Journal of Magnetism and Magnetic Materials 14g (i995) 206-208 ~ Journal of

magnetism and magnetic

JJ~a materials E I _ S ~ R

Magnetic polarization of interracial Pt in FePt/Pt multilayers witnessed by magneto-optical spectra

Katsuaki Sato a,*, Yasuhiro Tosaka a, Hiroshi Ikekame a Masato Watanabe b, Koki Takanashi ~, Hiroyasu Fujimori ~

a Tokyo Universir= o f Agriculture and Technology, Kogane~ Tokyo 184, Japan b The Research lnstilutefor Electric and Magnetic Materials, Taihak~-ku, Sel~lai 982, Japan

Institute o f Materials Research, Tohoku University, Aoba-lcu, Sendal 980, Japan

Abstract 1ffagaeto-optical spectra in FePt(x ~,) /Pt(50 ~,) multilayers prepared by the ion-beam sputtering technique were

measured between 1 and 6 eV. Simulation by the virtual optical constant method, assuming the existence of a magnetic Pt layer at the interface, provided a fairly" good explanation of the x-dependence of the magneto-optical spectra. The magnetic coutpling between FePt and Pt was found to be antiparallel for x >_ 50 and parallel for x < 50.

1. intr~duction 2. Experimental

:Pt-based and Pd-based multilayers ~ave been attracting interest as high-density magneto-optical recording materi- als for ~ e next generation [1]. The origin of the large magneto-optical effect has been attributed to either the magnetic polarization of Pt by the proximity effect, or to the existence of an interfacial alloy layer such as Pt-Co. We have shown in our previous study of P d / C o multilay- ers that both alloy formation and the magnetic polarization of lPd are responsible for the magneto-optical spectra [2]. Therefore we intentionally employ an FePt ~lloy layer as a counterpart of the Pt layer in the naultilayer instead of simple transition metal Fe, expecting a large magneto-opti- cal effect. We prepared mnltilayers consisting of FePt a lby and Pt on the MgO substrates by the ion-beam sp~-ttering technique. In the previous studies it was found by ~X-my' diffraction that X-ray peaks associated with the periodic structure disappear at a critical thickness of the FePt layer [3]. It was also found that below the critical thickness M s increases with FePt thickness, followed by a rapid decrease to nearly zero at the critical thickness, and then increases again with the layer thickness [3]. In order to elucidate the relation between the structural and mag- netic anomalies, the magneto-optical spectra were mea- sured.

' Corr=sponding author. Fax: +81.423-87-8151; e-mail: sat0:kats@¢e.tuaLae.jp.

FePI/Pt multilayers were prepared by the ion-beam sputtering technique through the Pt buffer layer on the MgO(100) crystal [,4]. The FePt la~er thickness x of the sample was varied from 20 to 200 A, while keeping the Pt layer thickness at 50 ,A. The surface layer of each sample was a FePt layer in all cases. The total thickness of the multilayers was kept at approximately at 2000 ,~ (between 1500 and 2700 A; thus the number of layers N L differed from sample to sample: N L was 5, 10, 13, 20, 28 and 35 for FePt thicknesses x of 200, 100, 75, 50, 35 and 20, respectively).

Kerr rotation and ellipticity spectra were measured between 1.2 and 6 eV by means of the polarization modu- lation technique described elsewhere [5]. Magnetic fields up to 1.7 T wore applied. Magnetic saturation was con- firmed by measuring the Kerr hysteresis loop every time before the spectrum measurement.

3. Results and discmsion

Spectra of the magneto-optical Kerr rotation and ellip- ticity are shown in Figs. l(a) and l(b), respectively. The sample with the thickest FePt layer, x = 200 A, shows a peak at 4.7 eV, the maximum value being 0.78 ° and another maximum at 1.5 eV with the value of Kerr rotation 0.42 °. On the other hand; the Kerr elliptieity spectra show the maximum at the highest photon energy, the maximum value reaching nearly 1.3 °. As the FePt thickness x de- creases, the peak energy positions of the Kerr rotation

031)4-8853/95/$09.50 © 1995 Elsevier S~i~nce B.V. All rights reserved SS£P2 0304-8853(95)00210-3

K. Sate et aL /Journal of Magnetism and Magnetic Materials 14~ (1995) 206-208 207

0.8

0.6

v

~- 0.~.

0.2

0.o

i i • i , i i

(a FePt (X-~) / PL (50A) ZOO : o .¢-o-xx

75: ~ ~ .Ir.~, 50: • ,(.d" ._.~. '~ 35: ,, /'/- ~ -~-,&

' ,,G 6

hu (ev)

0.8

0.6

"~ 0.4

o . 2

0.0

o : FesoP%o ~x : Fe,0P%o .j.o~,--..~:~.-,~,

' ' + ' ' ,G

h v (eV)

1.2 (b) eP*ixAi]P(( 0g) 2 0 0 : o / ? /

1.o too: . / j r - 75: ~ , / / /

o.8 50: • 35-- o

O + '~ 0.4

0.2

0.11

-0.2

hu (eV)

Fig. 1. Spectra of the magneto-optical Kerr effect in FePt/Pt multilayers. (a) Kerr rotation and (b) Kerr ellipticity.

1.5

1.0

= 0.5 g,,

0.(

b " i • l ' i • *

( )o : FesoPt~+ i Fe40P%o e r

-o.% , :+ =] , 6 h v (eV)

Fig. 2. Spectra of the magneto-optical Kerr effect in Fes0Pts0, Fe4oPt60 and Fe301PtTo alloys. (a) Kerr rotation and (b) Kerr elliptieity.

spectra move towards higher energies and the peak rota- tion decreases.

