UC Santa Barbara

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 Zn_1-xCo_xCr₂O₄ completely suppress the spin-Jahn-Teller distortion of ZnCr₂O₄ while, Cu²⁺ substitutions in Mg_1-xCu_xCr₂O₄ and Zn_1-xCu_xCr₂O₄ induce Jahn-Teller distortions at temperatures above their magnetic ordering temperatures. The Jahn-Teller distortions of Mg_1-xCu_xCr₂O₄ and Zn_1-xCu_xCr₂O₄ 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.