An anomalous magneto-optical Kerr effect in Pd-based multilayers

Journal of Magnetism and Magnetic Materials 128 (1993) 391-394 North-Holland

II

An anomalous magneto-optical Kerr effect in Pd-based multilayers S.M. Z h o u *, L.Y. Chen, Y. Su, X.W. Feng, Y.H. Qian, X.D. Xie Department of Physics, Fudan, University, Shanghai, 200433, China Received 5 January 1993; in revised form 13 April 1993

In this paper we present the calculated magneto-optical Kerr rotation spectra of F e / P d and C o / P d multilayers based on two different assumptions. Firstly the spin-polarization of the Pd layers and its additional magneto-optical Kerr effect in F e / P d and C o / P d multilayers are assumed negligibly small. Secondly the Pd layer is assumed to be spin-polarized. The calculated spectra with the latter assumption agree with the measured spectra by modifying the parameters. The Pd layers are suggested to induce an additional magneto-optical Kerr effect and thus enhance magneto-optical Kerr effect in these multilayers.

1. Introduction

Recently magnetic multilayers have received much attention, because of their important applications in magneto-optical storage and magneticfield detectors with basic problems, such as interlayer coupling etc. [1-7]. In magnetic/noble metal (Au, Ag, Cu) multilayers with a proper thickness ratio and modulation wavelength, there will be a magneto-optical Kerr effect (MOKE) enhancement near the plasma or the absorption edge, arising purely from the optical constant effect of the layered systems [8-9]. This phenomenon is not associated with spin-polarization of noble metals induced by the proximity effect of magnetic layers. For multilayers the Kerr rotation Ok will increase with increasing modulation wavelength if the non-magnetic layers are not spinpolarized, as indicated by the measured spectra of F e / C u multilayers and the calculated spectra of Co(Fe)/Pt multilayers with the assumption of no spin-polarization of Pt layers [10-11]. However Ok increases with decreasing modulation wavelength for F e / A g multilay.ers in the long wavelength range of 5500-8000 A and Co(Fe)/Pt

multilayers in the wavelength range of 3000-8000 A. As discussed previously [10,14], the normal metal layers in Co(Fe)/Pt and F e / A g multilayers are suggested to be spin-polarized and induce an additional MOKE. The Pd layers were also suggested to be spin-polarized in Fe-Si/Pd multilay-

Pd/Co 03 0

02

,,IF

(D

01 i

2O00

a

I

t

5OO0

i

I

SO00

WAVEL ENGTH(.) * Corresponding author.

Fig. 1. The measured Kerr rotation 0 k spectra of F e / P d (17 ,~,) multilayers.

0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

S.M. Zhou et aL / Anom~lous magneto-optical Kerr effect

392

tion and spin-orbit coupling of conduction electrons can be expressed as follows [10,19]:

O.3

Pd/Co 8.1/64

f '

5xy(W ) = (w~,X tr~ond/a.rr ) x

, O2

[

-" 0 2 + (i~o + 3,) 2

+eVo

O

0.1 I

2000

I

I

'

5000

'

'

8000

WAVELENGTH(nm)

Fig. 2. The measured Kerr rotation #k spectra of C o / P d multilayers.

"

(1)

Here ~, and /2 are the relaxation and skew scattering frequency, respectively, o)p is the plasma frequency, tr¢ond = ( n , - n , ) / ( n t + n , ), n ~ and n~ are the numbers of condition electrons with spin moment parallel and antiparallel to the total moment. P0 and V0 are the maximum of macroscopic dipole moment and the Fermi velocity. The relation between the dielectric tensor and the conductivity tensor elements is expressed as follows: - i4"rr × 5xy(o~) =

ers [15-16]. Anomalous MOKE behaviors of F e / P d and C o / P d multilayers were reported by Zhai and Nakamura et al. respectively, as shown in figs. 1 and 2 [17-18]. One can see that for F e / P d multilayers with increasing Fe layer thickness Ok increases in the long wavelength but decreases in the short wavelength range. For C o / P d multilayers with top Pd layer, with decreasing modulation wavelength Ok increases more rapidly in the short wavelength range than in the long wavelength range and approaches to that of CO-Pd alloys with small modulation wavelength. In this paper we calculated the Kerr rotation spectra of F e / P d and C o / P d multilayers by Fresnel's equations based on two different assumptions. Firstly the spin-polarization of Pd layers and their additional MOKE are assumed negligible. Secondly the Pd layers are assumed to be polarized and an additional MOKE is induced.

