Magnetoelasticity in ACr2O4 spinel oxides (A = Mn, Fe, Co, Ni, and Cu)

Dynamical properties of the lattice structure were studied by optical spectroscopy in ACr(2)O(4) chromium spinel oxide magnetic semiconductors over a broad temperature region of T = 10-335 K. The systematic change of the A-site ions (A = Mn, Fe, Co, Ni and Cu) showed that the occupancy of 3d orbitals on the A site has strong impact on the lattice dynamics. For compounds with orbital degeneracy (FeCr2O4, NiCr2O4, and CuCr2O4), clear splitting of infrared-active phonon modes and/or activation of silent vibrational modes have been observed upon the Jahn-Teller transition and at the onset of the subsequent long-range magnetic order. Although MnCr2O4 and CoCr2O4 show multiferroic and magnetoelectric character, no considerable magnetoelasticity was found in spinel compounds without orbital degeneracy as they closely preserve the high-temperature cubic spinel structure even in their magnetic ground state. Aside from lattice vibrations, intra-atomic 3d-3d transitions of the A(2+) ions were also investigated to determine the crystal field and Racah parameters and the strength of the spin-orbit coupling. DOI: 10.1103/PhysRevB.87.064416