A study of the magnetic properties of thin films, multilayers and oxides has been performed using Mössbauer spectroscopy and SQUID magnetometry. The systems studied are DyFe, HoFe and YFe cubic Laves Phase thin films, DyFe/Dy and DyFe/YFe multilayers; Ce/Fe and U/Fe multilayers; and iron oxide powders and thin films.
CEMS results at room temperature show a low symmetry magnetic easy axis for all of the Laves Phase samples studied. Analysis of the dipolar and contact hyperfine fields show that this axis is close to the and directions but cannot be fully determined. The spin moments lie out of plane in all samples by approximately , indicating a significant magneto-elastic anisotropy. inplane applied field measurements indicate a much larger magnitude of magnetocrystalline anisotropy in the DyFe system than in the YFe system. In the DyFe/YFe multilayer samples the anisotropy is dominated by the dysprosium single-ion anisotropy, propagated through the antiferromagnetic coupling with the iron moments and then through the YFe layers by the strong iron-iron exchange coupling. The DyFe(50Å)/YFe(50Å) sample shows average hyperfine fields consistent with the DyFe and YFe thin film results, whilst samples with thinner layers show an enhanced hyperfine field of up to . The DyFe/Dy multilayers have identical zero field properties to the DyFe thin film system down to a DyFe thickness of . In all samples studied under applied field the hyperfine fields were reduced from their zero field values.
SQUID magnetometry results from the Ce/Fe multilayers show that most of the samples exhibit antiferromagnetic coupling, with a ranging between and , dependent upon both cerium and iron layer thicknesses. The exchange coupling constant, , has been calculated for antiferromagnetically coupled samples and shows an oscillatory dependence. CEMS results from the U/Fe multilayers shows that each iron layer is composed of BCC iron, a poorly-crystalline iron layer with a reduced hyperfine field of up to , and a doublet from a paramagnetic UFe layer. The relative thicknesses of these layers scale nonlinearly with the thickness of the deposited iron layer below . Above this thickness the disordered iron and UFe layers reach maximum thicknesses of and respectively. Where the uranium layer has poor crystalline growth this is propagated into the iron layer and increases these thicknesses.
Room temperature Mössbauer spectroscopy results from a selection of printer toner powders were used to produce the ratio of magnetite to maghemite in the powders. CEMS results on magnetite thin films showed good crystal growth on a Pt(111) substrate, with some iron forming a non-magnetic layer diffused in the platinum. Magnetite deposited on AlO(0001) substrates showed good crystal growth when using an oxygen plasma source, but that from a normal sputtering source showed a distribution of hyperfine fields and a paramagnetic contribution from iron substituting for aluminium in the substrate.
Dr John Bland, 15/03/2003