Magnetization switching and spin-dependent transport in REFe2 exchange spring multilayers

Abstract

The properties of rare earth-transition metal Laves phase films and multilayers, grown by molecular beam epitaxy, form the subject of this thesis, hi particular, ErFe2/YFe2 superlattices, which order magnetically out of plane, are discussed in some detail. When a magnetic field is applied out of plane, magnetic exchange springs form in the soft YFe2 layers. These exchange springs result in broad reversible regions in the hysteresis curve, which can lead to negative coercivity in multilayers given sufficiently thick soft layers. However, there is a cross-over temperature, TQO, above which the coercivity of these multilayers becomes positive, with additional transitions at high fields. The spin configurations occurring in ErFe2/YFe2 multilayers during magnetic reversal have been studied using bulk magnetometry and micromagnetic modelling. At high fields and high temperatures, the magnetization of the hard layers points in-plane, at right angles to the applied magnetic field. In addition, more complex ErFe2/YFe2/DyFe2/YFe2 superlattices have also been created, where there is competition between the anisotropies of the ErFe2 and DyFe2 layers. The coupling between the different hard Er/Dy layers is easily changed by varying the soft YFe2 layer thickness. The interactions between the competing Er and Dy anisotropies are mediated both by soft layer thickness and temperature. A rich phase diagram of switching processes has been identified. Finally, the galvanomagnetic properties of ErFe2/YFe2 multilayers have been investigated using magneto-transport measurements. Both exchange spring giant magnetoresistance and the anomalous Hall effect are described in detail. The Hall effect data is complemented with results both from bulk magnetometry and micromagnetic modelling. It is shown that the anomalous Hall effect couples mainly to the Fe magnetisation in ErFe2/YFe2 multilayers. Nevertheless, very specific information about the magnetisation processes can be obtained from Hall effect measurements.

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