Stabilization of Metastable Phases by Epitaxy

Abstract

The interplay between magnetism and crystalline structure is illustrated in the stable structure of the transition metals. While in the second and in the third series of transition metals ranging from yttrium to palladium and lutetium to platinum respectively, there is a regular succesion of the stable phases (HCP BCC HCP FCC), the existence of spin polarized electronic ground states perturbs dramatically this simple scheme in the first series. Manganese has, thus, a complicated cubic structure, iron is BCC and cobalt HCP in their low temperature states instead of being HCP, HCP and FCC respectively. It is, then, very attractive to obtain these first series transition metals in new crystalline structures to modify their magnetic properties and understand the interplay between crystalline and electronic properties. On the other hand, a simple band structure argument predicts the enhancement of magnetic moments with the augmentation of interatomic distances. It is, then, also interesting to obtain these metals with large interatomic distances. Recent progress in molecular beam epitaxy of metals offers new ways to realise such a program. The pseudomorphic epitaxy may constrain deposited metals in non equilibrium structures and with interatomic distances very different from those of the usual stables phases. Since 1988, we have demonstrated the possibility of growing several pseudomorphic iron and manganese planes on various single crystalline surfaces of ruthenium or iridium. Single crystal superlattices with quasi ideal interfaces have also been realized. This paper will give some examples of our recent work in this field with two main different basic starting points. In the two first parts, we will describe the structure and magnetic properties of iron grown on dense plane of ruthenium and iridium.