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Magnetostructural and magnetodielectric coupling in spinel oxides

  • Author(s): Kemei, Moureen C
  • Advisor(s): Seshadri, Ram
  • et al.
Abstract

Spinels oxides are of great interest functionally as multiferroic, battery, and magnetic materials as well as fundamentally because they exhibit novel spin, structural, and orbital ground states. Competing interactions are at the heart of novel functional behavior in spinels. Here, we explore the intricate landscape of spin, lattice, and orbital interactions in magnetic spinels by employing variable-temperature high-resolution synchrotron x-ray powder diffraction, total neutron scattering, magnetic susceptibility, dielectric, and heat capacity measurements. We show that the onset of long-range magnetic interactions often gives rise to lattice distortions. We present the complete crystallographic descriptions of the ground state structures of several spinels, thereby paving the way for accurate modeling and design of structure-property relationships in these materials. We also report the emergence of magnetodielectric coupling in the magnetostructural phases of some of the studied spinels.

We begin by examining spin-lattice coupling in the Jahn-Teller active systems NiCr2O4 and CuCr2O4. Orbital ordering yields a cubic to tetragonal lattice distortion in these materials above their magnetic ordering temperatures, however, we find that magnetic ordering also drives structural distortions in these spinels through exchange striction. We provide the first orthorhombic structural descriptions of NiCr2O4 and CuCr2O4. Our observation of strong spin-lattice coupling in NiCr2O4 and CuCr2O4 inspired the study of magnetodielectric coupling in these spinels. Magnetocapacitance measurements of NiCr2O4 reveal multiferroic behavior and new magnetostructural distortions below the Néel temperature. This observation illustrates the sensitivity of dielectric measurements to magnetostructural transitions in spinel materials. Finally, in the examination of NiCr2O4 we show that magnetodielectric coupling is well described by Ginzburg-Landau theory.

In addition to exchange striction, geometric frustration couples spin interactions to the lattice of the spinels MgCr2O4 and ZnCr2O4. Novel spin ground states that are important for memory and quantum computing applications are predicted to exist in these spinels. However, their structural and spin ground states are not well understood. We find that tetragonal and orthorhombic phases coexist in antiferromagnetic MgCr2O4 and ZnCr2O4. The structural deformations in these materials lift spin degeneracy by primarily distorting the pyrochlore Cr sublattice. In subsequent studies, we probe the effect of adding dilute spins on the non-magnetic cation sites of MgCr2O4 and ZnCr2O4. Substitution of Co2+ cations in Zn1-xCoxCr2O4 completely suppress the spin-Jahn-Teller distortion of ZnCr2O4 while, Cu2+ substitutions in Mg1-xCuxCr2O4 and Zn1-xCuxCr2O4 induce Jahn-Teller distortions at temperatures above their magnetic ordering temperatures. The Jahn-Teller distortions of Mg1-xCuxCr2O4 and Zn1-xCuxCr2O4 do not lift spin degeneracy, therefore magnetic ordering is still suppressed down to low temperatures. We show that only more than 20% magnetic A substituents can lift spin degeneracy in MgCr2O4 and ZnCr2O4.

We have also examined the magnetostructural phase transition of the spinel Mn3O4. We show that Mn3O4 undergoes a magnetostructural phase transition from tetragonal I41/amd symmetry to a phase coexistence regime consisting of tetragonal I41/amd and orthorhombic Fddd symmetries. Phase coexistence in Mn3O4 is mediated by strain due to a significant lattice mismatch between the low temperature orthorhombic phase and the high temperature tetragonal phase. We propose that strain could be used to control the structure and properties of Mn3O4.

Our investigations of spin-driven lattice distortions in spinel oxides illustrate that structural phase coexistence is prevalent for spinels with Néel temperatures below 50 K.

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