Introduction

There has been much interest recently in layered magnets[34,35] particularly those composed of a two-phase distribution of hard and soft magnetic materials. One potential application of such systems is in permanent magnets where the hard magnetic layers provide high anisotropy and coercive fields, whilst the soft magnetic layers can enhance the magnetic field and reduce the rare earth content. These systems also have potential as GMR readheads with a reported $\nicefrac{\Delta R}{R}$ of $12\%$ in an applied field of $230\ensuremath{\unskip\,\mathrm{kOe}}$.[36]

A series of DyFe $_{2}(x\ensuremath{\unskip\,\mathrm{\AA{}}}$)/YFe $_{2}(y\ensuremath{\unskip\,\mathrm{\AA{}}}$) multilayers were investigated with varying DyFe$_2$ and YFe$_2$ layer thicknesses of $50\ensuremath{\unskip\,\mathrm{\AA{}}}$/ $50\ensuremath{\unskip\,\mathrm{\AA{}}}$, $40\ensuremath{\unskip\,\mathrm{\AA{}}}$/ $20\ensuremath{\unskip\,\mathrm{\AA{}}}$, $20\ensuremath{\unskip\,\mathrm{\AA{}}}$/ $40\ensuremath{\unskip\,\mathrm{\AA{}}}$ and $20\ensuremath{\unskip\,\mathrm{\AA{}}}$/ $20\ensuremath{\unskip\,\mathrm{\AA{}}}$. They have the same sample structure as for all the previous samples, shown in Figure 6.2.

In these multilayers the soft magnetic YFe$_2$ layers will be exchange-coupled to the DyFe$_2$ layers through the strong ( $\sim 6000\ensuremath{\unskip\,\mathrm{kOe}}$) iron-iron exchange coupling. This is illustrated in Figure 6.7. The strong single-ion anisotropy coupled antiferromagnetically through the dysprosium moments to the iron moments, as seen in the DyFe$_2$ thin film results in Section 6.2.2, will be transferred to the YFe$_2$ layers by the strong iron-iron exchange coupling. Hence we expect the system as a whole to be dominated by the dysprosium moments.

Figure 6.7: Exchange coupling between iron sublattices in DyFe$_2$/YFe$_2$ multilayers.

CEMS spectra were recorded at room temperature under in plane applied magnetic fields of $0\ensuremath{\unskip\,\mathrm{kOe}}$ and $2.5\ensuremath{\unskip\,\mathrm{kOe}}$. The applied field of $2.5\ensuremath{\unskip\,\mathrm{kOe}}$ will not be sufficient to induce exchange springs in the YFe$_2$ layers where the bending field, $B_{B}$, is of the order of $70\ensuremath{\unskip\,\mathrm{kOe}}$,[36] but there will be an exchange spring induced in the $800\ensuremath{\unskip\,\mathrm{\AA{}}}$ YFe$_2$ seed layer as discussed in Section 6.2.2.