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Abstract: Using x-ray magnetic circular and linear dichroism techniques, we demonstrate a collinear exchange coupling between an epitaxial antiferromagnet, tetragonal CuMnAs, and an Fe surface layer. A small uncompensated Mn magnetic moment is observed which is antiparallel to the Fe magnetization. The staggered magnetization of the 5 nm thick CuMnAs layer is rotatable under small magnetic fields, due to the interlayer exchange coupling. This allows us to obtain the x-ray magnetic linear dichroism spectra for different crystalline orientations of CuMnAs in the (001) plane. This is a key parameter for enabling the understanding of domain structures in CuMnAs imaged using x-ray magnetic linear dichroism microscopy techniques.

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Abstract: Recent breakthroughs in the electrical detection and manipulation of antiferromagnets have opened a new avenue in the research of non-volatile spintronic devices. Antiparallel spin sublattices in antiferromagnets, producing zero dipolar fields, lead to insensitivity to magnetic field perturbations, multi-level stability, ultrafast spin dynamics and other favourable characteristics, and may find utility in fields ranging from magnetic memories to optical signal processing. However, the absence of a net magnetic moment and ultrashort magnetization dynamics timescales make antiferromagnets notoriously difficult to study using common magnetometers or magnetic resonance techniques. Here, we demonstrate the experimental determination of the Néel vector in a thin film of antiferromagnetic CuMnAs (refs 9,10), a prominent material used in the first realization of antiferromagnetic memory chips10. We use a table-top femtosecond pump–probe magneto-optical experiment that is considerably more accessible than the traditionally employed large-scale-facility techniques such as neutron diffraction11 and X-ray magnetic dichroism measurements.

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Abstract: Tetragonal CuMnAs is an antiferromagnetic material with favourable properties for applications in spintronics. Using a combination of neutron diffraction and x-ray magnetic linear dichroism, we determine the spin axis and magnetic structure in tetragonal CuMnAs, and reveal the presence of an interfacial uniaxial magnetic anisotropy. From the temperature-dependence of the neutron diffraction intensities, the Néel temperature is shown to be (480 ± 5) K. Ab initio calculations indicate a weak anisotropy in the (ab) plane for bulk crystals, with a large anisotropy energy barrier between in-plane and perpendicular-to-plane directions.

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Abstract: Multiferroic composite materials, consisting of coupled ferromagnetic and piezoelectric phases, are of great importance in the drive towards creating faster, smaller and more energy efficient devices for information and communications technologies. Such devices require thin ferromagnetic films with large magnetostriction and narrow microwave resonance linewidths. Both properties are often degraded, compared to bulk materials, due to structural imperfections and interface effects in the thin films. We report the development of epitaxial thin films of Galfenol (Fe81Ga19) with magnetostriction as large as the best reported values for bulk material. This allows the magnetic anisotropy and microwave resonant frequency to be tuned by voltage-induced strain, with a larger magnetoelectric response and a narrower linewidth than any previously reported Galfenol thin films. The combination of these properties make epitaxial thin films excellent candidates for developing tunable devices for magnetic information storage, processing and microwave communications.

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Abstract: This thesis describes the characterisation of (Ga,Mn)As, related compounds and heterostructures. (Ga,Mn)As and other (III,Mn)V compounds have provided many interesting insights into fundamental physics, and are of considerable potential interest commercially in the eld 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 diraction (XRD), transport measurements and super conducting quantum interference device (SQUID) magnetometry were used as complimentary techniques. (Ga,Mn)As layers with epitaxial Fe grown on top, are shown to have a subnanometre interfacial layer which remains polarised above room temperature. A detailed understanding of these systems is obtained by applying the element speci c nature of XMCD in combination with two dierent 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 (Ga,Mn)As layer below the Curie temperature (TC). This coupling is also AF at low elds, leading to an exchange bias for the entire layer. Doping of (Ga,Mn)As with P is shown to have several eects on the magnetic properties of the (Ga,Mn)As layer. Changes in the layer strain are observed using high resolution XRD. This strain also manifests itself in the Mn L3;2 XMCD spectra and the relationship between the two is shown to be linear. A pronounced eect on the magnetic anisotropy is observed using SQUID measurements, with the easy axis switching from in-plane, in the compressively strained (Ga,Mn)As, to out-of-plane in the higher doped (Ga,Mn)(As,P) 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 eect 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 specic nature of unoccupied states near the Fermi level in a set of (Ga,Mn)As and (In,Ga,Mn)As samples with diering Mn doping levels . The character of the holes in low-doped samples is shown to be markedly dierent 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.