ELSEVIER Physica B 239 (1997) 53-55

Difference between the giant magnetoresistance of Fe/Cu and Co/Cu magnetic multilayers under high pressure

T. Sakai a'*, G. Oomi b, K. Okada c, K. Takanashi d, K. Saito d, H. Fuj imor i d

"Ariake National College of Technology, Omuta, Fukuoka 836, Japan b Department of Mechanical Engineering and Materials Science, Faculty of Engineering, Kumamoto University, Kumamoto 860, Japan

"Department of Physics, Faculty of Science, Kumamoto University, Kumamoto 860, Japan d lnstitute for Materials Research, Tohoku University, Sendai, Miyagi 980-77, Japan

Abstract

The giant magnetoresistance (GMR) of T/Cu (T = Fe, Co) magnetic multilayers (MML) has been investigated under high pressure at 4.2, 77 K and room temperature. It is found that the magnitudes of GMR at 77 K and at room temperature for both MMLs are almost independent of pressure up to 0.8 GPa, but 9.2% decrease in GMR is observed at 4.2 K under the pressure of 2 GPa. On the basis of these results, the difference between GMR for Fe/Cu and Co/Cu MMLs under high pressure is discussed briefly.

Keywords: Giant magnetoresistance; Magnetic multilayer; High pressure

The magnetic multilayers (MMLs) having the antiferromagnetic interlayer coupling between ad- jacent ferromagnetic layers have been studied ex- tensively since the giant magnetoresistance (GMR) was discovered in Fe/Cr by Baibich et al. [1]. The magnitude of G M R of Co/Cu was reported to be several tens percent even at room temperature, and oscillates as a function of the thickness of the para- magnetic layer tp 1-2] and also the ferromagnetic layer t m I-3]. The investigation of G M R for M M L under high pressure is considered to be one of the useful method to elucidate the origin of G M R be- cause we can control tin, tp and roughness at inter- faces continuously by applying pressure. In a recent work we measured the G M R of Fe/Cr [4] and Co/Cu [5] under high pressure, in which the behav- ior of G M R of Fe/Cr under pressure was found to

* Corresponding author.

be quite different from that of Co/Cu. In the present study, we made an attempt to measure the mag- netoresistance (MR) of Fe/Cu and Co/Cu under high pressure in order to clarify the influence of the elements of ferromagnetic layers on the pressure dependence of G M R in MML.

o

The .[Fe(10.1 A)/Cu(13.7 A)]15 (Fe/Cu) and [Co(10 A)/Cu(10 A)] 15 (Co/Cu) samples were pre- pared by means of multi-target-type ion-beam sputtering technique [6]. High pressure was gener- ated by a piston-cylinder apparatus. The pressure inside the cell was kept constant by controlling the load of hydraulic press within _+5% throughout the experiment. Magnetic field was applied up to 1.5 T. The measurement of MR was carried out up to 2 G P a at 4.2 K and up to 0.8 G P a at 77 K and room temperature. The details of the high-pressure apparatus was reported in our previous paper [7].

The pressure dependence of electrical resistivity p at room temperature for Fe/Cu and Co/Cu is

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54 12 Sakai et al. / Physica B 239 (1997) 53-55

45

40 I 35

30 zl.

25

20

15

F i

at room temperature

[Fe(10.1A)/Cu(13.7A)]15 // i I 1

1 2

P (GPa)

Fig. 1. Electrical resistivity p at room temperature for [Fe(10.1 A)/Cu(13.7A)Jls and [Co(10A)/Cu(10A)Jls mag- netic multilayers as a function of pressure.

shown in Fig. 1. Both samples show large p at room temperature compared with the resistivities of pure Fe, Co and Cu metals mainly due to the scattering by defects, interface roughness and so forth. By application of pressure, p of Fe/Cu de- creases smoothly with increasing pressure having the rate of -3 .1 x 10 -2 GPa -1 and tends to satu- rate above 2 GPa. While p of Co/Cu decreases linearly with increasing pressure. The decreasing rate ofp for Co/Cu is - 2 2 x 10 -2 GPa- 1, which is an order of magnitude larger than that of Fe/Cu.