For x = 5 0 ,~ the peak occurs at 5.2 eV. At this composition ratio, X-ray diffraction provides no periodic structure, as described in Section 1. For x < 50 A the periodic structure is recovered and the spectral features

o

undergo drastic changes. When x varies f rom 50 to 35 A, the peak position shifts toward lower energies and the peak rotation even increases. For smaller x the peak moves to lower epergies with rotation decreasing. Such a drastic change at the critical thickness can be more easily ob- served in the Kerr ellipticit~ spectra, in which three curves with x = 200, 100 and 75 A and the remaining two curves with x = 35 and 20 .~ show different spectral behavior especially in the low-energy region. These results show that the structural transition is accompanied by a change in the electronic structure or in the magnetic coupling.

For further analysis, magneto-optical spectra in three alloy samples with different composition ratios, i.e. Fcs0Pts0, Fe4oPt6o and Fe30PtT0, were measured. The results are given in Figs. 2(a) and (b) for Kerr rotation and Kerr elliptieity, respectively. It is clear that in FexPt~o0_ x the spectrum shows a low-energy shift as the Pt composi- tion increases, i.e. as x is reduced from 50 to 30. We attribute this shift to the increase in the magnetic polariza- tion of Pt. Reflectivity spectra o f these FePt alloys were

also measured to deduce the optical constants. The real and imaginary parts o f the off-diagonal elements of the dielec- tric permeability ~xr were obtained. The difference in e:~y for two different compositions were calculated and were normali2ed to 100% Pt composition. The resulting spectra o f ex:v o f 'magnetic Pt" are shown in Fig. 3.

Magneto-opticat spectra o f FePt (x) /P t (50) multilayers were siraulated by means o f the virtual optical constant method [6]. FeP t /P t without considering the existence of a

2.0

1.0

0.o

--1.0

".. Magnetic PL -

\ - - t:'xy i

~ 1 ! ......... Y

I 2 3 4 5 6

h u (eV)

Fig, 3. Spectra of ~xy of hypothetical 'magnetic I t ' .

208 K. gato et at,./ Journal of Magnetism and Magnetic Materials 148 (1995) 2(96-208

Calculation Non-magnetic Pt

FePt ( ~ ) / Pt (50.K) 00 1

3 5 : 0 / ~ '~ I 9 o . 4 ~ 0.2

0.0 2 3 4 5 6

h u (eV)

Fig. 4. Simulated magneto-Ol~tieal Kerr rotation spectrum in FePt/Pt without considering the existence of a "magnetic Pt' layer.

0.6

o.4

Q ~

0.2

0.01

Calculation Magnetic Pt (-a) . . . . '

FePt (XR) / Pt (50 A) 200: o 100: o

7 5 : 50: A 3 5 : o 2 0 : . /

|

2 3 4 5 6 i

h v (eV)

'magnet ic Pt ' layer only leads to a monotonous variation of the spectrum, as seen in Fig. 4.

Next, a hypothetical 'magnetic Pt ' with one-unit-cell thickness was assumed at each interface between the FePt and Pt layers. F e P t / P t without considering an existence o f 'magnet ic Pt ' layer, the magneto-optical permeability ~xy for the 'magnet ic Pt ' was used. The simulated magneto- optical spectra of FePl(x)/]Pt(50) with the magnetization o f interracial Pt antiparallel and parallel to that of FePt are shown in Figs. 5(a) and (b), respectively. In the antiparal- lel ease the peak position shifts toward higher energies as x decreases, while in the parallel case the peak shifts toward lower energies. If we assume that the magnetic coupling between Pt and FePt layers is antiferromagnetie for x >_ 50 ~, and ferromagnetic for x < 50 A, the mag- neto-optical spectra observed in the experiment can be explained.

it should be noted that similar results can also be obtained when the existence of an interfacial Pt-rich FePt layer o f one-unit-cell thick, hess with spins parallel and antiparallel to the bulk FePt layer is assamed. However, we believe that the variation of the magneto-optical spec- trum in the Pt-rich FePt layer also comes from the in- creased magnetic polarizatio~n of Pt. Therefore, it may be difficult to distinguish between these two cases. Neverthe- less, the change in the magnetic coupling between the interface and the bulk seems to be essential to explain the abrupt change in the spectra at the critical thiekness.

Further investigations are necessary to elucidate the relation between the structural anomaly and the magnetic coupling.

Calculation 1Vfagnetie Pt

(b FePt(XA) / Vt(50A) 0.6 2{)0 : o

100 : o 75: t~

I:D

0.2 "~

0.0 2 3 4 5 6 i~ v (ev)

Fig. 5. Simulated magneto-optical spectra of PcPt(.x)/Pt(50) with the magnetization of interracial Pt being (a) antiparallel and (b) parallel to that of the FePt bulk layer.

R e f e r e n c e s

[1] P.F. Carcia, J. Appl. Phys. 63 (1988) 5066. [2] Y. Tosaka, H. Ikekame, K. Urago, S. Kurosawa, K. Sato and

S.-C. Shin, J. Magn. Soe. Jpm. 18 (1994) 389. [3] M. Watanabe, K. Takanashi and H. Fajimori, J. Magn. Magn.

Mater. 113 (1991) I10. [4] M. Watanabe, K. Takanashi and H. Fujimori, J. Magn. See.

Jpn. 15 (1991) 415 (in Japanese). [5] K. Sato, H. Hongu, H. Ikekarne, Y. Tosaka, M. Watanabe, K.

Takanashi and H. Fujimori, llm. J. Appl. Phys. 32 Pt 1 (1993) 989.

[6] K. Ohta, A. Takahashi, T. I)cguchi, T. Hyuga, S. Kobayashi and H. Yamaoka, SPIE No. 382 (1983) 252.