Anomalous magnetic transition in multiferroic BiMnO$_{3}$ under high pressure

The magnetic-field-dependent dc magnetization and the pressure-dependent ($p_{max} \quad \sim $ 16 kbar) ac susceptibilities $\chi _{p}(T)$ on both powder and bulk multiferroic BiMnO$_{3}$ samples, synthesized in different batches under high pressure, are reported. The ferromagnetic (FM) transition ($T_{C }\sim $ 100 K) increases with higher magnetic field. The magnetic hysteresis shows the behavior as a soft ferromagnet. Ac susceptibility data indicate the following phenomena. (I) The FM peak (peak I) and its temperature ($T_{C})$ decrease simultaneously with increasing pressure. (II) Above a certain pressure (9 -11 kbar), another peak (peak II) appears at ($T_{p} \quad _{\sim }$93 K). (III) Peak II also decreases with increasing pressure. (IV) Both these peaks persist over some intermediate pressure range (9--13 kbar). (V) Peak I disappears with further application of pressure; however, the second peak survives until present pressure limit ($p_{max} \quad \sim $ 16 kbar). These features are considered to originate from the complex interplay of the magnetic and orbital structure of BiMnO$_{3}$ being affected by pressure.