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 R (3) over bar near room temperature and shows a structural phase transition at T-CO = 397 K to a cubic structure in space group Im (3) over bar; the Im (3) over bar -to-R (3) over bar transition is associated with charge ordering. Below T-CO = 294 K, a structural modulation transition associated with orbital ordering takes place. There are two magnetic transitions with Neel 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 similar to 37 and 65 K at 0 Oe), dielectric constant (between similar to 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 similar to 4 mu C/m(2). Field-induced transitions are detected at similar to 63 and similar to 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.