Correlations between the structural and magnetic properties of MBE-grown cobalt-based superlattices.

We describe the MBE (Molecular Beam Expitaxy) technique for growth of the first transition metal magnetic superlattices. The structural and magnetic properties of Co-Au and Co-Cu superlattices are characterized with various methods. The focus of this thesis is an attempt to reveal some of the relationship between them. The magnetic Co superlattices are grown epitaxially on GaAs substrates, as shown by in-situ RHEED (reflection high energy electron diffraction) studies. The in-plane crystallographic orientation of individual layers, which is preserved throughout the growth process, revealed with RHEED, TEM (transmission electron microscopy) and x-ray scattering measurements. The structural properties of Co superlattices are studied with cross-sectional and plan-view TEM imaging and diffraction techniques. The superlattice layers are found to be uniform with sharp interfaces across the entire film, with no evidence of interlayer mixing or disruption of the layers. Diffraction studies indicate that Co layers are grown in an hcp phase in Co-Au superlattices, while they are primarily stacked in the metastable fcc ordering in Co-Cu superlattices. Elastic strains exist in both systems, as determined by the x-ray scattering, and these are shown to be intimately related to the magnetic anisotropy in these epitaxial systems. SQUID and FMR (ferromagnetic resonance) measurements have been used to characterize the magnetic properties. The easy magnetization axis is found to change from in-plane to out-of-plane when the Co layer thickness reduces. Effective magnetic anisotropies are derived from both measurements. The cross-over thicknesses are 18A and 9A for Co-Au and Co-Cu superlattices, respectively. The low temperature perpendicular coercive field appears to oscillate as a function of the Au interlayer thickness in the Co-Au superlattices with constant Co layers. This may be the result of interlayer coupling interactions between individual Co layers. We suggest a simple calculation that includes the demagnetization energy, the magnetocrystalline, and magnetoelastic anisotropy energies, to account for the observed magnetic anisotropies in Co superlattices. The magnetic layer thickness dependence of the anisotropy is presented through the strains that have been measured by x-ray scattering. The calculation agrees quite well with experiments in the case of Co-Au superlattices; in particular, it can account for the saturation behavior of the effective anisotropy at smaller Co thickness. In the Co-Cu superlatice, the magnetocrystalline anisotropy constants have to be reduced by nearly 80% to obtain a satisfactory fit. This result is consistent with the growth of Co in a metastable fcc phase.