Pt multilayer films

Journal of Magnetism and Magnetic Materials 113 (1991) 110-114 North-Holland

Perpendicular magnetic anisotropy and magneto-optical Kerr effect of FePt/Pt multilayer films Masato W a t a n a b e , KSki T a k a n a s h i a n d Hiroyasu F u j i m o r i b~stitute for Materials Research, Tohoku University, 1-1, Katahira 2-chome, Aoba-ku, Sendai 980, Japan

Multilayer films (MLFs) composed of FePt ordered alloy and Pt were prepared by an ion beam sputtering method. The FePt layer thickness t d w e t) was varied in the range f l o r a , to 10 nm, keeping the Pt layer thickness to 5 nm. The torque and magnetization measurements ,'cvealed that all the F e P t / P t MLFs grown on MgO(100) substrates were perpendicularly magnetized films except for the case of dFePt around 5 nm, where a structural anomaly leading to the absence of the multilayer periodicity was seer, The main origin of the perpendicular magnetic anisotropy is considered to be the magnetocrystall!ne ani.~otropy af the FePt layers. When the total thickness was decreased to 19 nm, a 0 K remanence ratio of about 100% was obtained.

I. Introduction Recently, multilayer films (MLFs) composed of Co and noble metals have attracted much attention as candidates for magneto-optic recording media with higher storage densities since they have high magneto-optica! effect in shorter wavelengths and are also perpendicularly magnetized films with 100% remanence ratio [1,2]. Buschow et al. reported t~=at the FePt ordered alloy (abbreviated simply as FePt hereafter) has a large magneto-optical Kerr rotation OK in the region of wavelengths shorter than 300 nm [3]. Tb.is alloy has a large uniaxial magnetocrystalline anisotropy along the c axis which is larger than 2rrM 2, where M s is the saturation magnetization [3,4]. Taking advantage of these properties of FePt, we may expect the fabrication of FePt films with perpendicular magnetization and with large OK in shorter wavelengths. Multilayering of a ferromagnetic metal with a noble metal is available to control the crystal orientation perpendicular to the film plane. In addition, multilayering is known

Corresponde;:ce to: Dr. M. Watanabe, Institute for Materials Research, Tohoku University, 1-1, Katahira 2-chome, Aobaku, Sendai 980, Jaoan.

to enhance OK in several cases [5]. Thus, we have attempted to fabricate MLFs composed of FePt and a noble metal. Pt is chosen as the noble metal, since the lattice mismatch between FePt and Pt is relatively small (about 0.5%), leading to little structural disorder at the interface. Furthermore, Pt is a strongly enhanced paramagnetic element and also has a large spin-orbit interaction. Hence, it is expected that Pt produces a significant effect on magneto-optical properties. In the previous paper [6], the preliminary results of the structural and magnetic properties of the F e P t / P t MLFs were described. We will present more detailed results including magneto-optical spectra in this paper.

2. Experimental procedure F e P t / P t MLFs were prepared by an ion beam sputtering method on MgO(100) single crystal anu quartz suostrates. An re,-'t single layer t~lm (SLF) with the thickness of 100 nm were also prepared by the same method for reference. The base pressure and the pressure during deposition were 1 × 10 -6 and 5 x I0 -s Torr, respectively. The acceleration voltage of the Ar + ion beam was 1100 V. The substrate temperature was ele-

0304-8853/92/$05.00 ~~ 1992 - Elsevier Science Publishers B.V. All rights reserved

