X-ray magnetic circular dichroism studies of III-Mn-V compounds and heterostructures

This thesis describes the characterisation of GaMnAs, related compounds and heterostructures. GaMnAs and other (III,Mn)V compounds have provided many interesting insights into fundamental physics, and are of considerable potential interest commercially in the field of spintronics. This study examines a set of samples grown by molecular beam epitaxy and characterised using several techniques: primarily this study makes use of the x-ray absorption techniques, x-ray magnetic circular dichroism(XMCD) and x-ray absorption spectroscopy (XAS). In addition, x-ray diffraction (XRD), transport measurements and super conducting quantum interference device (SQUID) magnetometry were used as complimentary techniques. GaMnAs layers with epitaxial Fe grown on top, are shown to have a sub-nanometre interfacial layer which remains polarised above room temperature. A detailed understanding of these systems is obtained by applying the element specific nature of XMCD in combination with two different probing depths to explore separately the nature of the coupling of the bulk and interfacial region. The coupling between the interfacial layer and the Fe is shown to be strongly antiferromagnetic (AF). A weaker coupling is also shown to exist between the Fe and the bulk of the \gamnas layer below the Curie temperature (Tc). This coupling is also AF at low fields, leading to an exchange bias for the entire layer. Doping of GaMnAs with P is shown to have several effects on the magnetic properties of the GaMnAs layer. Changes in the layer strain are observed using high resolution XRD. This strain also manifests itself in the Mn L_2,3 XMCD spectra and the relationship between the two is shown to be linear. A pronounced effect on the magnetic anisotropy is observed using SQUID measurements, with the easy axis switching from in-plane, in the compressively strained GaMnAs, to out-of-plane in the higher doped GaMnAsP layers. A decrease in total magnetic moment per Mn atom and Tc are observed with i