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Abstract: We probe the current-induced magnetic switching of insulating antiferromagnet–heavy-metal systems, by electrical spin Hall magnetoresistance measurements and direct imaging, identifying a reversal occurring by domain wall (DW) motion. We observe switching of more than one-third of the antiferromagnetic domains by the application of current pulses. Our data reveal two different magnetic switching mechanisms leading together to an efficient switching, namely, the spin-current induced effective magnetic anisotropy variation and the action of the spin torque on the DWs.

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Abstract: Unveiling the physics that governs the intertwining between the nanoscale self-organization and the dynamics of insulator-to-metal transitions (IMTs) is key for controlling on demand the ultrafast switching in strongly correlated materials and nanodevices. A paradigmatic case is the IMT in V 2 O 3 , for which the mechanism that leads to the nucleation and growth of metallic nanodroplets out of the supposedly homogeneous Mott insulating phase is still a mystery. Here, we combine x-ray photoemission electron microscopy and ultrafast nonequilibrium optical spectroscopy to investigate the early-stage dynamics of isolated metallic nanodroplets across the IMT in V 2 O 3 thin films. Our experiments show that the low-temperature monoclinic antiferromagnetic insulating phase is characterized by the spontaneous formation of striped polydomains, with different lattice distortions. The insulating domain boundaries accommodate the birth of metallic nanodroplets, whose nonequilibrium expansion can be triggered by the photoinduced change of the 3 d -orbital occupation. We address the relation between the spontaneous nanotexture of the Mott insulating phase in V 2 O 3 and the timescale of the metallic seeds growth. We speculate that the photoinduced metallic growth can proceed along a nonthermal pathway in which the monoclinic lattice symmetry of the insulating phase is partially retained.

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Abstract: Large changes in the magnetization of ferromagnetic films can be electrically driven by non-180° ferroelectric domain switching in underlying substrates, but the shear components of the strains that mediate these magnetoelectric effects have not been considered so far. Here we reveal the presence of these shear strains in a polycrystalline film of Ni on a 0.68Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 substrate in the pseudo-cubic (011)pc orientation. Although vibrating sample magnetometry records giant magnetoelectric effects that are consistent with the hitherto expected 90° rotations of a global magnetic easy axis, high-resolution vector maps of magnetization (constructed from photoemission electron microscopy data, with contrast from X-ray magnetic circular dichroism) reveal that the local magnetization typically rotates through smaller angles of 62–84°. This shortfall with respect to 90° is a consequence of the shear strain associated with ferroelectric domain switching. The non-orthogonality represents both a challenge and an opportunity for the development and miniaturization of magnetoelectric devices.

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Abstract: Highly spin-polarised materials show great promise in applied spintronics as spin-filters and spin transfer torque magnetic random-access memory devices for data storage. However, controllable magnetisation switching requires materials with a low Gilbert damping parameter and stable magnetic properties at room temperature. Two highly spin-polarised magnetic materials with critical temperatures well above room temperature are characterised using a series of structural and magnetic analysis techniques. Correlation between the magnetic damping and microstructure is observed in both Fe3O4 and Co2FeAl0.5Si0.5 samples and both are seen to improve structurally and show more favourable damping parameters with annealing. Annealing Fe3O4 in CO/CO2 is shown to reduce the antiphase boundary density and decrease the two-magnon scattering-like extrinsic damping effects. The quality of the sample structure and the stoichiometry is also seen to improve considerably after the annealing although the defects are not completely eliminated. An anomalous peak in the damping of the annealed film is observed at 10GHz. Co2FeAl0.5Si0.5 grown on germanium and silicon substrates is seen to also improve with thermal annealing. The Gilbert damping is seen to be lower in the as-grown scheme using the silicon substrate but greater reduction of damping post-annealing is seen on germanium. In both cases the B2 order is observed in the Co2FeAl0.5Si0.5 thin films, and intermixing between the sample and substrate observed above 500oC is sufficient to disrupt the crystal structure and introduce significant extrinsic damping effects which increase the total damping. This prevents the Co2FeAl0.5Si0.5 from reaching the more desirable L21 structure.

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Abstract: The atomic-scale magnetism of Co2FeAl Heusler alloys has long been an outstanding question, and with the thickness down to the nanometer scale, this becomes even more sophisticated. Here, we report a direct measurement of the Co2FeAl epitaxial thin films on the GaAs(001) substrate with the in-situ magneto-optic Kerr effect and the synchrotron-based X-ray magnetic circular dichroism techniques. Strong uniaxial magnetic anisotropy has been observed from all thicknesses of the Co2FeAl thin films between 3 unit cells (uc) and 20 uc. A critical thickness of 3 uc has been identified, below which an anti-parallel spin component of the Co atoms occurs. This anti-parallel spin component can be responsible for the significantly reduced magnetic moment and the low spin-polarization near the Fermi level of the Co2FeAl.