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

The complex oxide BiMnFe2O6 crystallizing in a centrosymmetric orthorhombic structure (Pbcm) is known to exhibit a spiral antiferromagnetic ordering at T-N similar to 212 K and an anomaly in magnetization at low temperature (T-P similar to 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 T-P is associated with a reentrant spin-glass transition. From dc magnetization, we found that there exists another distinct temperature (T-irr), which corresponds to a strong thermomagnetic irreversibility. The field dependence of these two temperatures, T-P(H) and T-irr(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* similar to 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.