Journal of Magnetism and Magnetic Materials 126 (1993) 445-447 North-Holland

Magnetoresistance of C o - X / C u (X = Fe, Ni) multilayers

H. Kano, Y. Iwasaki, K. Hayashi and K. Aso Sony Corporation Research Center, 174 Fujitsuka-cho, Yokohama 240, Japan

We report the magnetoresistance (MR) of Co-X/Cu (X = Fe, Ni) multilayers prepared by dc magnetron sputtering. The maximum MR ratio was obtained with Co/Cu. The MR ratio of Co-Ni/Cu decreased monotonically with increasing Ni content, while that of Co-Fe/Cu decreased rapidly at the fcc-bcc crystal phase transition point.

1. Introduction

The giant magnetoresistance (GMR) effect is very attractive from both the physical and applications view- points. So far, many theoretical and experimental stud- ies have been made, but our understanding of GMR remains far from complete.

The magnetic layer material dependence of multi- layer MR is very important, because it might give a clue to the physical origin of the GMR. A theoretical study of C o - X / C u (X = Fe, Ni) multilayers has sug- gested that the continuous change in the MR ratio is similar to the Slater-Pauling curve [1]. It has also been reported that the MR ratios of samples prepared by ion beam sputtering (IBS) reached maxima at around 10-25% Fe content and decreased toward pure Co [2]. The absolute MR ratio for pure C o / C u by IBS was smaller than that obtained for dc-magnetron-sputtered multilayers [3,4], which is the highest MR ratio thus far.

In this paper, the GMR character of dc-sputtered C o - N i / C u and C o - F e / C u artificial multilayers is re- ported and the correlation between the GMR and the crystalline structure is discussed.

2. Experiment

The samples of [Co-X/Cu]30 (X = Ni, Fe) multilay- ers were prepared on glass substrates by dc magnetron sputtering. The base pressure prior to sputtering was below 3 x 10 -4 Pa and the Ar gas pressure during sputtering was 0.2 Pa. The sputtering rate of 5-50 n m / s was controlled by adjusting the discharge current and the rotation speed of the substrate holder. It has been reported that the Fe underlayer increases the

Correspondence to: H. Kano, Magnetic and Inorganic Materi- als Research Dept., Sony Corporation Research Center, 174 Fujitsuka-cho, Hodogayaku, Yokohama 240, Japan. Fax: 045- 334-6935.

MR ratio of C o / C u [4], but that a large MR ratio could be obtained without an underlayer with good reproducibility. Therefore, neither an underlayer nor overlayer was used in these experiments. The first layer on the substrate was a magnetic layer. The resistivity was measured by the dc four-terminal method with maximum external fields of 100 and 10 k A / m for the first and second peaks, respectively. The saturation magnetization (M~) was measured by a vibrating-sam- ple magnetometer. The crystal structure was defined by X-ray diffraction. All measurements were per- formed at room temperature.

3. Results and discussion

The MR ratios and M S of C o - N i / C u and Co- F e / C u multilayers are shown in fig. 1. The thickness of C o - X was 1 nm, and that of Cu was 1 nm for the first peak and 2.1 nm for the second peak. These values gave the maximum MR ratio for pure Co/Cu. It was not the optimum structure for Co-Fe; this problem will be discussed later.

The maximum MR ratio for pure C o / C u multilay- ers was 60% at the first peak, but it was not perfectly saturated even at an external field of 100 kA/m. It could be increased by a few percent with an adequate external field, to almost the same as the maximum MR ratio reported elsewhere [3,4]. The crystal structure of the films with large MR ratios was ( l l l ) -or ientcd fcc polycrystalline.

In C o - N i / C u multilayers, the MR ratio decreased monotonically with increasing Ni content. The pure N i / C u still showed a MR ratio of 6% at the first peak and 4% at the second peak. In the case of C o - F e / C u , the MR ratio decreased gradually as the Fe content was increased from 0 to 35%, and decreased rapidly above 35% Fe, to less than 5% in the region of more than 40% Fe. Calculations based on the random ex- change potential at the interfaces [1] support our re- suits only for C o - N i / C u . This theory suggests that a maximum MR ratio can be obtained in the C o - F c / C u

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446 H. Kano et al. / Magnetoresistance of Co-X/Cu

70 i(a) ]10 70 ~ ( b ) 10

60 8 60 M ~ ~ ' ~ - ~ s - 8

50 ~, 50 ~ ~ t s t p e - - ~" o~" 40 i "~ ~ ak) 6 ~ rr .6 ~ 40 .6

n-" 30

10 10

o - . . . . , o I I I ~ = = = : ~ o 0 20 40 60 80 100 0 10 20 30 40 50

Ni content (%) Fe content (at%) Fig. 1. Fe or Ni content dependence of MR ratios and M s in [Co-X/Cu]3o multilayers with (a) X - Ni, and (b) X - Fe. The magnetic layer thickness was 1 nm. Cu layer thickness of l and 2.1 nm for the first and second peak, respectively, were chosen so as

to obtain the maximum MR ratio in pure Co/Cu.

system, yet pure Co/Cu showed the largest M R ratio in these exper iments .

