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The linear magneto-optical Kerr effect (MOKE) is often used to probe magnetism of ferromagnetic materials, but MOKE cannot be applied to collinear antiferromagnets (AFs) due to the cancellation of sub-lattice magnetization. Magneto-optical constants that are quadratic in magnetization, however, provide an approach for studying AFs on picosecond time scales. Here, we combine transient measurements of optical reflectivity and birefringence to study the linear optical response of Fe2As to small ultrafast temperature excursions. We performed temperature dependent pump-probe measurements on crystallographically isotropic (001) and anisotropic (010) faces of Fe2As bulk crystals. We find the largest optical signals arise from changes in the index of refraction along the z-axis, i.e. perpendicular to the Néel vector. Both real and imaginary parts of the time-resolved optical birefringence rotation signal approximately follow the temperature dependence of the magnetic heat capacity, as expected if the changes in dielectric function are dominated by contributions of exchange interactions to the dielectric function. We conclude that under our experimental conditions, changes in the exchange interaction contribute more strongly to the temperature dependence of the magneto-optic constants than the Voigt effect. arXiv:1903.07810v3 [cond-mat.mtrl-sci]

We introduce an intermediate-temperature (350 °C) dry molten sodium hydroxide-mediated binder-free electrodeposition process to grow the previously electrochemically inaccessible air- and moisture-sensitive layered sodium transition metal oxides, NaxMO2 (M = Co, Mn, Ni, Fe), in both thin and thick film form, compounds which are conventionally synthesized in powder form by solid-state reactions at temperatures ≥700 °C. As a key motivation for this work, several of these oxides are of interest as cathode materials for emerging sodium-ion–based electrochemical energy storage systems. Despite the low synthesis temperature and short reaction times, our electrodeposited oxides retain the key structural and electrochemical performance observed in high-temperature bulk synthesized materials. We demonstrate that tens of micrometers thick >75% dense NaxCoO2 and NaxMnO2 can be deposited in under 1 h. When used as cathodes for sodium-ion batteries, these materials exhibit near theoretical gravimetric capacities, chemical diffusion coefficients of Na+ ions (∼10−12 cm2⋅s−1), and high reversible areal capacities in the range ∼0.25 to 0.76 mA⋅h⋅cm−2, values significantly higher than those reported for binder-free sodium cathodes deposited by other techniques. The method described here resolves longstanding intrinsic challenges associated with traditional aqueous solution-based electrodeposition of ceramic oxides and opens a general solution chemistry approach for electrochemical processing of hitherto unexplored air- and moisture-sensitive high valent multinary structures with extended frameworks.

We report the single-crystal growth and characterization of a new hexagonal phase, Cu0.82Mn1.18As, in the Cu-Mn-As system. This compound contains the same square-pyramidal MnAs5 units as the tetragonal and orthorhombic polymorphs of CuMnAs. Calorimetry, magnetometry, and neutron diffraction measurements reveal antiferromagnetic ordering at 270 K. The magnetic structure consists of a triangular arrangement of spins in the ab plane. Hexagonal Cu0.82Mn1.18As shows resistivity that varies only weakly from 5 K to 300 K, and is many times higher than tetragonal CuMnAs, indicative of a strongly-scattering metal. First-principles calculations confirm the metallic band structure with a small density of states at the Fermi energy. The neutron-refined magnetic ground state is close to the computationally-determined minimum energy configuration. This compound should serve as a clear control when disentangling the effects of current-driven Néel switching of metallic antiferromagnets since it exhibits in-plane spins but the magnetic ordering does not break degeneracy along the a and b directions, unlike tetragonal CuMnAs. arXiv:1908.01758v1 [cond-mat.mtrl-sci]