Incomplete transformations from ferromagnetic to charge ordered states in manganite perovskites lead to phase-separated microstructures showing colossal magnetoresistances. However, it is unclear whether electronic matter can show spontaneous separation into multiple phases distinct from the high temperature state. Here we show that paramagnetic CaFe3O5 undergoes separation into two phases with different electronic and spin orders below their joint magnetic transition at 302 K. One phase is charge, orbital and trimeron ordered similar to the ground state of magnetite, Fe3O4, while the other has Fe2+/Fe3+charge averaging. Lattice symmetry is unchanged but differing strains from the electronic orders probably drive the phase separation. Complex low symmetry materials like CaFe3O5 where charge can be redistributed between distinct cation sites offer possibilities for the generation and control of electronic phase separated nanostructures.

CoO has an odd number of electrons in its unit cell, and therefore is expected to be metallic. Yet, CoO is strongly insulating owing to significant electronic correlations, thus classifying it as a Mott insulator. We investigate the magnetic fluctuations in CoO using neutron spectroscopy. The strong and spatially far-reaching exchange constants reported in [Sarte et al. Phys. Rev. B 98 024415 (2018)], combined with the single-ion spin-orbit coupling of similar magnitude [Cowley et al. Phys. Rev. B 88, 205117 (2013)] results in significant mixing between j eff spin-orbit levels in the low temperature magnetically ordered phase. The high degree of entanglement, combined with the structural domains originating from the Jahn-Teller structural distortion at ∼ 300 K, make the magnetic excitation spectrum highly structured in both energy and momentum. We extend previous theoretical work on PrTl3 [Buyers et al. Phys. Rev. B 11, 266 (1975)] to construct a mean-field and multi-level spin exciton model employing the aforementioned spin exchange and spin-orbit coupling parameters for coupled Co 2+ ions on a rocksalt lattice. This parameterization, based on a tetragonally distorted type-II antiferromagnetic unit cell, captures both the sharp low energy excitations at the magnetic zone center, and the energy broadened peaks at the zone boundary. However, the model fails to describe the momentum dependence of the excitations at high energy transfers, where the neutron response decays faster with momentum than the Co 2+ form factor. We discuss such a failure in terms of a possible breakdown of localized spin-orbit excitons at high energy transfers. arXiv:1908.00459v2 [cond-mat.str-el] 2 Aug 2019Motivated by previous work on PrTl 3 27,58 , the theoretical portion of this paper begins by first writing down the equations-of-motion for the response function in terms of commutators involving the magnetic HamiltonianĤ. We

The high pressure material MnFe3O5 displays a rich variety of magnetically ordered states on cooling through three separate phase transitions.

The antiferromagnetic mixed valence ternary oxide α-CoV3O8 displays disorder on the Co 2+ site that is inherent to the Ibam space group resulting in a local selection rule requiring one Co 2+ and one V 4+ reside next to each other, thus giving rise to an intrinsically disordered magnet without the need for any external influences such as chemical dopants or porous media. The zero field structural and dynamic properties of α-CoV3O8 have been investigated using a combination of neutron and x-ray diffraction, DC susceptibility, and neutron spectroscopy. The low temperature magnetic and structural properties are consistent with a random macroscopic distribution of Co 2+ over the 16k metal sites. However, by applying the sum rules of neutron scattering we observe the collective magnetic excitations are parameterized with an ordered Co 2+ arrangement and critical scattering consistent with a three dimensional Ising universality class. The low energy spectrum is well-described by Co 2+ cations coupled via a three dimensional network composed of competing ferromagnetic and stronger antiferromagnetic superexchange within the ab plane and along c, respectively. While the extrapolated Weiss temperature is near zero, the 3D dimensionality results in long range antiferromagnetic order at T N ∼ 19 K. A crystal field analysis finds two bands of excitations separated in energy at ω ∼ 5 meV and 25 meV, consistent with a j eff = 1 2 ground state with little mixing between spin-orbit split Kramers doublets. A comparison of our results to the random 3D Ising magnets and other compounds where spin-orbit coupling is present indicate that the presence of an orbital degree of freedom, in combination with strong crystal field effects and well-separated j eff manifolds may play a key role in making the dynamics largely insensitive to disorder.

MnFe3O5 was synthesized under a pressure of 10 GPa at 1400 °C. MnFe3O5 has an orthorhombic structure (space group Cmcm, a = 2.9137(1), b = 9.8565(7) and c = 12.6143(6) Å at 300 K) and is isostructural with Fe4O5. Magnetic measurements reveal an antiferromagnetic transition at 350 K and a broad Curie transition at 150 K, similar to the spin ordering temperatures of Fe4O5. Variable temperature synchrotron X‐ray diffraction shows that the structure undergoes anisotropic thermal expansion below 350 K, but no long range charge ordering is observed in the crystal structure.