The Magnetic Properties of Ultra-Thin Iron-Silver Superlattices Fabricated by Molecular Beam Epitaxy.

The magnetic properties of high quality Fe(110)/Ag(111) heterostructures and superlattices have been examined by Mossbauer spectroscopy. These samples were fabricated by the recently developed technique of molecular beam epitaxy (MBE) which had the capability of characterizing the films in situ with an electron diffraction technique as they were being grown. This characterization method was used to determine the optimal growth conditions and it also confirmed that flat, crystalline and singly-oriented layers were produced. The quality of these MBE samples was significantly higher than Fe/Ag films previously grown by other methods from which rough, multiply-oriented, noncontinuous layers resulted. It was shown that alternating layers of Fe and Ag could be deposited in order to form a superlattice without a significant deterioration of the individual layers. Three different superlattice series were fabricated with the Fe component 8, 5 and 2 monolayers thick. Within each series the thickness of the Ag component was varied. The Mossbauer spectra of the 8 and 5 monolayer Fe superlattices at temperatures between 4.2K and 300K were consistent with the predictions of spin-wave theory if the effects of surface spin-waves are included. From this analysis it was concluded that there is a magnetic coupling between neighboring Fe components separated by an Ag layer. The superlattices with the Fe component 2 monolayers thick were shown to consist of small islands rather than continuous layers. The behavior of these islands dominated the magnetic excitations of the samples, resulting in a magnetization with a linear temperature dependence rather than the T^{3/2} behavior predicted by spin-wave theory. In the past, this linear temperature dependence has often been attributed to two -dimensional magnetic effects. This work demonstrates conclusively that many of those results were simply due to poor quality samples that were not flat and continuous but rather were composed of small Fe islands. The development of MBE has for the first time allowed those magnetic characteristics inherent in two -dimensional structures to be separated from the magnetic behavior that results from poor film quality. This study represents only a first step in this most recent phase of research on two-dimensional ferromagnetism.