig. 3. The resistivity in the figure is depicted in arbitrary unitso It in- creases first with temperature and drops off significantly around 400 ·C. Following this large drop the resistivity de- creases linearly with de~rease of temperature as observed in usual metal-metalloid or metal-meta! amorphous aHoy rib~

bons.⁶•7 However, it is worth knowing that small bumps of resistivity are observed at 320·C. This small change of res is- tivity is also observed for most ofTb based RE~TM amor- phous films. However. we failed to detect any indications of phase changes by using a conventional x-ray diffraction analysis. But this small increase of resistivity might be relat- ed to some structural relaxation in amorphous structure.

Furthermore, even at temperatures where large drops of re- sistivity take place, no indications of phase transformation can be detected from a conventional x-ray diffraction analy- sis. The x-ray diffraction curves still show a typical diffrac- tion profile indicating an amorphous phase. Of course, such iarge drops of resistivity could also relate to some structure change in amorphous structure. Nevertheless, in this report, we define a crystallization temperature of Tb-Fe-Co amor- phous films as the temperature at which resistivity starts to drop significantly with increase of temperature.

First of all, to know the effect of Co addition to Th-Fe amorphous films, we made a series of samples replacing only

I I I I

"1--_~:0~-

ill ___ /--~---

$

'"---i:,.,---Too----3tO----~~----

T ('e)

FIG. 3. Temperature dependence of resistivity for the Tb₂₁Fe₆₀Co₁₉ amor- phous filmo Temperature indicated by a large arrow is defined as a crystalli- zation temperature Tx.

Takayama et al. 2611 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

Fe by Co under a fixed Tb content. The change of magnetic and magneto-opticai properties by such replacement is de- picted in Fig. 4. All compositions of these amorphous films were RE-rich side. As shown in the figure, crystallization temperature Tx tends to increase with the increase of Co content so that thermal stability of amorphous structure ap- pears to increase by the way of such replacement. On the other hand, both Kerr rotation angle Ok and Curie tempera- ture Tc increase with increase of Co content, Since Ok is nearly proportional to room temperature sublattice magne- tization of transition metals, this increase of Ok also reflects on the increase of sublattice magnetization oftransition met- als. This can be clearly noticed from the result of the de- crease of saturation magnetization Ms with substituting Co for Fe as depicted in the figure. Since the present Th-Pe-Co amorphous films are ferrimagnetic, the increase of sublattice magnetization of transition metals due to the above replace- ment causes the total saturation magnetization to decrease at the present RE-rich side composition. As a result, composi- tions of the amorphous films shift to the transition-metal- rich side approaching a compensation composition.

Figure 5 shows representative curves of Ms versus tem- perature for the same Tb-Fe-Co amorphous samples as shown in Fig. 4. As Co content increases, the shape of the curves change dramatically, and the compensation tempera- ture Tcomp below Tc begins to appear above room tempera- ture as represented in the curves of C0₂₉ (hereafter denoted by 29 at. % Co).

We also investigate the effect of replacement of Fe by Th. Figure 6 shows the change of Ms. Ok, To and Tx as a function ofTb content of TbxFe_{S2 __} x Co_IS amorphous films.

One should notice that as in the case of replacement of Fe by Co, T" tends to increase slightly with increase of substitution of Tb for Fe. Hence, thermal stability of Tb-Fe-Co amor- phous films appears to increase with increase of Th content in a manner of the above replacement. JUdging from the polarity of the Kerr hysteresis loop, the starting composition at 21 at. % Th was on the TM -rich side. Then, as Tb content increases, the polarities of the Kerr hysteresis loop change from the sign of TM-rich side to that of RE-rich side, indi- cating that the compositions shift to RE-rich side. There-

Co Cone. x(at%)

FIG. 4, The change of M" Curie temperature Tc , T" fJk as a function of Co content in Tb₂₈Fen ._., Cox amorphous films.

