Magnetic Phase Transition in the Csci Fesi Spacer in Fe/FeSi Multilayers
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J. Dekoster, A. Vantomme, S. Degroote, R. Moons and G. Langouche Katholieke Universiteit Leuven, Instituut voor Kern- en Stralingsfysica, Celestijnenlaan 200 D, B-3001 Leuven, Belgium
ABSTRACT We report the formation of the CsCI FeSi phase in Fe/FeSi multilayers with constant Fe thickness and varying FeSi layer thickness. The crystallographic and magnetic properties of this metastable FeSi phase are determined in thick (62 nm) epitaxial silicide layers with ion channeling, X-ray diffraction and Mossbauer spectroscopy measurements which clearly show that the CsCl phase becomes increasingly magnetic upon cooling down to 4.2 K. These results indicate that the transition from antiferromagnetic to ferromagnetic coupling observed at low temperatures in such multilayers is controlled by a magnetic phase transition in the FeSi spacer layer.
INTRODUCTION An increasing amount of experimental and theoretical work is performed on magnetic multilayers since the discovery of antiferromagnetic (AF) coupling of ferromagnetic layers between metallic spacer layers [1]. The coupling mechanism is generally believed to be associated with the polarization of the conduction electrons in the spacer layer via a Ruderman Kittel Kasuya Yosida (RKKY) interaction [2]. The situation for coupling through non metallic , insulating or semiconducting spacer layers is not at all clear. Recently a series of experiments indicated the
presence of AF coupling between Fe layers through semiconducting crystalline Fe silicide layers [31, amorphous silicon layers [4] and a mixture between both [5],[6]. The situation is confusing especially for the Fe silicide spacer materials where the spacer was interpreted as being either the stable 6-FeSi phase or a metastable CsCl FeSi phase in Fe/FeSi or Fe/Si multilayers respectively [3],[5]. Moreover for the coupling through the FeSi spacer the degree of AF coupling was strongly decreasing upon cooling and could be restored by photo-induction. These effects were attributed to charge carriers in the FeSi spacer layer, which when thermally or optically generated are capable of communicating spin information between the Fe layers [3]. The purpose of the present report is twofold, first we want to determine the structural and magnetic properties of a thick epitaxial metastable CsCl FeSi layer and secondly we will demonstrate that this phase can be formed in an FeSi/Fe multilayer. This silicide phase does not appear in the thermodynamic phase diagram of Fe and Si and can be stabilized up to large thickness onto both Si(l 11) and Fe(001) substrates [7] which suggests that it is a metastable phase. The observation of a magnetic phase transition at low temperatures in such a thick well defined metastable single crystal phase is unique and would yield valuable information for comparison with ab initio theoretical calculation on the magnetic and structural properties of crystal phases but would also have important consequences for the interpretation of the AF to ferromagnetic (FM) coupling transition observed in Fe/Si and F
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