bct to bcc iron in (001) FeIr superlattices: Relation between structure and magnetism

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Abstract
The analysis of the growth, structure, and magnetic properties of iron in (001) FeIr superlattices grown by molecular beam epitaxy is presented. The growth was analyzed by reflection high-energy electron diffraction (RHEED), and the morphology of the superlattices observed by transmission electron microscopy and low-angle x-ray diffraction. The structure was determined by RHEED, θ-2θ and grazing x-ray diffraction, extended x-ray-absorption fine structure, diffraction anomalous fine structure, and anomalous diffraction. The magnetic properties were determined by hysteresis loop and magnetization curve measurements. It is shown that up to five atomic Fe planes on Ir, the growth of Fe on Ir, and Ir on Fe is two-dimensional and the obtained superlattices are uniformly strained. This leads to extremely flat superlattices. In this case, Fe is in a bct structure with a c/a ratio near 1.25. It is thus shown that this result can be well explained by the elastic theory. A transition from a nonmagnetic to a low-spin ferromagnetic state was also observed. The variation of the average moment is related to the atomic volume variation and is consistent with theoretical predictions. For larger Fe thicknesses, flat superlattices are also produced if the iridium thickness does not exceed three atomic planes. Again, the elastic theory well describes the results. However, no clear relation between magnetism and structure is found. For larger Fe and Ir thicknesses, the quality of the superlattices was damaged, because of the disappearance of two-dimensional growth. Some roughness thus appears, which can be explained by the occurrence of three-dimensional growth or of the Grinfel’d instability.