Magnetodielectric effects inA-site cation-ordered chromate spinels LiMCr₄O₈ (M=Ga and In)

Abstract

We report the occurrence of a magnetodielectric effect and its correlation with structure and magnetism in the A -site ordered chromate spinel oxides LiMCr₄O₈ ( M = Ga , In). In addition to magnetic and dielectric measurements, temperature dependent synchrotron and neutron diffraction experiments have been carried out for the Ga compound. The results are compared and contrasted with that of a corresponding conventional B -site magnetic chromate spinel oxide, ZnCr₂O₄. Like ZnCr₂O₄, the A -site ordered chromate spinels exhibit a magnetodielectric effect at the magnetic ordering temperature (TN ∼ 13 – 15 K), resulting from magnetoelastic coupling through a spin Jahn-Teller effect. While the presence of a broad magnetic anomaly, associated with a short-range magnetic ordering (TSO ∼ 45 K) in ZnCr2O₄, does not cause any dielectric anomaly, a sharp change in dielectric constant has been observed in LiInCr₄O₈ at the magnetic anomaly, which is associated with the opening of a spin gap (TSG ∼ 60K). Contrary to the In compound, a broad dielectric anomaly exists at the onset of short-range antiferromagnetic ordering (TSO ∼ 55K) in LiGaCr₄O₈. The differences in dielectric behavior of these compounds have been discussed in terms of breathing distortion of the Cr₄ tetrahedra.