Magnetic Anisotropy in Epitaxial Ni/Cu (100) Thin Films
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MAGNETIC ANISOTROPY IN EPITAXIAL Ni/Cu (100) THIN FILMS
G. BOCHI, C. A. BALLENTINE, H. E. INGLEFIELD, S. S. BOGOMOLOV, C. V. THOMPSON, and R. C. O'HANDLEY Massachusetts Institute of Technology, Cambridge MA 02139
ABSTRACT Epitaxial Ni/Cu (001) films grown on Si (001) by Molecular Beam Epitaxy were studied in-situ using the Surface Magneto-optic Kerr Effect (SMOKE) and ex-situ with a Vibrating Sample Magnetometer (VSM). Perpendicular magnetization is observed for Ni thicknesses 15 A < h < 60 A and fully in-plane magnetization for h > 70 A when the films are characterized in-situ. The reversal in magnetic anisotropy observed insitu at 60 A shifts to 125 A when the films are exposed to air. 100 A Ni films deposited on Cul-xNix alloy substrates also show a reversal in magnetic anisotropy as x is changed. These results suggest that changes in magnetic anisotropy correlate with misfit strain accommodation. INTRODUCTION Since the theoretical predictions of Gay and Richter [1], considerable experimental effort has been focused on the issue of perpendicular magnetic anisotropy in ultrathin ferromagnetic films grown epitaxially on non-magnetic substrates. The systems that have been investigated most extensively are Fe/Ag (100) [2-5] and Fe/Cu (100) [6-10]. For Fe/Ag (100) at 100 K, the magnetization switches from perpendicular to in-plane above 2 monolayers (ML) of Fe [5]. In the case of Fe/Cu (100) the switching behavior has been observed even at T=300 K where the magnetization goes from perpendicular to in-plane at a thickness of 6 ML [7]. The change in magnetic anisotropy in the Ni/Cu (100) system has received less attention than in the Fe/Cu and Fe/Ag systems. SMOKE measurements [11] at T=100 K indicate that the easy axis of magnetization lies in the plane of the film for Ni thicknesses up to 3.1 ML and a preference for perpendicular magnetization is observed above 4.7 ML. An interesting reversal in magnetic anisotropy has also been observed in epitaxial Cu/Ni/Cu (100) sandwiches (Cu thickness=1000 A) deposited on Si (100) and characterized ex-situ by VSM [12]. The magnetization was found to lie in plane for Ni thicknesses h>500 A and out-of-plane for h=50 A with a transition thickness h=100 A. In the present paper, we report on the behavior of magnetic anisotropy in epitaxial Ni/Cu/Si (100) at T=300 K characterized under UHV conditions by SMOKE and in air by VSM. The Ni film thicknesses extend from 15 A to 200 A. We also present preliminary results showing the correlation between magnetic anisotropy changes and misfit strain in the Mat. Res. Soc. Symp. Proc. Vol. 313. ©1993 Materials Research Society
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magnetic film. With lattice parameters of 3.52 A and 3.61 A for Ni and Cu, respectively, the misfit between the film and the substrate is T! =2.5 %. The misfit can be reduced to 1.5 % by depositing Ni on Cul-xNix/Cu alloy substrates, where x is varied between 0 and 0.4 (above x=0.6 bulk Ni-Cu alloys exhibit a ferromagnetic order). Even though the importance of strain has already been pointed out by Chappert and Bru
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