Magnetism of Bct Iron Grown In (001) FeIr Superlattices
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ract In this paper, we show that iron can be grown by MBE in a body centered tetragonal structurein (001) FeIr superlattices.The growth, structure and morphology of these superlattices are briefly resumed. A variationof the BCT Fe magnetic moment depending on the Jr thickness is observed.This variationis demonstratedto come from a variation of the BCT Fe atomic volume, due to the competition of the Fe and Ir stresses. A magnetic transitionfrom a non-magnetic to a low spinferromagnetic state depending on the atomic volume is thus observed.
The heteroepitaxy of metals with unusual cristallographic structure has became possible using the Molecular Beam Epitaxy technique (MBE). This is of great interrest for the understanding of the relationship between the cristallographic structure and magnetic properties of metals of the first transition series. Indeed, if striking structural similarities can be noticed between the metals belonging to the second and first series, it does not extend to the first transition series. These discreepancies are due to the magnetic contribution to the total energy of the system. For instance, ruthenium and osmium exhibit a non-magnetic HCP structure, but a ferromagnetic BCC structure is observed for iron. This paper reports a work done to improve our understanding of the interrelation between the crystalline structure and magnetism of iron. Several bands calculation methods have been used in order to study the magnetic behavior of BCC and FCC iron phases 1-5. As the BCC iron phase was always found to be ferromagnetic whatever the value of the lattice parameter is, the situation is more complex for the FCC phase : by taking into account an isotropic variation of the atomic volume, several authors found that FCC iron can be non-magnetic (NM), low-spin ferromagnetic (LS), high spin ferromagnetic (HS), or antiferromagnetic (AF). Peng and Hansen 5 also predicted these magnetic behaviour for a tetragonalisation of the FCC phase.
253 Mat. Res. Soc. Symp. Proc. Vol. 384 01995 Materials Research Society
In order to check these theoretical predictions, iron with unusual structure must be prepared. For this purpose, the MBE technique is used here. It is wellknown that BCC iron can be grown on (001) Ag 6 ((100) BCC Fe 1/ (110) FCC Ag) and FCC iron on (001) Cu 7 ((100) FCC Fe / (100) FCC Cu), because of the very small mismatch in both cases. The main idea of this work is to grow iron on a non-magnetic metal with a first neighbour distance in the (001) plane intermediate between those of Ag and Cu, i.e. intermediate between the (110) FCC Fe distance (2.54A at room temperature) and the (100) BCC Fe distance (2.8664A). FCC Ir is a good candidate since its (110) distance is equal to 2.715A. A strained BCC or FCC Fe structure was thus expected to grow on (001) Ir. The growth and structure of iron on this (100) Ir surface was ever reported in several papers 8,9. The main results are briefly summarized. Two-dimensional (2D) growth was observed during the epitaxy of Fe on (001) Ir up to a minimum of 4 atomic
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