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Abstract: Magnetic vortex cores in polycrystalline Ni discs underwent non-volatile displacements due to voltage-driven ferroelectric domain switching in single-crystal BaTiO3. This behaviour was observed using photoemission electron microscopy to image both the ferromagnetism and ferroelectricity, while varying in-plane sample orientation. The resulting vector maps of disc magnetization match well with micromagnetic simulations, which show that the vortex core is translated by the transit of a ferroelectric domain wall, and thus the inhomogeneous strain with which it is associated. The non-volatility is attributed to pinning inside the discs. Voltage-driven displacement of magnetic vortex cores is novel, and opens the way for studying voltage-driven vortex dynamics.

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Abstract: In antiferromagnetic spintronics the Néel vector corresponding to the staggered magnetization is used to encode information. In the framework of spin-based electronics, antiferromagnets as active elements have a number of advantages compared to ferromagnetic materials: The natural spin dynamics of antiferromagnets in the THz range is of major significance as it makes this class of magnetic materials promising for ultrafast switching as well as for the generation and detection of THz radiation. Furthermore, the lack of net magnetization of antiferromagnets makes their Néel vector oriented state robust with respect to external magnetic fields. This property results in a high stability of the information encoded in the Néel vector orientation, which cannot be destroyed by external magnetic fields. Last but not least, antiferromagnets do not produce stray fields, which are limiting the minimum feasible size of data storage units based on ferromagnets. However, the lack of net magnetization makes the manipulation of the Néel vector orientation much more challenging than the corresponding manipulation of the magnetization of ferromagnets. One of the possible approaches is based on a current induced bulk spin-orbit torque, which was predicted to exist for very specific crystal structures of collinear antiferromagnets, the so-called Neel spin-orbit torque. Only two antiferromagnetic compounds with the required crystal structure are known up to now: Mn2Au, on which this work is focused, and CuMnAs. In my thesis, I investigate the effects of current pulses on the antiferromagnetic domain structure which is modified by a Néel vector reorientation originating from Néel spin-orbit torques and alternative effects. As an alternative Néel vector manipulation method, I also investigate the switching of the Néel vector orientation in Mn2Au by the application of very large high magnetic field pulses causing a spin-flop transition. To investigate current induced Néel vector switching in Mn2Au, I performed electric transport measurements where I measured current-induced changes of the resistance of up to 6% and compared the results with calculations of our collaboration partners. In order to directly visualize the Néel vector reorientation, we performed photoelectron emission microscopy experiments with magnetic contrast combined with transport measurements. During this experiment, massive changes of the AFM domain pattern induced by current pulses were observed. Also the reorientation of the Néel vector induced by magnetic field pulse driven spin-flop transitions was correlated with magnetotransport measurements. Here, persistent changes of the resistance associated with anisotropic magnetoresistance and a transient drop of the resistance in the range of 2% were obtained. These experiments demonstrated Néel vector switching in Mn2Au both by a current induced Néel spin-orbit torque mechanism and by a magnetic field induced spin-flop transition. It was shown that the magnetoresistance of the antiferromagnetic domain walls in Mn2Au plays a crucial role in the understanding of current-induced resistance modifications associated with Néel vector switching processes. Additional to transient effects, there are as well persistent resistance modifications induced by the Neel vector reorientation , which is an essential property for data storage applications.

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Abstract: Biogenic nanoscale vanadium magnetite is produced by converting V(V)- bearing ferrihydrites through reductive transformation using the metal- reducing bacterium Geobacter sulfurreducens. With increasing vanadium in the ferrihydrite, the amount of V-doped magnetite produced decreased due to V-toxicity which interrupted the reduction pathway ferrihydrite – magnetite, resulting in siderite or goethite formation. Fe L2,3 and V L2,3 X-ray absorption spectra and data from X-ray magnetic circular dichroism analysis revealed the magnetite to contain the V in the Fe(III) Oh site, predominately as V(III) but always with a component of V(VI) present a consistent V(IV)/V(III) ratio in the range 0.28 to 0.33. The bacteriogenic production of V-doped magnetite nanoparticles from V-doped ferrihydrite is confirmed and the work reveals that microbial reduction of contaminant V(V) to V(III)/V(IV) in the environment will occur below the Fe-redox boundary where it will be immobilised in biomagnetite nanoparticles.

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Abstract: Exchange-bias has been reported in bulk nanocrystalline Fe2MnAl, but individual thin films of this Heusler alloy have never been studied so far. Here we study the structural and magnetic properties of nanocrystalline thin films of Fe2–xMn1+xAl (x = –0.25, 0, 0.25) obtained by sputtering and ex-situ post-deposition annealing. We find that Fe2MnAl films display exchange-bias, and that varying Mn concentration determines the magnitude of the effect, which can be either enhanced (in Fe1.75Mn1.25Al) or suppressed (in Fe2.25Mn0.75Al). X-ray diffraction (XRD) shows that our films present a mixed L21-B2 Heusler structure where increasing Mn concentration favors the partial transformation of the L21 phase into the B2 phase. Scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX) reveal that this composition-driven L21→B2 transformation is accompanied by phase segregation at the nanoscale. As a result, the Fe2–xMn1+xAl films that show exchange-bias (x = 0, 0.25) are heterogeneous, with nanograins of an Fe-rich phase embedded in a Mn-rich matrix (a non-negative matrix factorisation algorithm was used to give an indication of the phase composition from EDX data). Our comparative analysis of XRD, magnetometry and X-ray magnetic circular dichroism (XMCD), shows that Fe-rich nanograins and Mn-rich matrix are composed of a ferromagnetic L21 phase and an antiferromagnetic B2 phase, respectively, thus revealing that exchange-coupling between these two phases is the cause of the exchange-bias effect. Our work should inspire the development of single-layer, environmentally-friendly spin valve devices based on nanocomposite Heusler films.

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Abstract: Epitaxial strain provides important pathways to control the magnetic and electronic states in transition-metal oxides. However, the large strain is usually accompanied by a strong reduction of the oxygen-vacancy formation energy, which hinders the direct manipulation of their intrinsic properties. Here, using a postdeposition ozone annealing method, we obtain a series of oxygen stoichiometric SrCoO 3 thin films with the tensile strain up to 3.0%. We observe a robust ferromagnetic ground state in all strained thin films, while interestingly the tensile strain triggers a distinct metal-to-insulator transition along with the increase of the tensile strain. The persistent ferromagnetic state across the electrical transition therefore suggests that the magnetic state is directly correlated with the localized electrons, rather than the itinerant ones, which then calls for further investigation of the intrinsic mechanism of this magnetic compound beyond the double-exchange mechanism.