M. Watanabe et aL / Anisotropy and Kerr e]fect of FePt / Pt multilayer

vated to 600°C during deposition in order to raise the degree of order in FePt layers. A Pt buffer layer with a thickness of 100 nm was first deposited on the substrate. The FePt layer thickness (dvePt) was varied from 1 to 10 nm, keeping the Pt layer thickness (dpt) to 5 nm. Repetition number was varied in the range from 10 to 45. The uppermost layer was an FePt layer in all cases. X-ray diffractometer (CuK~ radiation) was used for the structural analysis. The saturation magnetization M s and the Curie temperature Tc were measured by a vibrating sample magnetometer. The perpendicular magnetic anisohopy was measured by a torque magnetometer. The maximum field applied was 20 kOe. Because the field of 20 kOe was not large enough to saturate the magnetization along the perpendicular direction to the film plane, we used Miyajima's method [7] to evaluate the uniaxial magnetic anisotropy constant K u. The polar magneto-optical Kerr rotation OK was measured from the film plane side in the 230-800 nm wavelength range.

3. Results and discussion

The multilayer periodicity was confirmed from V-ray diffraction in a low angle region except for the case around dvept = 5 nm. Fig. 1 shows the X-ray diffractograms in a low angle region for the F e P t / P t MLFs on MgO(100) substrates with dye m=3.5, 5 and 10 nm. In spite of the high substrate temperature, the Bragg peaks corresponding to the multilayer periodicity are observed up to fairly high orders in the case of dvept = 3.5 and 10 nm. However, in the case of dvcr, t = 5 nm, no peaks were observed in a low angle region. This means that the multilayer periodicity is absent only around dvePt = 5 nm, although the origin of this structural anomaly is not clear at present. It should be noted here that the ~ t l U ~ , L U I _~I

OLIIIJIIIOLIy

I~

I l l Jr.

~L~.,~,IU~.~IILOLI

uu~.¢

"2.

111

6

dFePt=3"5nnl 9

2 3 7

41s.,+ ~

8

10

dFePt=Snm

~

Io

5

10

5

20 ( deg. ) Fig. 1. X-ray diffractograms in a low angle region of FePt(dFe m nm)/Pt(5 nm) MLFs on MgO(100) substrates with dFcpt = 3.5 nm, dFcpt = 5 nm and dFept = 10 nm.

The X-ray diffraction measurements in a high angle region indicated that the [001] axis of FePt layers and the [100] axis of Pt layers in the F e P t / P t MLFs on MgO(100) substrates were preferred orientations along the perpendicular direction to the film plane except for the case around dFcpt = 5 nm. In the case of the F e P t / P t MLFs on quartz substrates, on the other hand, the [111] axis was found to be preferred orientation. Satellite peaks corresponding to the multilayer periodicity were observed in both the cases of MgO(100) and quartz substrates except for the case of dFePt around 5 nm. Torque measurements revealed that the easy axis of magnetization was perpendicular to the film plane i~] all the F e P t / P t MLFs on MgO(100) substrates except for the case of dFept around 5

t , ~ , r ~'t,tJ

instrumental problem. We tried to fabricate F e P t / P t MLFs 3 times. In each instance, the absence of the multilayer periodicity was seen at only around dyer, t = 5 nm. The structural anomaly affects the magnetic and magneto-optical properties, which will be described later.

along the perpendicular di;ection to the film plane for the F e P t / P t MLFs on MgO(100) and quartz substrates. K U is normalized by the volume of the FePt layers and includes the demagnetizing energy. Ku's of all the F e P t / P t MEFs on MgO(100) substrates are positive except for

112

M. Watam~be et al. / Anisotropy and Kerr effi'ct of FePt / Pt muhffayer 1000 6t

MgO(iOO)

,-¢

-

= ~ 500

o

~-2-

quartz

-:oo -4

[] 0

5

j/

/

;.; 1o0

10

dFePt [ n m ] Fig. 2. dFcPt dependence of uniaxial perpendicular magnetic anisotropy constant K u of FePt(dF~Pt nm)/Pt(5 nm) MLFs on MgO(100) and quartz substrates.