In o rder to clarify the reason for the M R decrease in C o - F c / C u , the magnet ic layer thickness depen- dence of the MR ratio for various Fc conten ts was investigated; the results are shown in fig. 2. The maxi- mum M R ratio of Co-Fe /Cu was smaller than that of pure C o / C u , but the M R ratio of C o - F e / C u was almost thc same as tha t of pure C o / C u in the thin magnet ic layer region. The maximum M R ratio of C o - F e / C u was ob ta ined at th inner magnet ic layer than that of pure C o / C u . In case of the op t imum film thickness for each composi t ion, the M R ratio increased especially in the Fe-rich region.

The X-ray diffract ion pa t te rns of C o - 3 5 % F e / C u at the second peak with magnet ic layer thicknesses of l and 2 nm are shown in fig. 3. The M R ratios of these films were 15% and 3%, respectively. The crystal struc- ture of films with large M R ratios was fcc, whereas

30

25

~- 2O == 8 15

o lO

~ 5

0 0.5

I u

7 1 1.5

Magnetic layer thickness (nm)

Fig. 2. Magnetic layer thickness dependence of MR ratios in [Co/Cu]3~ I, [Co-35% Fe/Cu]30 and [Co-50% Fe/Cu]~l

multilayers.

those with small M R ratios were bcc. Also the Co-5(1~ F e / C u with a small M R ratio was found to have a bcc s tructure. Thus the decrease in the M R ratio for C o - 40% F c / C u was caused by a fcc-bcc crystal s t ructure transi t ion. From X-ray analysis, it was found that the fcc -bcc phase t ransi t ion point in multi laycrs was shifted to an Fe-rich composi t ion as compared with a bulk C o - F e alloy and it varied with magnet ic layer thick- ness.

The sa tura t ion field at the first peak was about 100 k A / m for 1 nm C o - 3 5 % Fe, and less than 25 k A / m for 2 nm C o - 3 5 % Fe. Thus the inter layer exchange coupling energy of films with bcc s t ructure was lower than that of fcc-structurcd film. It could be thought tha t the bee-s t ructured magnet ic layer reduced the

~ ~ ~ dco Fe 2 0 nm

~ , ,. j ~ . dCo_Fe : 1.0 nm

I I 10 40 70 100

2e Fig. 3. X-ray diffraction patterns of [Co 35~ Fe (1.0 nm) / Cu (2.1 nm)]11,~ and [Co-35% Fe (2.0 nm)/Cu (2.1 nm)]E, . multilayers, which showed MR ratios of 15"/, and 35~.

respectively.

H. Kano et al. / Magnetoresistance of Co-X~ Cu 447

interlayer exchange coupling and that the antiferro- magnetic coupling at zero external field was imperfect, thus reducing the MR ratio. The structural phase tran- sition should therefore be taken into account in discus- sions of the MR ratio of C o - F e / C u multilayers.

The results obtained from dc-magnetron-sputtered C o - F e / C u are different from those of samples pre- pared by IBS [2], in which the maximum MR ratio was obtained at 10-25% Fe. The experiment on sputtered Co/Cu shows that the MR ratio depends strongly on the film preparation conditions and the effect of re- sputtering or mixing is important for obtaining large MR ratios [5]. These effects are likely to be different between IBS and dc magnetron sputtering, and thus the MR of dc-sputtered Co/Cu might be large.

4. Conclus ion

We have investigated the composition dependence of the MR ratio in dc-magnetron-sputtered C o - X / C u (X = Ni, Fe) multilayers. The maximum MR ratio was obtained for pure Co/Cu multilayers. The MR ratio of

C o - N i / C u multilayers decreased monotonically with increasing Ni content. In C o - F e / C u multilayers, the MR ratio decreased rapidly at 40% Fe content; this transition point was found to correspond with the fcc-bcc crystal phase transition, and to vary with the magnetic layer film thickness. Films with thick or Fe- rich magnetic layers had bcc structure and showed small MR ratios. The structural phase transition is thus important in discussions of the MR ratio of C o - F e / C u multilayers.

References

[1] J. lnoue, H. Itoh and S. Maekawa, J. Phys. Soc. Jpn. 61 (1992) 1149.

[2] Y. Saito, S. Hashimoto and K. Inomata, Appl. Phys. Lett. 60 (1992) 2436.

[3] S.S.P. Parkin, R. Bhadra and K.P. Roche, Phys. Rev. Lett. 66 (1991) 2152.

[4] S.S.P. Parkin, Z.G. Li and David J. Smith, Appl. Phys. Lett. 58 (1991) 2710.

[5] R.J. Highmore, W.C. Shih, R.E. Somekh and J.E. Evetts, J. Magn. Magn. Mater. 116 (1992) 249.