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Abstract: The effects of current induced Néel spin-orbit torques on the antiferromagnetic domain structure of epitaxial Mn2Au thin films were investigated by x-ray magnetic linear dichroism–photoemission electron microscopy.We observed current induced switching of antiferromagnetic domains essentially corresponding to morphological features of the samples. Reversible as well as irreversible Néel vector reorientation was obtained in different parts of the samples and the switching of up to 30% of all domains in the field of view of 10 μm is demonstrated. Our direct microscopical observations are compared to and fully consistent with anisotropic magnetoresistance effects previously attributed to current induced Néel vector switching in Mn2Au.

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Abstract: The redox behaviour modification following the addition of zirconia to ceria nanostructures supported on Rh(111) has been investigated using a combination of Low Energy Electron Diffraction (LEED) and X-ray Photoemission Electron Microscopy (XPEEM). Soft X-ray irradiation was employed to reduce ZrO2-x(111) supported on Rh(111) and, by introducing oxygen, the reoxidation process of the thin film was monitored. CeO2(111) was then deposited on zirconia/Rh(111) and, using XPEEM, we determined that the mixed metal oxide formed a phase-separated structure with CeO2(111) nanoparticles on top of the zirconia. Upon exposure of CeO2-x/ZrO2-x/Rh(111) to X-ray illumination, the zirconia no longer undergoes any observable reduction while at the same time the ceria is reduced. Our results indicate a synergy between the zirconia and ceria in the phase-separated system expected in the working catalyst, with oxygen transfer between the metal oxides. This sheds light on the mechanism of the enhancement of catalytic properties seen with the addition of zirconia to ceria and highlights the oxygen storage and release ability of ceria.

Open Access

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Abstract: A dissipative magnetic soliton, or magnetic droplet, is a structure that has been predicted to exist within a thin magnetic layer when non-linearity is balanced by dispersion, and a driving force counteracts the inherent damping of the spin precession. Such a soliton can be formed beneath a nano-contact (NC) that delivers a large spin-polarized current density into a magnetic layer with perpendicular magnetic anisotropy. Although the existence of droplets has been confirmed from electrical measurements and by micro-magnetic simulations, only a few attempts have been made to directly observe the magnetic landscape that sustains these structures, and then only for a restricted set of experimental parameter values. In this work we use and x-ray holography technique HERALDO, to image the magnetic structure of the [Co/Ni]x4 multilayer within a NC orthogonal pseudo spin-valve, for different range of magnetic fields and injected electric currents. The magnetic configuration imaged at −33 mA and 0.3 T for devices with 90 nm NC diameter reveals a structure that is within the range of current where the droplet soliton exist based on our electrical measurements and have it is consistent with the expected size of the droplet (∼100 nm diameter) and its spatial position within the sample. We also report the magnetisation configurations observed at lower DC currents in the presence of fields (0–50 mT), where it is expected to observe regimes of the unstable droplet formation.

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Abstract: This dissertation investigates the growth and magnetic properties of magnetic thin films deposited on semiconductor GaAs and the insulator MgO, which could be useful for devices such as Spin-FET and MRAM. CoFeB amorphous films were grown on both GaAs and MgO. We have studied the origin of the uniaxial magnetic anisotropy (UMA) and perpendicular magnetic anisotropy (PMA) with TEM, VSM and XMCD. Our results demonstrated that the orbital moment of Co atoms play an important role to both UMA and PMA. The origin of UMA in Fe/GaAs (100) system with Cr interlayers is explored. The values of UMA in the Fe/GaAs systems were found to be dependent on the thickness of Cr interlayer by the SQUID-VSM measurements. RHEED patterns and TEM images offered the morphology and crystalline structure information of different layers in the samples. Our results show that the UMA disappears when the interlayer Cr forms continuous film of around 5 ML. This offers direct evidence for the first time that the origin of UMA is from the interface bonding rather than the lattice mismatch related film stress. Finally, Fe films were deposited onto GaAs (100) substrate with a heavy metal element Au interface layer. The enhancement of UMA was found in the Fe/Au/GaAs system by the VSM measurements. The XMCD results show that the orbital moment of Fe is enhanced when the Au interlayer is under 0.5 ML, which leads to the enhancement of the UMA in the Fe/Au/GaAs system. Results from the three different systems provide an important understanding of the research into the interface magnetic properties of ferromagnetic metal/semiconductors. These interesting discoveries are very useful for the development of next generation electronic devices like MRAM and SpinFET.

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Abstract: Vortices, occurring whenever a flow field ‘whirls’ around a one-dimensional core, are among the simplest topological structures, ubiquitous to many branches of physics. In the crystalline state, vortex formation is rare, since it is generally hampered by long-range interactions: in ferroic materials (ferromagnetic and ferroelectric), vortices are observed only when the effects of the dipole–dipole interaction are modified by confinement at the nanoscale1,2,3, or when the parameter associated with the vorticity does not couple directly with strain4. Here, we observe an unprecedented form of vortices in antiferromagnetic haematite (α-Fe2O3) epitaxial films, in which the primary whirling parameter is the staggered magnetization. Remarkably, ferromagnetic topological objects with the same vorticity and winding number as the α-Fe2O3 vortices are imprinted onto an ultra-thin Co ferromagnetic over-layer by interfacial exchange. Our data suggest that the ferromagnetic vortices may be merons (half-skyrmions, carrying an out-of plane core magnetization), and indicate that the vortex/meron pairs can be manipulated by the application of an in-plane magnetic field, giving rise to large-scale vortex–antivortex annihilation.