2612 J. Appl. Phys., Vol. 61, No.7, 1 April i S87

Tbz6Fen-x Cox

FIG. 5. Temperature dependence of saturation magnetization Ms for films with x = 0, x = 8, and x = 29 in a Th2SFen _ ^x Cox alloy system.

fore, M, first decreases and then increases after passing through a compensation composition at 2S at. % Th as shown in the figure. On the other hand, both Ok and Tc decrease as Fe is replaced by Th. From foregoing discussion.

this decrease of Ok can be explained by the decrease of sublat- tice magnetization of transition metal as a result of increase ofTb content. One should notice here that Ok decreases sig- nificantly with increase of Th content in Th-Fe-Co amor- phous films.

These temperature dependencies of Ms are shown in Fig. 7. From the previous figure, samples ofTb₁₇ (hereafter denoted by 17 at. % Tb) and Th22 are of a TM-rich side composition, while those OfTb23 and Tb₃₄ belong to the RE- rich side one. From the figure, one can immediately see that samples with low Th content (Tb₁₇ and Th22 samples) show Tc above room temperature. But as Tb content increases, T_cQmp starts to appear above room temperature, With further increase of Tb content, Tcomp disappears again and only Tc is observed, as exhibited in the curve ofTb_w Note that at high temperatures, saturation magnetization on the TM-rich side samples shows larger values than that of RE- rich ones. In the case of the Th22 sample, .Ms at room tem- perature is very low. This is due to the fact that its composi-

FlG. 6. The change ofM" Te, Tx, and Bkas a function ofTh content in ThxFe_{gZ _ x}Co'8 amorphous films.

Takayama et 81. 2612 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

we)

FIG. 7. Temperature dependence of M, for amorphous films with x = 17, 22,28, and 34 in a TbxFe'2_xCaJg alloy system.

tioD is close to the compensation composition, and also its compensation temperature is not too far below room tem~

perature. However, even though Ms is low at room tempera- ture, it increases with increase of temperature and reaches a maximum value near Te. These temperature dependencies of M, could have a large influence on the formation of mag- netic domains on writing as already pointed out by Tanaka and Imamura.⁸ This will be discussed later.

Next, we summarize Tc and T com!, obtained in Th-Pe- Co films as functions of both Tb and Co concentrations as shown in Fig. 8. For the sake of comparison, the data ofTh- Fe films reported by Mimura and Imamura are also included in the figure.⁹ In the figure, the solid points indicate a com- pensation temperature. It should be noticed that films hav- ing only Tc (not Tcomp) above room temperature are ob- tained at either lower or higher Tb concentration. At other Tb concentrations, both Tc and Team!' appear above room temperature. General features of Tc and Teamp versus com- position are that as Co content increases, Tc increases rapid~

ly at lower Tb concentration, whereas the increase of Tc is small at higher Tb concentration. Also, the Th concentra-

400

300

:? ₂₀₀

a. E 100

0

<.) I -

~ 0 -100

o

0"-0 ~

"

'0, \.

~ 1\

'®-if!

/0 -0

/ l

o ~

•

'/

fill

0 5 10.3 17.5 30.4

! !

•

10 20 30 40 50

Tb Cone,)( (at.·f.)

Tc Tcomp

0

•

®

<>

0 II!

6

'"

FIG. 8. The change of Curie temperature Tc and compensation tempera- ture T,omp as functions ofCa and 1b contents in ThxFe1oo _ x .. yOly amor- phous films.

2613 J. Appl. Phys., Vol. 61, No.7, 1 April i987

tion at which Tc is equal to Tcomp tends to shift to higher values with increase of Co content. It is important to know that from this figure, one can immediately design alloy com- positions with desired values of Tc and Tc.omp taking account of their temperature dependence of Ms as shown in Figs. 5 and 7.

B. Dynamic writing and reading characteristics

Now, let us examine how the above magnetic properties of films affect the dynamic writing and erasing character- istics in magneto-optical recording. To get insight into this, we selected two representative Tb-Fe-Co amorphous films as magneto-optical recording media. One is a TM-rich side ThZZFe69Co9 film and the other an RE-rich side Tb26Fes9CoJ5 film. For this purpose, 5-in. magneto-optical disks were made. The disk structure consists of a glass sub- strate, pregrooved UV resin layer, SiO layer, Tb--Fe-Co film layer, and SiOz protective layer, respectively. Pregrooves of 1.6 p;m pitch are fabricated by a 2P (photopolymerization) method.

