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

Wadley, Peter (2012) X-ray magnetic circular dichroism studies of III-Mn-V compounds and heterostructures. PhD thesis, University of Nottingham.

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Abstract

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 increased doping. This is inferred not to be due to a direct effect of the P on the local surrounding of the Mn ions, owing to the striking similarity of the XMCD spectra. This is instead attributed to reduced participation of Mn ions in the magnetic ordering.

Finally, K edge XMCD is used to reveal the element specific nature of unoccupied states near the Fermi level in a set of GaMnAs and (In,Ga,Mn)As samples with differing Mn doping levels . The character of the holes in low-doped samples is shown to be markedly different than for those in the highly doped metallic samples. A transfer of orbital magnetic moment from the Mn to the As sites is observed on crossing the metal-insulator transition, with the large XMCD on Mn sites in low doped samples interpreted as a sign of hole localisation around the Mn ion.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Edmonds, K.
Gallagher, B.L.
Keywords: Semiconductors magnetism xmcd
Subjects: Q Science > QC Physics > QC501 Electricity and magnetism
Faculties/Schools: UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID: 13451
Depositing User: EP, Services
Date Deposited: 16 Jul 2013 08:53
Last Modified: 13 Sep 2016 20:13
URI: http://eprints.nottingham.ac.uk/id/eprint/13451

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