High-quality MoFe 2 O 4 epitaxial films, exhibiting p-type conductivity and room-temperature ferrimagnetism, were successfully fabricated by a pulsed laser deposition technique. In this experiment, the MoFe 2 O 4 and MoFe 2 O 6 targets prepared via a vacuum-sealed annealing and air annealing were used. The result shows that the spinel MoFe 2 O 4 epitaxial films are obtained by using the MoFe 2 O 4 target, while films with cation-vacant spinel structures exhibiting n-type conductivity are obtained by using the MoFe 2 O 6 target even under reductive deposition conditions. Thus, controlling the oxygen content in the target is crucial to realize p-type conductivity probably related to the requirement of highly reductive Mo 3+ ions. The MoFe 2 O 4 film shows room-temperature ferrimagnetism with a large in-plane magnetic anisotropy (5 × 10 5 erg/cm 3 ) and high saturation magnetization (0.7 μ B /f.u.). In addition, the film exhibits an anomalous Hall effect and magnetoresistance at room temperature. Density functional theory calculations reveal that the p-type conductivity of MoFe 2 O 4 is derived from the holes generated in the occupied Mo 3+ 4d 3 t 2g majority spin orbitals. Article pubs.acs.org/crystal

We investigated the electronic structure of a layered perovskite oxyfluoride Sr 2 RuO 3 F 2 thin film by hard X-ray photoemission spectroscopy (HAXPES) and soft X-ray absorption spectroscopy (XAS) as well as density functional theory (DFT)-based calculations. The core-level HAXPES spectra suggested that Sr 2 RuO 3 F 2 is a Mott insulator. The DFT calculations described the total and site-projected density of states and the band dispersion for the optimized crystal structure of Sr 2 RuO 3 F 2 , predicting that Ru 4+ takes a high-spin configuration of (xy) ↑ (yz, zx) ↑↑ (3z 2-r 2) ↑ and that Sr 2 RuO 3 F 2 has an indirect band gap of 0.7 eV with minima at the M,A and X,R points. HAXPES spectra near the Fermi level and the angular-dependent O 1s XAS spectra of the Sr 2 RuO 3 F 2 thin film, corresponding to the valence band and conduction band density of states, respectively, were drastically different compared to those of the Sr 2 RuO 4 film, suggesting that the changes in the electronic states were mainly driven by the substitution of an oxygen atom coordinated to Ru by fluorine and subsequent modification of crystal field.

Most perovskite oxyfluorides synthesized to date have cubic structures, wherein fluorine atoms tend to reside at every oxygen site randomly. In this theoretical study, we show a twodimensional fluorine arrangement in a perovskite nickel oxyfluoride (NdNiO 2 F) with a large orthorhombic distortion. The site selectivity in the perovskite lattice is due to the orthorhombicity, which stabilizes the two-dimensional cis configurations with shorter Ni−O and longer Ni−F bonds to minimize the electrostatic energy. The electronic structure of NdNiO 2 F is characterized by its large octahedral rotation and the higher electronegativity of fluorine than oxygen. We also observed how the anion arrangement was affected by the biaxial strain by modeling epitaxial strained thin film on substrate and found that the 2D cis structure remains the most stable. However, the orientation of the two-dimensional structure containing F depends on the magnitude of the biaxial strain. Our findings suggest that fluorine doping in orthorhombic perovskite oxides effectively yields oxyfluorides with anisotropic anion ordering.

