Studies of Exchange Coupling in Fe(001) Whisker/Cr/Fe Structures Using bls and Rheed Techniques.
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STUDIES OF EXCHANGE COUPLING IN Fe(001) WHISKER/Cr/Fe STRUCTURES USING BLS AND RHEED TECHNIQUES. B. Heinrich, M. From, J.F. Cochran, L. X. Liao, Z. Celinski, C.M. Schneider+ and K.Myrtle, Physics Department, Simon Fraser University, BURNABY, Canada, V5A1 S6.
ABSTRACT The conditions for an almost perfect growth of smooth Cr(00 1) films on an iron whisker substrate have been investigated by means of reflection high energy electron diffraction (RHEED). The exchange interaction between 20 monolayer thick Fe(001) films separated from a bulk whisker Fe(001) substrate by a variable number of Cr(001) monolayers (ML) has been investigated by means of Brillouin light scattering experiments (BLS). These experiments show unambiguously that the exchange coupling strength between the iron film and the iron whisker can be described by a short wavelength oscillatory term superposed on a slowly varying antiferromagnetic background. The BLS data enabled one to separate the bilinear and the biquadratic contributions to the antiferromagnetic exchange coupling terms. Both the bilinear and the biquadratic coupling strengths exhibited a short period oscillatory dependence on the Cr interlayer thickness (-2 monolayers). Maxima in the bilinear antiferromagnetic coupling strength occur for an odd number of Cr monolayers. This observation is not in agreement with first principles calculations. The first phase inversion has been found to occur between 4 and 5 ML of Cr.
INTRODUCTION Fe/Cr/Fe(001) structures have played an immense role in the study of the exchange coupling between ferromagnetic films separated by a non-ferromagnetic interlayer. A new era in the study of the exchange coupling was begun with the discovery of short wavelength oscillations (X=2. 11ML) in Fe/Cr/Fe(001) systems [ 1,2]. Smooth interfaces are a necessary prerequisite for the observation of short wavelength oscillations, and Fe[001] whiskers have provided substrates which allow one to grow very smooth interfaces. By choosing the right growth conditions one can observe short wavelength oscillations up to thicknesses as large as 80 ML of Cr [3]. This result is certainly not universal even for samples grown on Fe[001] whisker templates. In their quantitative measurements Purcell et al [2], and Griinberg et al [4], observed only a few oscillations. The measurements by Purcell et al [2] and Grunberg et al [4] indicated that the maxima of the antiferromagnetic coupling (AFM) occured at an even number of Cr atomic layers (ML). However, in the thickness range studied by Purcell et al, and by Griinberg et al, [2,4], the NIST group showed [3], and our BLS studies [5] on Fe/whisker/Cr/Fe samples showed, that the maxima of the AFM coupling appear at an odd number of ML. Recent results by Griinberg et al [6] on improved interfaces produced a larger number of oscillations (up to 26 ML) than their earlier studies on Fe whiskers [4]. The maxima of the AFM coupling reported in this latest work were observed to occur somewhere between an even and an odd number of Cr ML. The phase of th
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