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Register a new userLocal Structural Evidence for Strong Electronic Correlations in LiRh$_2$O$_4$ Spinel
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The local structure of the spinel LiRh$_2$O$_4$ has been studied using atomic pair distribution function (PDF) analysis of powder x-ray diffraction data. This measurement is sensitive to the presence of short Rh-Rh bonds that form due to dimerization of Rh$^{4+}$ ions on the pyrochlore sublattice, independent of the existence of long range order. We show that structural dimers exist in the low-temperature phase, as previously supposed, with a bond shortening of $Delta r sim 0.15$ AA . The dimers persist up to 350 K, well above the insulator-metal transition, with $Delta r$ decreasing in magnitude on warming. Such behavior is inconsistent with the Fermi surface nesting-driven Peierls transition model. Instead, we argue that LiRh$_2$O$_4$ should properly be described as a strongly correlated system.
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A structural phase transition from cubic $Fdbar{3}m$ to tetragonal $I$4$_1$/$amd$ symmetry with $c/a >$ 1 is observed at $T_{rm{S}}$ = 16 K in spinel GeCo$_2$O$_4$ below the Neel temperature $T_N$ = 21 K. Structural and magnetic ordering appear to be decoupled with the structural distortion occurring at 16 K while magnetic order occurs at 21 K as determined by magnetic susceptibility and heat capacity measurements. An elongation of CoO$_6$ octahedra is observed in the tetragonal phase of GeCo$_2$O$_4$. We present the complete crystallographic description of GeCo$_2$O$_4$ in the tetragonal $I$4$_1$/$amd$ space group and discuss the possible origin of this distortion in the context of known structural transitions in magnetic spinels. GeCo$_2$O$_4$ exhibits magnetodielectric coupling below $T_{rm{N}}$. The related spinels GeFe$_2$O$_4$ and GeNi$_2$O$_4$ have also been examined for comparison. Structural transitions were not detected in either compound down to $T approx$ 8 K. Magnetometry experiments reveal in GeFe$_2$O$_4$ a second antiferromagnetic transition, with $T_{rm{N1}}$ = 7.9 K and $T_{rm{N2}}$ = 6.2 K, that was previously unknown, and that bear a similarity to the magnetism of GeNi$_2$O$_4$.
The importance of electronic correlation effects in the layered perovskite Sr$_2$RuO$_4$ is evidenced. To this end we use state-of-the-art LDA+DMFT (Local Density Approximation + Dynamical Mean-Field Theory) in the basis of Wannier functions to compute spectral functions and the quasiparticle dispersion of Sr$_2$RuO$_4$. The spectra are found to be in good agreement with various spectroscopic experiments. We also calculate the $textbf{k}$-dependence of the quasiparticle bands and compare the results with new angle resolved photoemission (ARPES) data. Two typical manifestations of strong Coulomb correlations are revealed: (i) the calculated quasiparticle mass enhancement of $m^*/m approx2.5$ agrees with various experimental results, and (ii) the satellite structure at about 3 eV binding energy observed in photoemission experiments is shown to be the lower Hubbard band. For these reasons Sr$_2$RuO$_4$ is identified as a strongly correlated 4$d$ electron material.
AB$_2$O$_4$ normal spinels with a magnetic B site can host a variety of magnetic and orbital frustrations leading to spin-liquid phases and field-induced phase transitions. Here we report the first epitaxial growth of (111)-oriented MgCr$_2$O$_4$ thin films. By characterizing the structural and electronic properties of films grown along (001) and (111) directions, the influence of growth orientation has been studied. Despite distinctly different growth modes observed during deposition, the comprehensive characterization reveals no measurable disorder in the cation distribution nor multivalency issue for Cr ions in either orientation. Contrary to a naive expectation, the (111) stabilized films exhibit a smoother surface and a higher degree of crystallinity than (001)-oriented films. The preference in growth orientation is explained within the framework of heteroepitaxial stabilization in connection to a significantly lower (111) surface energy. These findings open broad opportunities in the fabrication of 2D kagome-triangular heterostructures with emergent magnetic behavior inaccessible in bulk crystals.
We experimentally investigated the magnetic properties of NiCo$_2$O$_4$ epitaxial films known to be conductive oxides with perpendicular magnetic anisotropy (PMA) at room temperature. Both magneto-torque and magnetization measurements at various temperatures provide clear experimental evidence of the spin reorientation transition at which the MA changes from PMA to easy-cone magnetic anisotropy (ECMA) at a certain temperature ($T_{rm{SR}}$). ECMA was commonly observed in films grown by pulsed laser deposition and reactive radio frequency magnetron sputtering, although $T_{mathrm{SR}}$ is dependent on the growth method as well as the conditions. The cone angles measured from the $c$-axis increased successively at $T_{mathrm{SR}}$ and approached a maximum of 45-50 degrees at the lowest measurement temperature of 5 K. Calculation with the cluster model suggests that the Ni$^{3+}$ ions occupying the $T_d$ site could be the origin of the ECMA. Both the magnetic properties and the results of the calculation based on the cluster model indicate that the ECMA is attributable to the cation anti-site distribution of Ni$^{3+}$, which is possibly formed during the growth process of the thin films.
The spinel-structured lithium manganese oxide (LiMn$_2$O$_4$) is a material currently used as cathode for secondary lithium-ion batteries, but whose properties are not yet fully understood. Here, we report a computational investigation of the inversion thermodynamics and electronic behaviour of LiMn$_2$O$_4$ derived from spin-polarised density functional theory calculations with a Hubbard Hamiltonian and long-range dispersion corrections (DFT+$U-$D3). Based on the analysis of the configurational free energy, we have elucidated a partially inverse equilibrium cation distribution for the LiMn$_2$O$_4$ spinel. This equilibrium degree of inversion is rationalised in terms of the crystal field stabilisation effects and the difference between the size of the cations. We compare the atomic charges with the oxidation numbers for each degree of inversion. We found segregation of the Mn charge once these ions occupy the tetrahedral and octahedral sites of the spinel. We have obtained the atomic projections of the electronic band structure and density of states, showing that the normal LiMn$_2$O$_4$ has half-metallic properties, while the fully inverse spinel is an insulator. This material is in the ferrimagnetic state for the inverse and partially inverse cation arrangement. The optimised lattice and oxygen parameters, as well as the equilibrium degree of inversion, are in agreement with the available experimental data. The partially inverse equilibrium degree of inversion is important in the interpretation of the lithium ion migration and surface properties of the LiMn$_2$O$_4$ spinel.
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