Figures 9 and 10 show the temperature dependence of M s' demagnetization field 41TM" and coercivity He for TM- rich TbzzFe69Co9 and RE-rich Tb26Fe59Coj5 amorphous films, respectively. In the case of Tb22Fe69Cog film in Fig. 9, as temperature increases, He monotonically decreases, whereas l'W_s first increases and then decreases, showing a maximum near the Curie temperature (Tc = 210 ·C).

Note that demagnetization field 41TMs greatly exceeds He from 40 ·C up to Curie temperature Tc. On the other hand, Tbz6Fe59Col5 film in Fig. 10 has a compensation point at 120 ·C so that as temperature increases, He increases first and decreases rapidly after changing the polarity of Kerr hysteresis loop, while Ms shows a minimum at the compen- sation point and becomes to zero at Tc = 240 ·Co As stated above, the values of .'Ad_s are relatively low up to Te. There- fore, the demagnetization field is relatively low and lower than He up to 180°C.

To evaluate the above two kinds of films as a magneto~

optical recording medium, thermomagnetic writing and reading at 1 MHz were carried out on the disks rotating at

FIG. 9. Temperature dependence of saturation magnetization M". demag- netization field 41TM, and coercivity He for ThnFe69Co9 amorphous film (TM-rich side).

Takayama fit a/. 2613 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

r---~7 G 5

o 40 80 120 1 GO 200 240 280 i (·C )

FIG. iO. Temperature dependence of M"47rM,, and He for Th26Fe59Co[s amorpholls film (RE-rich side).

speed of 900 rpm. Figures 11 and 12 show carrier C and modulation noise N mod as a function of external field in writ- ing for Tb22Fe69Co9 (TM-richside) and Thz6Fes9Col5 (RE- rich side) films, respectively. These C and N_mod were mea- sured at the resolution bandwidth of 30 kHz. In the case of Tb2zFe69Co9 film, writing and reading laser powers at the medium were 7 and 1.5 roW, respectively, while pulse dura- tion in writing was 170 ns as included in Fig. 11. On the other hand, for the Tbz6Fes9Col5 films, a bit was written with laser power of9 mW and 150 ns pulse duration and read with 2.5 m W as depicted in Fig. 12. The schematic cross sections of the disk structures are also exhibited in both figures, includ- ing the initial direction of magnetization in films and signs of an external field, He~ applied to the disk. Here, the modula- tion noise was measured as a difference in noise before and after writing. Thus, this N mod is believed to mainly result from the irregular shape of written bits (hereafter also caned reversed domains) formed by thermomagnetic writing. For magneto-optical recording media, the larger this difference between C and N ^mod' the higher readout carrier to noise ratio

( C IN) can be obtained.

It is interesting to know in both Figs. 11 and 12, that readout carrier signal is observed even at zero external field.

50r

~4+

'830 z E

~ 20 10

Write: 7mw, nOns Read: 1.5 mw

• SiOz

1

Ms SiO

- UV resin

Hext substrate

Nmod

o ~----.---

-GQO-400 -200 0 200 400 600 800 Hext (Oe)

FIG. 1!. Dependence of readout carrier C and modulation noise N moo on external bias field He, for a ThnFe₆₉Co₉ magneto-optical disk.

2614 J. Appl. Phys., Vol. 61, No.7, 1 April 1 S87

50

r

1~Si~~:Fe5!1cOj!)

+ SID

H t UV resin

ex substrat~

'-

" " Nmod

, ,

/ "... _ ... ---_ ... -

Or-~---~-~-=-=-=-~--~-~-~----~

-400 -200 0 200 400 GOO 800 Hext(Oe)

FIG. 12. Dependence of readout carrier C and modulation noise Nmod on external bias field He, for a Th26FeS9CoJ5 magneto-optical disk.