the case of dFePt around 5 nm. On the other hand, Ku's of all the F e P t / P t MLFs on quartz substrates are negative. This indicates that the positive Ku's of the F e P t / P t MLFs on MgO(100) substrates originate mainly from the magnetocrystalline anisotropy along the c-axis of FePt layers and not from the interface anisotropy as was reported in many other MLFs previously. [8] Fig. 3 shows the dFePt dependence of M s and Tc. Tc was obtained by a M 2 - T plot. M s and T c. decreases with decreasing dFePt in the region of dFcpt < 4 nm. In the region of dFcpt < 3 nm, Tc values sufficiently low for application to thermomagnetic recording were obtained. M s and T c have minima around dFeet = 5 nm, which is the same behavior as that of K u. The minima of K u, M s and Tc at about 5 nm are considered to be ascribed to the structural anomaly mentioned above. In the previous paper [6], the OK remanence ratio was found to increase with decreasing dFePt. Fig. 4 shows the OK hysteresis loops of the FePt (3 nm)/Pt(5 nm) MLFs with the total thickness equal to 19, 91 and 187 nm. In the case where the total thickness equal~ 19 nm, a 0~: remazence ratio about 100% is obtained, although the coercive force is somewhat reduced. The total thickness dependence of the 0~: rernanence ratio is similar to that of the C o / P t MLFs reported by

0

5 drePt [nm]

10

Fig. 3. drcpt dependence of saturation magnetization at room temperature M s (a) and Curie temperature Tc (b) of FePt(dFcpt nm)/Pt(5 nm) MLFs on MgO(100) substrates.

[

~

.| "~ O'l t

~

-

T

'

-

-

-

~ total thickness - ~--/=19nm

~ -o. 1 I

',

'~ I

0.1 la

I

I

o

J

i

I -I

I

I

I

I

t

t

I

total thickness -91nm

~

\

~:~ - o . 1 t

..

0.1----

-6

~

I

t

I

I

.

-4

!

total th~ckuess =187nm

-2 H

0

2

4

O

[kOel

Fig. 4. OK hysteresis loops of FePt(3 nm)/Pt(5 nm) MLFs on MgO(100) substrates with total thickness of 19, 91 and 187 nm.

M. Watanabe et ai. / Anisotropy and Kerr effect of FePt/Pt multilayer Hashimoto et al. [9]. They found that C o / P t MLFs showed a 100% remanence ratio when the total thickness is smaller than a critical value of the order of tens of nm. The OK spectra of the F e P t / P t MLFs on MgO(100) substrates with dFept of 10, 7.5 and 3 nm are shown in fig. 5. For comparison, The OK spectrum of the FePt SLF is also shown. In the region of A < 400 nm, 0K'S for both the FePt SLF and the F e P t / P t M L F s increase with decreasing wavelength and have maxima at about 250 nm. In the region of A > 400 nm, 0~ of the FePt SLF increase with increasing wavelength. O n the other hand, the increase in OK for the F e P t / P t MLFs is not so remarkable. The dvePt d e p e n d e n c e of O K at A = 300 and 633 nm are shown in fig. 6. OK, as well as M s, Tc and K o m e n t i o n e d above, has a m i n i m u m around dFePt = 5 rim, which is considered to be due to the structural anomaly. Except for the dip around dFePt = 5 nm, the decrease in OK with decreasing dFePt can be seen. If no magneto-optic activity is assigned to Pt layers, we can calculate OK using the following formula [10].

OK= [dvept/(dvePt + d p t ) ] O ~ ePt),

(1)

where 0 ~ ~Pt~ is OK of the FePt SLF. In this calculation, we assume the same refractive indices for FePt and Pt layers [10]. This assumption

0

,

I

'

'

"

I

'

'

'

I

J

..°""......................................... .4; ] -0.2

:/"ii//"~'---"-~, : ,,' ! /

ldl:eet=3nn~

'v

e, s L F

-o6I,

i

X=300nm i

{).4

"I:I ,..=.,

,,~0.2 /-

........ © 0

;k=633nm 0.4

,.~1).2

5

10

dFePt [rim] Fig. 6. dvcpt dependence of OK of FePt(dvept nm)/Pt(5 nm) MLFs on MgO(100) substrates at ~ -- 300 and 033 nm.