Oxynitride MnTaO2N exhibits a helical spin order in contrast to the isostructural oxide MnTiO3 with a G-type antiferromagnetism. To understand the role of the nitride ions on the magnetism, in this study, we theoretically investigated the structural and magnetic properties of MnTaO2N. Band calculations based on the density functional theory revealed that besides the most stable anion coordinations of cis-MO4N2 octahedra (M = Mn and Ta), the other coordinations such as trans-MO4N2, MO3N3, and MO5N are also considered to coexist due to the small total energy difference. This results in random existence of the nitride ions in MnTaO2N. The magnetic properties were investigated using the Heisenberg model to quantify the coupling energy (J) between Mn 3d5 local moments. Interestingly, the average J of the Mn–N–Mn bonds (J 1N) was four times larger than that of the Mn–O–Mn bonds (J 1O), mainly due to the covalent nature of Mn–N bonds. In comparison to MnTiO3, which contains only one type of magnetic interaction, J 1O, MnTaO2N has various kinds of magnetic interactions due to the random existence of the nitride ions. In addition, the J 1O value became comparable to the next-nearest-neighbor interaction (J 2). These results caused spin frustration, which is a prerequisite to the emergence of the helical spin order in MnTaO2N.

Epitaxial thin films of nitrogen-substituted Fe 3 O 4 (Fe 3 O 4−y N y ) were synthesized on MgO (001) substrates by using nitrogen-plasma-assisted pulsed laser deposition. The obtained Fe 3 O 4−y N y thin films showed ferrimagnetic behavior at room temperature. The carrier density of the films was decreased by nitrogen substitution up to y = 0.4. These properties were similar to those of A-site (tetrahedral site)-substituted Fe 3 O 4 , such as Zn x Fe 3−x O 4 or Mn x Fe 3−x O 4 , previously reported. On the other hand, the electrical resistivity of the Fe 3 O 4−y N y thin films at room temperature was rather insensitive to y, in sharp contrast to Zn x Fe 3−x O 4 or Mn x Fe 3−x O 4 , of which the resistivity drastically increased with increase of x. These results indicated that nitrogen substitution is an effective method to control the carrier density of Fe 3 O 4 without disturbing the carrier conduction through the B-site (octahedral site) Fe ion network. Density functional theory-based first-principles calculation predicted that Fe 3 O 4−y N y is half-metallic.

Double-perovskite GdBaCo2O x is a promising material owing to its fascinating electronic and magnetic features, such as spin crossover, charge and spin-state ordering, magnetic-field or photo-induced antiferromagnetic-to-ferromagnetic phase transition, and large magnetoresistance (MR). These properties can be controlled by changing the oxygen content (x), and an increase of x is effective for stabilizing ferromagnetic order. However, a stoichiometric phase (x = 6) has not yet been obtained because the distorted coordination geometry prefers oxygen vacancies. In this study, we report the successful synthesis of GdBaCo2O6 epitaxial films via the topotactic oxidation of a film with x = 5.5. The transformation between x = 5.5 and 6 phases was reversible via low-temperature redox reactions. The x = 6 film exhibited ferromagnetic and metallic behavior below the Curie temperature (T C) of 110 K and semiconducting behavior above T C. The spontaneous magnetization of the x = 6 film was 4.8 μB/f.u., which is the largest in the double-perovskite cobaltite system owing to the ferromagnetic interaction between Co3.5+ and Gd3+. It also exhibited strong magnetic anisotropy along the a axis (1.7 × 107 erg/cm3) and anisotropic MR behavior associated with the Gd/Ba ordering along the c axis. Furthermore, theoretical calculations predicted that the half-metallicity of the x = 6 film originates from the double exchange interaction between intermediate-spin Co3+ and low-spin Co4+.

It has been recently reported that H-doping of SrFeO 2 induces an insulator-to-metal transition but generates only a small amount of carrier electrons. To investigate the crystal structure of H-doped SrFeO 2 (SFOH) and the origin of its peculiar transport properties, we performed DFT-based first-principles calculations. Through structural sampling and total energy calculations, we showed that the doped hydrogen atoms exist in hydride form (H − ). Incorporation of the hydride drastically changed the valence state or the d-band configuration of the Fe ions adjacent to the doped hydride, resulting in a metallic density of states in specific hydride configurations. Thermodynamic analysis revealed that the formation of an insulating phase with O-site hydride was energetically preferable, but metallic phases with O-site hydride or interstitial hydride could also be present at reaction temperature, suggesting that SFOH is a mixture of an insulating matrix and metallic domains. This two-phase model accounted for the observed low carrier density as well as the metallic transport properties.

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