Ni-multilayers

Ni-multilayers

Journal of Magnetism and Magnetic Materials 104-107 (1992) 1831-1832 North-Holland IHI Perpendicular magnetic anisotropy of Pd/Coand Pd/Ni-multilaye...

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Journal of Magnetism and Magnetic Materials 104-107 (1992) 1831-1832 North-Holland

IHI

Perpendicular magnetic anisotropy of Pd/Coand Pd/Ni-multilayers H. Takahashi, S. Fukatsu, S. Tsunashima and S. Uchiyama Department of Electronics, Nagoya Univ., 464-01 Nagoya, Japan

Pd/Co-, Pd/PdCo- and Fd/Ni-multilayers (MLs)with (111) and (100)orientations have been prepared by the RF sputtering method. It was found that the perpendicular magnetic anisotropy of these MLs strongly depends on the crystal orientation. In both Pd/Co- and Pd/PdCo-(100)MLs, perpendicular magnetic anisotropy is induced only when magnetic layers are very thin. In Pd/Ni-MLs, perpendicular magnetic anisotropy is larger in (100)MLs than in (IlI)MLs. The mechanism of the anisotropy is discussed from the viewpoint of magnetoelastic surface anisotropy. It is known that many kinds of noble metal/Co-MLs, such as Pd/Co, P t / C o and A u / C o , have large perpendicular magnetic anisotropy [1-3], which depends on the crystal orientation [4,5]. This magnetic anisotropy seems to originate from at least two sources, the magnetocrystalline surface anisotropy a n d / o r the magnetoelastic surface anisotropy [6]. In this paper, we investigate the relation between the perpendicular magnetic anisotropy and the crystal orientation in Pd/Co-, P d / P d C o - and P d / N i - M L s considering the contribution of the magnetoelastic surface anisotropy. Films were prcparcd on water-cooled glass and MgO(100)-substratcs from two sel~arate targets by RF sputtering. For (100)MLs, 500 A-thick Ag was deposited as an underlayer. The pressure of Ar during sputtering was 10 mTorr. The deposition rate was about 3-7/~,/s for Pd and 3 ,~/s for Co, PdCo and Ni. Total thickness of MLs was about 500 .A, for P d / C o and Pd/PdCo, and 2000 ,~, for Pd/Ni. From the X-ray diffraction patterns, it was found that M k s "~n glass have a (i 11~ orientation and MLs on MgO have a (100) orientation. The half-width of rocking curves was about 1.5 ° in (100) MLs and 8 ° in (111) MLs. Fig. 1 shows the effective perpendicular magnetic anisotropy K c° eft (per Co-layer) multiplied by Co-layer thickness tco of P d / C o - M L s as a function of tco. In (100)MLs, tco X Ke,, is enhanced deviatingofrOm the straight line when tco is smaller than 10 A. This is quite similar to the result in ref. [4]. In the case of Pd/Co-MLs, the stress-induced anisotropy due to the lattice misfit between Pd- and PdCo-aiioy formed at the interface seems to be one of the sources of the perpendicular magnetic anisotropy [7]. Here we point out that the peculiar behavior of (100)MLs can be explained by considering the anisotropic magnetostriction constants of PdCo-alloys, which depend on the composition as shown in fig. 2 [8]: A ~ is negative in the entire composition range, while AHI0 is negative only in the Pd-rich region. It can be assumed that extremely thin Co layers have magnetostriction constants similar to Pd-rich alloys, namely

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tco (A) Fig. 1. Effective perpendicular magnetic anisotropy K c('' f l (per Co-layer) multiplied by Co-layer thickness t(.o in Pd/Co-MLs with (111) and (100) orientation as a function of tco. both A 111 and A,~0 are negative, while a slightly thicker Co-layer might have those of Co-rich alloy, A ~ is negative but AlOO is positive. Thus we can expect positive magnetoelastic anisotropy in ( l l l ) M L s as well as in (100)MLs with an extremely thin Co-layer, but negative anisotropy in (100)MLs with rather thick Co-layers. In order to confirm the effect of PdCo-alloying at the interface, we prepared MLs consisting of Pd- and PdCo-alloy layers with (111) and (100) orientations. Fig. 3 shows the effective perpendicular magnetic anisotropy /d'POCo (per PdCo-layer) in Pd/PdCo-MLs "~'cff as a function of /PdCo" in (ilI)MLs, Ke, is larger t~han in (100)MLs. This result might be related to the difference in magnetostriction constants. In Pd/PdCo-MLs, A~t~ of PdCo-layers is large and negative, while AH,~ is very small and positive as shown in fig. 2. The magnetoelastic anisotropy of the PdCo-alloy layer due to the lattice misfit is expected to be large in (lll)MLs and small in (100)MLs. When PdCo-layers are made thinner, Keff of (100)MLs become very large and comparable to the

0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

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Fig. 2. Compositional dependence of magnetostriction constant in PdCo- and PdNi-alloys at 0 K (extrapolation) [8]. value of (111)MLs. This behavior could be explained in a quite similar way to that of Pd/Co-MLs, considering the mixture of atoms at the interface. In P d / P d C o MLs, the alloy at the interface is supposed to be more Pd-rich than PdCo-sublayers and to have a negative and very large magnetostriction both for (111) and (100) (see fig. 2). Then magnetoelastic anisotropy due to the lattice misfit is expected to become large even in the (100) orientation. For further understanding of the magnetoelastic

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Fig. 4. Magnetization curves of (100)- and (lll)-oriented Pd/Ni (7 ,~,/4 2%)-Mhs with a field perpendicular (.L) and parallel ( / / ) to the film plane. surface anisotropy, we investigated the Pd/Ni-system. Fig. 4 shows magnetization curves of (100) and (111) Pd/Ni-MLs. ( l l l ) M L s exhibit very small perpendiculm anisotropy, around 1 x 10 s e r g / c m 3 or less [9], while (100)MLs exhibit large magnetic anisotropy up to 5 x 10 5 e r g / c m 3. This dependence of the anisotropy on the crystal orientation might be related to the fact that A100 is about two times larger than A~I~ in PdNialloys as shown in fig. 2 [8]. The authors are grateful to Mr. S. Adachi for EPMA measurements and to Prof. M. Matsui for the measurement using a VSM. The authors would like to thank Dr. K. Nakamura and Mr. M. Hasegawa for valuable discussion. This work was supported by the Grant-inAid for scientific research from the Ministry of Education, Science and Culture of Japan. References

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[1] P.F. Garcia, A.D. Meinhald and A. Suna, Appl. Phys. Lett. 47 (1985) 178. [2] P.F. Garcia, J. Appl. Phys. 63 (1988) 5066. [3] F.J.A. den Broeder, D. Kuiper, A.P. van de Mosselaer and W. Hoving, Phys. Rev. Lett. 60 (1988) 2769. [4] F.J.A. den Broeder, D. Kuiper, H.C. Donkersloot and W. Hoving, Appl. Phys. A 49 (1989) 507. [5] C.H. Lee, R.F.C. Farrow, C.L Lin and E.E. Marinero, Phys. Rev. B 42 (1990) 11384. [6] P. Bruno and J. Seiden, J. de Phys. C8 (1988) 1645. [7] S. Tsunashima, K. Nakamura and S. Uchiyama, IEEE Trans. magn. 25 (1990) 2724. [8] T. Tokunaga, M. Kohri, H. Kadomatsu and H. Fujiwara, J. Phys. Soc. Jpn. 50 (1981) 1411. [9] H. Takahashi, S. Fukatsu, S. Tsunashima and S. Uchiyama, J. Magn. Magn. Mater. 93 (1991) 469.