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

Dynamical properties of the lattice structure were studied by optical spectroscopy in ACr₂O₄ 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, NiCr ₂O₄, 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 A2⁺ ions were also investigated to determine the crystal field and Racah parameters and the strength of the spin-orbit coupling.