Orientational Dependence of Oscillatory Exchange Coupling
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ORIENTATIONAL DEPENDENCE OF OSCILLATORY EXCHANGE COUPLING M.T. JOHNSON (1), P.J.H. BLOEMEN (2), R. COEHOORN (1), J.J. de VRIES (2), N.W.E. McGEE 0), R. JUNGBLUT 0), A. REINDERS (1) AND J. aan de STEGGE (1) (1) Philips Research, P.O. Box 80000, 5600 JA Eindhoven, The Netherlands. (2) Physics Department, Eindhoven University of Technology, 5600 MB Eindhoven,
The Netherlands. ABSTRACT The role of the interlayer Fermi surface in driving oscillatory exchange coupling of ferromagnetic layers has been critically tested in a series of MBE-grown coherent epitaxial sandwiches prepared on single-crystal substrates. The wedge-shaped Cu (or Cu-based) interlayers have been modified in a number of different ways, achieving orientational, compositional and structural alteration of the Cu Fermi surface. Observed phenomena include multiperiodic coupling oscillations in both the ferromagnetically and antiferromagnetically coupled regions of (100)-oriented Co/Cu/Co samples, extended oscillatory periods across CuNi alloy interlayers, and extremely short-period oscillations in a bcc (100) Fe/Cu/Fe system. These phenomena are straightforwardly explained in terms of calculated Fermi surface variations. INTRODUCTION The discovery that the exchange coupling across non-magnetic interlayers in a wide variety of magnetic multilayers can oscillate between antiferromagnetic (AF) and ferromagnetic (F) as a function of the interlayer thickness [1-4] has presented a considerable challenge to solid state physicists. The long period A (8-22 A) of these oscillations was quickly ascribed to the aliasing effect, whereby rapid RKKY oscillations with period kF are only sampled at discrete thicknesses of the interlayer (spacing d) resulting in A = 27r/12kF--nlI [5]. This argument subsequently led to a prediction of orientation-dependent oscillationperiods in the coupling, even in the case of a nearly-free-electron metallic interlayer [6]. Further refinement by Bruno and Chappert [7] emphasized the role of the Fermi surface extremal points of which, connected by vectors q parallel to the growth-direction, generate oscillation-periods equal to 27r/Iql. In this light, a multiplicity of oscillation-periods may be encountered, the number and values of which are predicted to depend upon the growth- direction of the samples. Oscillatory exchange coupling across Cu interlayers has attracted much attention from both the fundamental and application viewpoint. On the one hand, the relatively simple nature of the Cu Fermi surface makes this a suitable candidate for the verification of exchange coupling theories; on the other hand, the discovery of sizeable antiferromagnetic (AF) coupling and giant magnetoresistance (MR) effects in sputtered (111 )-textured Co/Cu [1,2] and Fe/Cu [8] multilayers is of great interest to theorists and application engineers alike. In this paper, we test the validity of the Fermi surface approach. Through the observation of oscillatory coupling in a series of samples, we have probed several aspects of the Fermi surface of Cu (or Cu-based)
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