New BCT Iron Phase In (100) Felr Superlattices
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NEW BCT IRON PHASE IN (100) FeIr SUPERLATTICES S. ANDRIEU*, J. HUBSCH**, M. PIECUCH*, L. HENNET*, H. FISCHER* * Laboratoire mixte CNRS-St GOBAIN, 54704, Pont-A-Mousson France ** Laboratoire de Cristallographie, Univ. Nancy I, 54506, Vandoeuvre France ABSTRACT: In this paper, the synthesis of a new structure of iron in (100) FeIr superlattices grown by Molecular Beam Epitaxy is reported. Two-dimensional growth up to 4 planes at 400K occurs, as shown by RHEED oscillations. Electron diffraction also gives evidence that the Fe lattice is pseudomorphic to the Ir one during the 2D-growth, and relaxes to the bcc lattice for thicker deposits. An X-Ray analysis shows that the Fe structure is a body centered tetragonal one with a c/a ratio close to 1.25. This phase is observed to be non-magnetic at room temperature, as a weak ferromagnetic behaviour is observed at low temperatures, except at small Ir thicknesses. This is the evidence of the existence of a low spin phase which seems to undergo a second order phase transition with the atomic volume of iron. INTRODUCTION: The heteroepitaxy of metals is now usually realised using the Molecular Beam Epitaxy (MBE) technique. The synthesis of 3d metals in new crystallographic structures is thus possible. This opens a new field: the experimental study of the correlation between the structure and the magnetic behavior. Indeed, a comparison of the structures between 3d (magnetic) and 4d or 5d (non-magnetic) metals indicates that the magnetic behavior is probably responsible for the anomalous structures in the 3d series. Between the systems which have been studied up to now, the epitaxy of 3d metals on square lattices seems to be particularly interesting since the growth of tetragonal stuctures is possible. Indeed, in (100) Co/Cu I11, Cu/Pd 121 and Mn/Pd 131, the mismatch between the interatomic distances leads to body centered tetragonal (bct) structures with c/a ratio different from I (bcc) and '/2 (fcc). In the case of Fe, three systems were studied ((1(X)) FeCu 141, FePd 151 and FeNi 161) but a largely tetragonized structure was only observed for the FeNi one. The magnetic properties are thus difficult to carry out since Ni is also magnetic. The main goal of this study is to synthesized Fe in a bct phase and to determine its magnetic properties. We want to travel along the Bain path in order to see if there exists some magnetic transition versus the iron volume and (or) the c/a ratio. The choice of Ir as a substrate is motivated by the fact that its distance between first neighbours in the (100) plane (2.715A) is located between those of bcc (2.866A) and fcc (around 2.5A at RT) Fe. We thus expect to be able to vary the volume and c/a ratio and to record the existence and the nature of the magnetic transition. EXPERIMENTAL RESULTS: The epitaxies are perfornied in a MBE chamber with vacuum in the range of 10-10 torr. An Ir buffer layer of 200A is firstly realised at 500'C on (100) MgO substrates. Ir is evaporated by an e-gun at 0.5 A/s, and Fe using a Knudsen cell heated tIp to 1350
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