Abstract

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$_2$, HoFe$_2$ and YFe$_2$ cubic Laves Phase thin films, DyFe$_2$/Dy and DyFe$_2$/YFe$_2$ 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 $\left[\bar{2}41\right]$ and $\left[\bar{3}51\right]$ directions but cannot be fully determined. The spin moments lie out of plane in all samples by approximately $22^{\circ}$, indicating a significant magneto-elastic anisotropy. $2.5\ensuremath{\unskip\,\mathrm{kG}}$ inplane applied field measurements indicate a much larger magnitude of magnetocrystalline anisotropy in the DyFe$_2$ system than in the YFe$_2$ system. In the DyFe$_2$/YFe$_2$ 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$_2$ layers by the strong iron-iron exchange coupling. The DyFe$_2$(50Å)/YFe$_2$(50Å) sample shows average hyperfine fields consistent with the DyFe$_2$ and YFe$_2$ thin film results, whilst samples with thinner layers show an enhanced hyperfine field of up to $17\%$. The DyFe$_2$/Dy multilayers have identical zero field properties to the DyFe$_2$ thin film system down to a DyFe$_2$ thickness of $50\ensuremath{\unskip\,\mathrm{\AA{}}}$. 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 $T_{N}$ ranging between $125\ensuremath{\unskip\,\mathrm{K}}$ and $190\ensuremath{\unskip\,\mathrm{K}}$, dependent upon both cerium and iron layer thicknesses. The exchange coupling constant, $J(z)$, has been calculated for antiferromagnetically coupled samples and shows an oscillatory $z$ 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 $3\%$, and a doublet from a paramagnetic UFe$_2$ layer. The relative thicknesses of these layers scale nonlinearly with the thickness of the deposited iron layer below $60\ensuremath{\unskip\,\mathrm{\AA{}}}$. Above this thickness the disordered iron and UFe$_2$ layers reach maximum thicknesses of $20\ensuremath{\unskip\,\mathrm{\AA{}}}$ and $18\ensuremath{\unskip\,\mathrm{\AA{}}}$ 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 Al$_2$O$_3$(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.