BCT TO BCC IRON IN (001)FEIR SUPERLATTICES - RELATION BETWEEN STRUCTURE AND MAGNETISM

The analysis of the growth, structure, and magnetic properties of iron in (001) FeIr superlattices grown by molecular beam epitaxy is presen ted. The growth was analyzed by reflection high-energy electron diffra ction (RHEED), and the morphology of the superlattices observed by tra nsmission electron microscopy and low-angle x-ray diffraction. The str ucture was determined by RHEED, theta-2 theta and grazing x-ray diffra ction, extended x-ray-absorption fine structure, diffraction anomalous fine structure, and anomalous diffraction. The magnetic properties we re 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 bet structure with a c/a ratio near 1.25. It is thus shown that this result can be well explained by the elastic theo ry. A transition from a nonmagnetic to a low-spin ferromagnetic state was also observed. The variation of the average moment is related to t he atomic volume variation and is consistent with theoretical predicti ons. For larger Fe thicknesses, flat superlattices are also produced i f the iridium thickness does not exceed three atomic planes. Again, th e elastic theory well describes the results. However, no clear relatio n between magnetism and structure is found. For larger Fe and Ir thick nesses, the quality of the superlattices was damaged, because of the d isappearance of two-dimensional growth. Some roughness thus appears, w hich can be explained by the occurrence of three-dimensional growth or of the Grinfel'd instability.