2. Numerical calculations and discussions

The off-diagonal element of the conductivity tensor 5xy(tO) contributed by the spin-polariza-

(.0

(2)

Further the complex optical constants for right ( + ) and left ( - ) circularly polarized light are related to the dielectric tensor elements by IV + 2 = ~ ± = ~xx + i~xy"

(3)

If the parameters in Eq. (1) are known the optical constants A~± of the Pd layers can be obtained. With the first assumption the spin-polarization of Pd layer and its contribution to #xy in F e / P d and C o / P d multilayers are negligiblysmall, /V ± remain as ordinary optical constants N. The magneto-optical and optical constants of Fe, Co and Pd are taken from the literature [20-22]. In this paper the calculated procedure is the same as Eqs. (1)-(9) used in our previous work [23]. The calculated MOKE spectra of Fe/Pd(17 A) multilayers are shown in fig. 3. It is obvious that in the whole wavelength range of 2000-8000 .A Ok increases with increasing Fe layer thickness. This does not coincide with the experimental results in the short wavelength range. The calculated Ok spectra of C o / P d multilayers with top Pd layer

S.M. Zhou et aL / Anomalous magneto-optical Kerr effect

30.C

200

Fe/Pd - - - - - 85/17 150 - - ~ 16/17 22/17 e, 100 (1)

,

/ -'-""-~

~10.~

0.1

01

~800

6000 WAVELENGTH(~)

8000

Fig. 3. The calculated Kerr rotation spectra of F e / P d (17 A) multilayers with the assumption of no Pd atom polarization.

are displayed in fig. 4. It is found that although the calculated Ok increases with decreasing modulation wavelength, like the measured spectra, there are two differences between the calculated and the measured spectra for C o / P d multilayers. First the calculated Ok is quite smaller than the measured one. Secondly the increment of Ok in the short wavelength region is quite smaller than the measured results, as shown in fig. 2. This

15.0 Pd/Co c

Fe /Pd

----- 8.5/17 16/17 ...... 22/17 .-. 2OC

50

°

393

10.0

~

E

5.0 C\--8.1/6.4/ 0.0

,

'

2000 4000 6000 8000 VC,VELE NGTI-.t(/)

Fig. 4. The calculated Kerr rotation spectra of C o / P d multilayers with the assumption of no Pd atom polarization.

4( )0

!

6000 WAVELENGTI-I(/~)

8000

Fig. 5. The calculated Kerr rotation spectra of F e / P d (17 .~) multilayers with the spin-polarization of Pd atoms.

implies that the Pd layers in F e / P d and C o / P d multilayers might be spin-polarized and an additional MOKE is induced by the Pd layer, especially in the short wavelength region. Thus we modify the parameters in eq. (1) by assuming that the conduction electrons of the Pd layers are spin-polarized. In terms of O'¢o,dX~ 2, Po/eVo, 3' and 12, the optical constants N + of the Pd layers and then the Kerr rotation spectra can be modeled using Eqs. (1)-(9) in Ref. [23], where N is replaced by/V ±. The calculated spectra of F e / P d and C o / P d multilayers are displayed in figs. 5 and 6 respectively. For Fe/Pd(17 A) multilayers 1,, C&o.dX tO2 and Po/eVo are taken as 0.04 eV, - 1 5 . 0 (eV) 2 and 3.0; and 12 is taken as 0.5, 0.6 and 0.65 eV for F e / P d multilayers with the Fe layer thicknesses of 8.5, 16, 22 A respectively. For C o / P d multilayers 12, 3' and Po/eVo are taken as 0.4 eV, 0.04 eV, 3.0, respectively. ~rco.dX to2 is taken as -30.0 and -25.0(eV) 2 for C o / P d multilayers with the modulation wavelengths of 14.5 o and 50.7 A respectively. The calculated spectra of F e / P d and C o / P d multilayers with the latter assumption are found to fit to the experimental results and thus the Pd layers in F e / P d and C o / P d multilayers are suggested to induce an additional MOKE and thus enhance the MOKE of multilayers without any reflectivity reduction, unlike what happened in layered films and some multilayered films [1-3,11]. So the spin-polariza-

394

S.M. Zhou et aL / Anomalous magneto-optical Kerr effect

200

References

Pd/Co

15.0 ~ 8 . 1 / 6 4 .c_

E

1QO 5.( 30

i

I

4000

GO00

WAVELENGTH(/I)

8000

Fig. 6. The calculated Kerr rotation spectra o f Co/Pd multilayers with the spin-polarization of Pd atoms.

tion in multilayers can provide an important way to improve magneto-optical properties of multilayers. It is noted that although eq. (1) was deduced with the intraband mode and is only suitable for conduction electrons and d electrons of Pd and Pt layers are partly localized, and the additional MOKE induced by Pd and Pt layers in Fe (or Co)/Pt (or Pd) multilayers can not be described by eq. (1) completely, the calculated spectra with the second assumption agree with the measured spectra for F e / P d and Co/Pd multilayers. The Pd atoms have a large Stoner factor of 10, that is the threshold value of becoming ferromagnetic. When Pd atoms form alloys such as Co-Pd, Fe-Pd [24,25], Pd atoms are spin-polarized to enhance the magnetization. For Fe (or Co)/Pd multilayers Pd atoms are actually surrounded by ferromagnetic atoms and acquire induced magnetic moments because of the interface interdiffusion. So the spin-polarization of Pd atoms in F e / P d and Co/Pd multilayers may arise from the proximity effect of magnetic layers as pointed by Nakamura and Liu et al. [18,15].

Acknowledgement This work was supported by State Education Commission and National Science Foundation of China.

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