would be reasonable for a rough estimate, since the reflectance of the FePt SLF is similar to that of the Pt SLF in the 200-900 nm wavelength range. Broken curves in fig. 6 indicate the calculated dvcpt d e p e n d e n c e of OK. The calculation agrees fairly well with the experiment at A = 633 nm except for the dip around dvept = 5 nm. However, at A = 300 nm, the experimental values of OK are larger than the calculated ones except for the dip around dveet = 5 nm. This suggests the possibility that the effect of magneto-optic activity in Pt layers appears in shorter wavelengths.

4. Conclusion

//

I

I).6

~\dvept=7.5nni

°41 ..# 200

113

,

I

400

,

t

,

l

,

J

600

Wavelength

,

I

800

Into]

i:ig. 5. 0 k spectra of FePt(dv~Pt nm)/Pt(5 nm) MLFs on MgO(100) substrates with dF~pt= 10. 7.5 and 3 nm. The spectrum of the FePt SLF on a MgO(100) substrate is also shown for comparison.

We have prepared MLFs composed of FePt o r d e r e d alloy and Pt on MgO(100) substrates by an ion beam sputtering m e t h o d The F e P t / P t MLFs on MgO(100) substrates exhibited an easy axis of magnetization perpendicular to the film plane. The main origin of the perpendicular magnetic anisotropy is considered to be the magnetocrystalline anisotropy of FePt layers. A OK r e m a n e n c e ratio about 100% was

114

M. Watanabe et al. / Anisotropy and Kerr effect of FePt / Pt rnultilayer

obtained when the total thickness was decreased to 19 nm. The 0 K spectrum shapes of the F e P t / P t MLFs suggest enhancement of 0 K in the region of h < 400 nm due to the magneto-optic activity in Pt layers. However, it is necessary to investigate the dielectric tensor in order to elucidate the magneto-optic activity in Pt layers more clearly. The F e P t / P t MLFs showed a structural anomaly leading to an absence of the multilayer periodicity around dFer,t ----5 nm. Accordingly, Ktj, M s and Tc had minima around dFePt = 5 nm. The details of the structural anomaly will be described elsewhere, including transmission electron microscope observation results.

Acknowledgements We would like to thank Drs. T. Katayama and Y. Suzuki at Electrotechnical Laboratory for measurements of OK spectra.

References [1] W.B. Zeper, F.J.A.M. Greidanus, P.F. Carcia and C.R. Fincher, J. Appl. Phys. 65 (1989) 4971. [2] Y. Ochiai, S. Hashimoto and K. Aso, IEEE Trans. Magn. MAG-25 (1989) 3755. [3] K.H.J. Buschow, P.G. van Engen and R. Jongebreur, J. Magn. Magn. Mater. 38 (1983) 1. [4] O.A. Ivanov, L.V. Solina, V.A. Demshina and L.M. Magat, Fiz. Metal. Metalloved 35 (1973) 92. [5] T. Katayama, H. Awano, Y. Nishihara and N. Koshizuka, IEEE Trans. Magn. MAG-23 (1987) 2949. [6] M. Watanabe, K. Takanashi and H. Fujimori, J. Magn. Soc. Jpn. 15 (1991) 415 (in Japanese). [7] H. Miyajima, K. Sate and T, Mizoguchi, J. Appl. Phys. 47 (1976) 4669. [8] F.J.A. den Breeder, W. Hoving and P.J.H. Bloemen, J. Magn. Magn. Mater. 93 (1991) 562. [o] S. Hashimoto, Y. Ochiai and K. Asc, J. Appl. Phys. 67 (1990) 4429. [10] E.R. Moog, J. Zak and S.D. Bader, J. Appl. Phys. 69 ( 1991 ) 880.