Reentrant spin-glass state and magnetodielectric effect in the spiral magnet BiMnFe₂O₆

Abstract

The complex oxide BiMnFe₂O₆ crystallizing in a centrosymmetric orthorhombic structure (Pbcm) is known to exhibit a spiral antiferromagnetic ordering at TN∼212 K and an anomaly in magnetization at low temperature (TP∼34 K). To understand the origin of the low temperature anomaly and to verify experimentally whether or not the spiral magnetism induces ferroelectricity, we have carried out detailed magnetization and electrical measurements on a polycrystalline sample. Frequency-dependent ac susceptibility, dc magnetic memory effect, and dc magnetization relaxation studies show that the low temperature anomaly at TP is associated with a reentrant spin-glass transition. From dc magnetization, we found that there exists another distinct temperature (Tirr), which corresponds to a strong thermomagnetic irreversibility. The field dependence of these two temperatures, TP(H) and Tirr(H), follow Gabay-Toulouse and de Almeida–Thouless lines, respectively, which are indicative of the Heisenberg spin-glass state. We also observe another magnetic anomaly at T∗∼170 K, where a dielectric anomaly with a significant magnetocapacitance effect is observed. However, pyroelectric current and positive-up-negative-down measurements reveal the absence of ferroelectricity in this compound.