ELSEVIER

Journal of Magnetism and Magnetic Materials 156 (1996) 247-249 Journal of magnetism

. ~ and magnetic

~ i materials

Oscillations of Hall resistivity and thermoelectric power in Co(Fe)/Cu multilayers

H. Sato a,,, y . Kobayashi ~, Y. Aoki a, Y. Saito b, K. Inomata b ~' Department of Physics, Tokyo Metropolitan University Hachioji-shi, Tokyo 192-03, Japan

Toshiba Corporation, Adt'anced Research Laboratory, Research and Development Center, Saiwai-ku, Kawasaki 210, Japan

Abstract We have observed, tbr the first time, a clear oscillation in the extraordinary Hall resistivity and thermoelectric power as a

function of Cu layer thickness correlated with the giant magnetoresistance in CogoFej0/Cu multilayers.

Recently, quite a few experiments on various transport properties have been reported on magnetic multilayers [1-3]. There remains a controversy regarding the field dependence of the extraordinary Hall resistivity which is expressed as:

p g = Rs M = ( ap + bp 2) M, ( I )

where the extraordinary Hall coefficient R s consists of the skew component proportional to the resistivity p and the side-jump components proportional to p2 and M is the magnetization. A debatable point is whether R s for the giant magnetoresistance (GMR) systems depends on the field strength ( H ) or not. For ordinary magnetic materials, R s can be assumed to be field independent, since 19 is only weakly field dependent. However, for the GMR system, we naturally expect R s to be field dependent, since p depends largely on H.

The first measurement of the Hall effect in GMR system was reported by Song et al. on F e / C r multilayer [3]. They reported an anomalous peak in the field depen- dence of p~t though no clear explanation of the origin has been shown. If R s is field-independent, we expect p ~ to depend on H in the same manner as M as predicted from Eq. (1) and hardly expect such a peak. We have asserted that the peak comes from the field dependence of R s and is a common feature to the systems exhibiting the GMR effect including the multilayers and granular alloys, based on our simultaneous measurements of p~, magnetoresis- tivity (MR) and M in a same geometry and on a same sample [1]. However, many reports on the Hall effect in the GMR system have ignored the field dependence and some reports emphasized that R s is field independent [2].

* Corresponding author. Fax: +81-426-77-2483; sato@ phys.metro-u.ac.jp.

0304-8853/96/$15.00 © 1996 Elsevier Science B.V. SSDI 0304-885 3(95 )00855-1

email:

To clarify the disagreement, we have measured the Cu layer thickness (dcu) dependence of Hall resistivity on Co9oFe lo /Cu multilayers which are known to exhibit clear oscillation in MR ratio [4]. In order to further support the Hall effect result, we also performed thermoelectric power measurements on the same samples.

0.2

E ¢,3

=I. 0.1

"I- O..

T=4.2K

dc0=10.5A o ~ • • ! ea

°Q e

o~O ~ 21A

A I I I

T=4.2K 25-,,,,,,

,_.. " ~ d c ~ = l O , 5 A

o 20 " . . ".

. -~ 14A " -

. . . . . . . . .

- ~ . . . . A A A A A A ~ A • • • • • • •

I I 1 I

1 2 3 4 5

H (m) Fig. 1. The field dependence of p~t and p at 4.2 K in CogoFe]0/Cu multilayers for Cu layer thickness de.-= 10.5, 14. 21 and 27 A..

0.15

~ 0.1 0

0.05

8

6

co 4 <1

248 H. Sato et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 247-249

8C

6C 0

n- 4c

2(

l \ o 273K

T=296K

//\ i ^ 1 i i ~

o o 4.2K D n 77K

'a 1 ~'G' 273K

o o RI I R 1=0 2tO 30 dcu (A)

Fig. 2. The normalized Hall coefficient, thermoelectric power, and magnetoresistance as a function of dcu.

Samples were prepared by IBS on MgO (110) single- crystal substrates. Four dcu values were selected at the first and the second maxima, and the first and the second minima of the MR oscillation to observe the oscillation most clearly [4].

Fig. 1 shows the field dependence of p ~ and p at 4.2 K for H perpendicular to the layers. At higher fields, nothing related to the GMR effect has been observed in p~. However, below the saturation field, there can be seen a clear difference in the field dependence depending on the MR ratio; for the large MR samples p ~ shows a bump, while for the smaller MR ones, pMn(H) mimics the magne- tization M(H) as expected from Eq. (1) with a field independent R s as usually reported for the ordinary ferro- magnetic films. The bump in the field dependences of pn M correlated with the MR in Fig. 1 directly evidences that R s does depend on the field strength. At low fields, the larger conduction electron scattering enhancing p predominates in p~ , which leads to the enhanced p ~ at lower fields for the large MR samples. At higher fields, on the other hand, the scattering responsible for the GMR effect disappears and has no influence on p~.

To show the GMR contribution to p ~ as a function of dcu, we calculated the field dependence of R s from Eq.

-10 1 1 T

~ 4 A

-15

> :=L v

O') -2O

-25

0cu_-,0 2,A fA

T=296K

30

E o

"~ 20

~ 21A

27A i I i

101- - -0.5 0 0.5

H (T) Fig. 3. The field dependence of thermoelectric power and resistiv- ity for H JlMgO [100] at 296 K.

(1) using the measured M(H) in the same geometry. We plotted the field dependent part

A R s = Rs(0 T) - Rs(5 T)

in Fig. 2 along with the MR ratio, where AR s is found to oscillate synchronously with the MR ratio.

Fig. 3 shows the field dependence of thermoelectric power S at 296 K for the field parallel to the easy-axis ( H r] MgO [100]) along with the MR in the same condition; the MR was measured immediately after the thermopower measurement in the same experimental set-up. The correla- tion of the field dependences of the S and the MR is apparent. The same comparison was made also at 80 K, where we obtained the similar results. The result for dcu = 10.5 ,~ is consistent with that reported by Nishimura et al. [5] where the anisotropy in S was investigated on the sample with dcu = 10 A. The field dependent part

A S = S (0T) - S ( l T )

was also plotted in Fig. 2. All the properties oscillate as a function of dcu synchronously, evidencing the dominant role of the same conduction electron scattering mechanism responsible for the GMR effect in the extraordinary Hall effect and thermoelectric power.

Acknowledgements: This work was partly supported by a Grant-in-Aid for Scientific Research from The Ministry of Education, Science and Culture of Japan.

H. Sato et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 247-249 249

References

[1] H. Sato, Mater. Sci. and Engng. B 31 (1995) 101. [2] C.L. Chien, Mater. Sci. and Engng. B 31 (1995) 127. [3] S.N. Son, C. Sellers and J.B. Ketterson, Appl. Phys. Lett. 59

(1991) 479.

[4] K. Inomata and Y. Saito, J. Magn. Magn. Mater. 126 (1993) 425.

[5] K. Nishimura, J. Sakurai, K. Hasegawa, Y. Saito, K. Inomata and T. Shinjo, J. Phys. Soc. Jpn. 63 (1994) 2685.