This means that a reversed domain is formed with no exter-

?al fi.el~. Fu~hermore, one can also see that a readout signal

1S obtamed m the range of negative external field. In other words, a reversed domain is formed with a field in the direc- tion of erasing a written bit. As seen in the figure, negative external field at which a carrier signal starts to be observed is larger in the Tb22Fe69Co9 film than in the _Thz Fe Co film.

S h 1· . 6 59 15

ue Imlt values of negative external field are considered to be an intensity of magnetic field required to erase a written bit entirely. It is also worth noting that carrier signal C in- creases more gradually in the Thz2Fe69Co9 film than in the Thz6Fes9Co15 film with increase of external field. Since car- rier ~evels vary wit? the size of written bits, such change of carner level most hkely results from the change of a size of :vritten bits. This size of a written bit, of course, is largely mfiuenced by a strength of external field and temperature dependence of both coercivity and demagnetization field (that is, saturation magnetization) near writing tempera-

~ure, as demonstrated by Takahashi et ai.^lO Therefore, judg- mg from the result of above carrier curves, bit size is expect- ed to increase more rapidly in the Tb26Fe59Col5 film than in the Tb₂₂Fe₆₉Coy one with increase of external field. There·

fore, one can state that field dependence of bit size in writing is larger in the Th22Fe69Co9 film than in the Th₂₆Fes₉Co!5 one. To get a steady carrier signal, the smaller the variation of a written bit in writing, of course, is the better. Thus, in this sense, the medium whose carrier signal C quickly satu- rates with external field appears to be better for practical use, as represented in Fig. 12.

Now, focusing on modulation noise in the above both figures, N ^mod of more than a few dB always exists in reading.

Here, in the case of the Tbz6Fes9Co15 film in Fig. 12, high readout C IN more than 50 dB is obtained at Hex = 300 Oe where N ^mod is a minimum. This film shows a large N ^mod at a negative external field of - 100 Oe. On the other hand, the Tb2zFe69Co9 film also shows a maximum of N 00 at Hex = - 200 Oe and a minimum at Hex = 200 Oe. But, its maximum and minimum values are smaller and larger, re- spectively, comparing with those of the Th26Fe59Co15 film.

Such modulation noise, as mentioned above, is expected to

Takayama et al. 2614 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

result mainly from the irregular shape of written bits formed during thermal writing, This is clearly seen in Fig. 13. The left photos show the micrographic observation of domain bits written under the conditions of (a) large NtnOd for the Th19Fe76Cos film (TM rich) and (b) small Nmod for the Th2sFe6sColO film (RE rich). The schematic figures oftheir written bits are also exhibited on the right-hand side of the figure. As expected, written bits at large Nmod show an irreg- ular shape and are erased in part at their center area, whereas those of small N ^mod are of regular shape and have no erased part inside of reversed domains.

IV. DISCUSSION

We already mentioned that the formation of bits in writ- ing depends largely on the intensity of demagnetization field at temperature reached by irradiation of laser beam. Since this demagnetization field arises from the saturation magne- tization of films themselves, it changes as a function of tem- perature dependence of M. as shown in Figs. 9 and 10.

Therefore, as clearly seen in both figures, the temperature dependence of the demagnetization field differs greatly from TM-rich films to RE-rich ones. Particularly, at writing tem- peratures near Curie temperature, the demagnetization field of Tb22Fe69Co9 film (TM rich) is larger than that of the Th26Fe59Co15 film (RE rich), and much larger than its own coercivity. Here, it is apparent that the demagnetization field of a TM-rich side films always encourages reversal of an initial direction of magnetization in a film. Due to this effect, a reversed domain can be easily formed in writing even with no external field, and even, at times, with negative external field as shown in Fig. 11.

(0)

Domain for large Nmod

( b)

Domain for small Nmod

FIG. 13. Recorded magnetic domains observed by a polarized microscope:

(a) domains for a large N.noo and (b) domains for a small N_{moo '} 2615 J. Appl. Phys .• Va!. 61, No.7, 1 April 1987

On the other hand, in the case of thermomagnetic writ- ing for RE-rich films, a bit (reversed domain) can be formed with the external field paraHel to the initial direction of mag- netization in a film. Thus, the demagnetization field of the RE-rich films exerts little help to create a reversed domain.

