Magnetic Interactions Within and among Ultrathin Epitaxial Iron Films

Two types of magnetic interactions have been investigated in this work: the coupling of ferromagnetic bcc Fe layers through nonmagnetic noble metal spacers, and the direct exchange between Fe atoms in a metastable fcc phase of Fe. The samples were all grown epitaxially by Molecular Beam Epitaxy (MBE), and magnetic measurements were performed using ^{57}Fe transmission Mossbauer spectroscopy, Vibrating Sample Magnetometry (VSM), and Superconducting Quantum Interference Device (SQUID) magnetometry. In the interlayer coupling experiments, a site specific Mossbauer spectroscopy technique was used to measure the spin wave spectrum at the interface between the Fe and the noble metal. As the effective exchange at the interface is modified by the indirect exchange interaction through the noble metal spacer with the adjacent Fe layer, the spin wave spectrum is modified in a very definite way. Using this technique, an oscillatory coupling with a period of 6 ML was detected through Ag(111) interlayers, in good agreement with theoretical predictions. Experiments have also been performed on Cu(111) interlayers, with preliminary results that also show evidence of oscillatory coupling. These results represent the first observation of oscillatory coupling and measurement of the period in Ag(111). In the fcc Fe work, the relation between structure and magnetic properties was investigated for the metastable fcc phase of Fe that can be stabilized on a Cu substrate. The substrate lattice parameter was varied by using CuAu alloy single crystals with variable composition. The alloy substrates were grown by MBE by co-deposition from two pure metal sources, and the composition could be varied by changing one or both of the deposition rates. X-ray diffraction provided lattice parameter data for the substrates. As the substrate lattice parameter was increased, a transition from antiferromagnetic to ferromagnetic Fe was observed, but the magnetic moments did not show the expected dependence on the lattice parameter. Rather, all the samples consisted of the same two metastable states (one AF and one FM), and the relative amount of Fe in the FM state increased at the expense of the AF state as the lattice parameter was increased. SQUID magnetometry was performed on these samples and showed a {~5} -fold increase in the saturation moment as the lattice parameter was increased. (Abstract shortened by UMI.).