Magnetoelasticity in ACr2O4 spinel oxides
  Dynamical properties of the lattice structure was studied by optical spectroscopy in ACr2O4 chromium spinel oxide magnetic semiconductors over a broad temperature region of T=10-335K. 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. Besides 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. 
V. Kocsis, S. Bord\'acs, D. Varjas, K. Penc, A. Abouelsayed, C. A.   Kuntscher, K. Ohgushi, Y. Tokura and I. K\'ezsm\'arki
cond-mat.str-el cond-mat.mtrl-sci