However, if writing laser power is high enough to raise the temperature within a relatively large area of films above the compensation temperature, and if demagnetization field in- side of a written bit is large enough to exceed the coercivity of the film during cooling process, a reversed domain can be fonned even at zero external field. This is most likely the case of Tb2sFe65ColO film where a carrier signal C is observed at zero external field.

Now, comparing the change of temperature dependence of Ms in Fig. 9 with that in Fig, 10, one can immediately see that the demagnetization field ofTb22Fe69Co9 film at writing temperature near Curie temperature is much larger than that ofTbz2Fc69C09 film. This seems to be a main reason why the erasing external field of Th2zFe69Co9 film can be more negative than that of Th2sFe6sColO film. That is, the larger the demagnetization field at writing temperature becomes, the more negative erasing external field can be tolerated.

On the other hand, from above arguments, it is easily expected that the shape of a written bit other than its size is also greatly influenced by demagnetization field. Since mod- ulation noise in reading arises mainly from the irregularity of the shape of ^Ii written bit, N ^mod is also thought to relate intimately to the intensity of demagnetization field in writ- ing. In fact, in the case of writing, there is a temperature difference between internal and external portions within an area irradiated by a laser beam. Due to this fact, there occurs a difference in the intensity of saturation magnetization, that is, the demagnetization field intensity with a bit correspond- ing to the temperature dependence of Ms in a film during cooling process. In the case of TbnFe69Cog film as shown in Fig. 9, demagnetization field intensity is quite large and sig- nificantly exceeds the coercive force near Curie temperature.

Thus, during the cooling process near Curie temperature, the demagnetization field intensity at the periphery of a writ- ten bit is larger than that in the center. Under this condition, if demagnetization field intensity at the periphery of a writ- ten bit overcomes the coercive force at the center portion of a written bit during cooling process, an internal area of a writ- ten bit would be erased to some extent. Therefore, it is intu- itively expected that the larger the area of a written bit, the larger the influence of demagnetization field during cooling process, However, in the case of Tb26FeS9Col5 film, large N ^mod is also observed at negative external field. In this case, with negative external field, a reversed domain can be formed only by demagnetization field inside of area irradiat- ed by laser beam. These detailed discussions will be reported elsewhere by Takahashi et al. ^!O

Consequently, it is important to know that temperature dependence of both Ms and He plays an important role in writing and erasing characteristics of magneto-optical re- cording. Thus, for thermomagnetic writing, these tempera- ture dependencies, including Tc and T_{comp '} should also be taken into account to obtain high readout carrier-to-noise ratio (C IN).

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V.SUMMARY

In order to design alloy composition of Th_FCbCo amorb phous films for magneto-optical recording medium, Curie temperature Tc and compensation temperature Teomp were summarized as functions of both Th and Co contents. It was found that films with either higher or lower Th concentra- tion show only Tc above room temperature, whereas those with other Th concentrations has both Tc and T ^camp above room temperature. Crystallization temperature Tx of Th- FebCo amorphous films was also measured to be about 400 'C and tends to increase with replacing Fe by either Co orTh.

Dynamic writing and reading characteristics were ex- amined on two typical Th-Fe-Co amorphous films (that is, TM-rich side and RE-rich side). It was found that written bits of large N_mod show an irregular shape while those of small Nmou are more regular. To reduce Nmod and obtain high readout C IN, films with lower saturation Ms (that is, low demagnetization field) at just below writing tempera- ture are preferred.

2616 J, Appl. Phys., Vol. 61, No.7, 1 April 1987

ACKNOWLEDGMENTS

The authors are very grateful to A. Saito and M. Y oshi- hiro for their evaluation of magneto-optical disks. They also gratefully acknowledge Dr. N. Ohta and Dr. Y. Tsunoda for valuable discussions throughout this work.

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