Magnetostructural coupling and magnetodielectric effects in the A -site cation-ordered spinel LiFeCr₄O₈

Abstract

The A-site cation-ordered spinel LiFeCr₄O₈ is a unique system which manifests properties of the frustrated pyrochlore lattice, including the breathing distortion and antiferromagnetic coupling between A and B sites. This compound crystallizes in a cubic spinel structure with a noncentrosymmetric space group (F43m), where Li⁺ and Fe³⁺ ions are ordered in a zinc-blende-type manner at the A site and Cr³⁺ ions at the B site form a breathing pyrochlore lattice. It undergoes a ferrimagnetic transition at T_N ∼94 K, below which temperature-dependent magnetization data exhibit a maximum at T_SG ∼60 K, corresponding to a spin-gap arising from breathing distortion. At low temperature (T_MS ∼23 K), concurrent structural (I4m2) and magnetic phase transitions occur because of significant geometrical frustration of the pyrochlore lattice, giving rise to a complex conical magnetic order. Intriguingly, the observed dielectric anomalies at all three magnetic transition temperatures in zero applied magnetic field indicate the occurrence of magnetodielectric effects. Further, the temperature dependence of Raman modes shows three distinct anomalous behaviors that correlate with the dielectric anomalies at T_N,T_SG and T_MS, which suggest the presence of spin-lattice coupling.