Multilayer films formed from transition metals and semiconductors have long been studied because of their unusual superconducting properties and because of possible application as x-ray optical elements. Many unusual phenomena have been produced, ranging from the observation of dimensional crossover in weakly coupled superconducting Nb layers in Nb/Ge multilayers to the occurrence of bcc Ge in short-period Mo/Ge multilayers. Unusual magnetic properties have recently been observed in Fe/Si multilayers by workers at ETH and Argonne. A large antiferromagnetic (AF) interlayer coupling in these multilayers manifests itself in hysteresis loops as a high saturation field and a low remanent magnetization. Similar magnetization curves are associated with large interlayer coupling in metal/metal multilayers like Fe/Cr and Co/Cu.[6,7] Much consideration has been given to whether the coupling in the Fe/Si system has the same origin as in the metal/metal multilayers.[8,9] Therefore the question of whether the spacer layer in the Fe/Si multilayers is a metal or semiconductor is of particular interest.
Previous work on Nb/Si, Co/Si, Ni/Si, and Mo/Si multilayers have shown that there is a strong tendency towards compound formation at the metal/silicon interface. In general these multilayers consist of polycrystalline metal layers separated by an amorphous silicon layer which is bounded on either side by a layer of intermixed material. The intermixed silicide layers in these films were amorphous unless they were annealed at several hundred °C.[13,14] These previously studied multilayers were therefore likely in their as-grown state to have metal/semiconductor character because of the presence of the amorphous silicon layer.
In order to investigate the character of the spacer layer in the Fe/Si multilayer system, we have grown a large number of films with different substrate temperatures, substrate types, and layer thicknesses. When the Si spacer layer thickness is greater than about 20Å, we find that the metal layers are crystalline but that the spacer layers are amorphous, similar to the situation in other transition metal/silicon systems. When the Si spacer layer thickness is less than about 20Å thick, the iron silicide spacer layer forms a crystalline silicide with either the B2 or DO3 structure. The B2 structure consists of two interpenetrating simple cubic sublattices and is identical to the CsCl structure for a 1:1 ratio of Fe and Si, while the DO3 structure is an fcc lattice with two inequivalent Fe sites. Extensive growth experiments, described below, suggest that crystallinity of the spacer layer is crucial for occurrence of the antiferromagnetic interlayer coupling, in keeping with previous suggestions. Since both the B2 and DO3 phases are metallic,[15,16] the fact that crystallinity is required for antiferromagnetic coupling suggests that the coupling in Fe/Si has a common origin with that observed in metal/metal multilayers.