Crystal Structure Dependence of Antiferromagnetic Coupling in FE/SI Multilayers
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ABSTRACT Recent reports of temperature dependent antiferromagnetic coupling in Fe/Si multilayers have motivated the generalization of models describing magnetic coupling in metal/metal multilayers to metal/insulator and metal/semiconductor layered systems. Interesting dependence of the magnetic properties on layer thickness and temperature are predicted. We report measurements that show the antiferromagnetic (AF) coupling observed in Fe/Si multilayers is strongly dependent on the crystalline coherence of the silicide interlayer. Electron diffraction images show the silicide interlayer has a CsCl structure. It is not clear at this time whether the interlayer is a poor metallic conductor or a semiconductor so the relevance of generalized coupling theories is unclear.
INTRODUCTION The magnetic coupling of adjacent ferromagnetic layers separated by a broad range of non-magnetic metal spacer layers oscillate from anti-ferromagnetic to ferromagnetic as the spacer layer thickness increases 1 . The variation of the coupling can result in oscillations in easily measured quantities such as the saturation field and the magneto-resistance as a function of interlayer thickness. Most features of the oscillating exchange coupling have been successfully explained by applying RKKY type interactions to the layered geometry and using the Fermi surface characteristics of the interlayer metal 2 . Recent experimental observations of anti-ferromagnetic coupling in Fe/Si multilayers3'4 have motivated generalization of models of interlayer exchange to include systems without well defined fermi surfaces such as semiconductors and insulators 5 ' 67. Among the most pronounced predicted differences between metallic and nonmetallic interlayer systems is the strong temperature dependence of the coupling in multilayers with a non-metallic interlayer due to the thermally activated nature of the carriers which carry the exchange. In this paper we present data describing structural and magnetic characteristics of Fe/Si multilayers deposited using ion beam sputtering and discuss their significance to the theories of Bruno6 and Zhang 5 . Consistent with previous studies we find that increasing the Si interlayer thickness from 14A to 20A, while keeping the Fe thickness fixed at about 30A, has a dramatic effect on the magnetic properties and the morphology of the multilayer. We find that for Si layers around 14A thick, the multilayer maintains crystalline coherence in the growth direction through more than one bilayer period and magnetically the Fe layers are anti-ferroma.gnetically coupled resulting in a high saturation field. For slightly thicker Si layers (around 20A) the crystalline coherence in the growth direction is only as thick as a single bilayer, and the saturation field is small consistent with either ferromagnetically coupled or uncoupled Fe layers. Further, for Fe layers sufficiently thin, crystalline coherence is not achieved in the multilayer. We find that even for 14A thick Si interlayers, disordered ferromagnetic Fe layers are either
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