Spin-Driven Multiferroic Properties of PbMn₇O₁₂ Perovskite

We synthesize PbMn₇O₁₂ perovskite under high-pressure (6 GPa) and high-temperature (1373 K) conditions and investigate its structural, magnetic, dielectric, and ferroelectric properties. We find that PbMn₇O₁₂ exhibits rich physical properties from interplay among charge, orbital, and spin degrees of freedom and rich structural properties. PbMn₇O₁₂ crystallizes in space group R3̅ near room temperature and shows a structural phase transition at T_CO = 397 K to a cubic structure in space group Im3̅; the Im3̅-to-R3̅ transition is associated with charge ordering. Below T_OO = 294 K, a structural modulation transition associated with orbital ordering takes place. There are two magnetic transitions with Néel temperatures of T_N1 = 83 K and T_N2 = 77 K and probably a lock-in transition at T_N3 = 43 K (on cooling). There is huge hysteresis on specific heat (between ∼37 and 65 K at 0 Oe), dielectric constant (between ∼20 and 70 K at 0 Oe), and dc and ac magnetic susceptibilities around the lock-in transition. Sharp dielectric constant, dielectric loss, and pyroelectric current anomalies are observed at T_N2, indicating that electric polarization is developed at this magnetic transition, and PbMn₇O₁₂ perovskite is a spin-driven multiferroic. Polarization of PbMn₇O₁₂ is measured to be ∼4 μC/m². Field-induced transitions are detected at ∼63 and ∼170 kOe at 1.6–2 K; similar high-magnetic field properties are also found for CdMn₇O₁₂, CaMn₇O₁₂, and SrMn₇O₁₂. PbMn₇O₁₂ exhibits a quite small magnetodielectric effect, reaching approximately −1.3 to −1.7% at 10 K and 90 kOe.