Magnetoelasticity in ACr2O4 spinel oxides

TL;DR

This study investigates the lattice dynamics and magnetoelastic properties of ACr2O4 spinel oxides using optical spectroscopy, revealing the influence of orbital degeneracy and Jahn-Teller transitions on phonon modes and magnetic order.

Contribution

It provides new insights into how orbital degeneracy affects lattice vibrations and magnetoelasticity in ACr2O4 spinel oxides, highlighting the role of Jahn-Teller transitions.

Findings

Orbital degeneracy leads to phonon mode splitting and activation of silent modes.

No significant magnetoelasticity observed in compounds without orbital degeneracy.

Intra-atomic 3d-3d transitions help determine crystal field and spin-orbit coupling parameters.

Abstract

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.