In Fig. 2 we show the MR ratio Ap/p~ = [p(H) - Ps]/Ps versus applied field curves at room temperature at 0, 0.8 GPa for Fe/Cu and Co/Cu, with the applied field and the electric current both in-plane but perpendicular to each other, where p(H) and ps are the electrical resistivities below and above the saturation field H~. At ambient pressure, the magnitude of Ap/p~ at room temperature is 9.7% for Fe/Cu and 32% for Co/Cu. By applica- tion of 0.8 GPa, the magnitudes of MR and H~ of Fe/Cu and Co/Cu are almost independent of pres- sure. The effect of pressure on the MR-curve is very

3o+ at room temperature J ± H

[Fe(10.1A)/Cu(13.7A)]t 5

o P = 0 GPa 6~o zx P=0.8GPa

20 1 a ~° ~///[C°(10fl0/Cu(10A)]15

0 0.0 0.4 0.8 1.2

H(T)

10,

Fig. 2. Magnetoresistance ratio Ap/ps(H) at room temperature at various pressure for Fe/Cu and Co/Cu.

small below 0.8 GPa. It indicates that the electronic state and/or roughness at interfaces dominating the magnitude of GMR of these samples is not strongly affected by pressure below 0.8 GPa. Similar results of MR are obtained at 77 K as well. The data at 4.2 K for Fe/Cu and Co/Cu was reported in a pre- vious paper [-8].

We show the values of Ap/ps of Fe/Cu and Co/Cu in Fig. 3 as a function of pressure. At 4.2 K the value of Ap/ps of Fe/Cu is 25.1% at ambient pressure, and it decreases smoothly with increasing pressure and becomes 22.8% (9.2% decrease) at 2 GPa. At 77 K and room temperature the values of Ap/ps are almost constant below 0.8 GPa. At 4.2 K, Co/Cu has a large value of Ap/p~, 53.5% at 0 G P a and becomes 50.0% (6.5% decrease) at 2 GPa. The overall behavior of pressure depend- ence of Ap/ps is almost similar to that of Fe/Cu as shown in Fig. 3.

The main result of our present study is that there is only a slight difference in the pressure depend- ence of Ap/p~ between Fe/Cu and Co/Cu. The small pressure effects on GMR for these MMLs com- pared with that of Fe/Cr is probably due to the difference in the interface magnetic disorder be- tween Fe/Cu (Co/Cu) and Fe/Cr, which is affected strongly by applying pressure [8]. Furthermore, considering that the bulk moduli of Fe, Co and Cu layers are probably larger than that of each bulk

T. Sakai et al. / Physica B 239 (1997) 53-55 55

,---, 3O

6 0 ] ' - . ~ . ,

|

-

)< s R . T .

20 - - ~ 4.2 K

~'~"~ 77 K

10

R.T.

- [ " [Fe( 10.1A)/Cu( 13.7A)]L ~

L , I

1 2

P (GPa)

Fig. 3. Magnetoresistance ratio Ap/ps at H = 0 T at 4.2, 77 K and room temperature for Fe/Cu and Co/Cu as a function of pressure.

sample [9], the contribution of the change in the thickness of ferro- or paramagnetic layer by applying pressure to GMR is considered to be negligibly small.

In summary, we observed the pressure effect on the GMR in Fe/Cu and Co/Cu MMLs. It has been

found that the overall behaviour of GMR under pressure for Fe/Cu is quite similar to that for Co/Cu at room temperature, 77 and 4.2K. An important result of our present data is that GMR in MML under high pressure up to 2 GPa is not strongly affected by the difference of electronic or magnetic state in ferromagnetic layer elements (Fe and Co) and the change in the thickness of ferro- or paramagnetic layers.

References

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