Multiple magnetic transitions in multiferroic BiMnO3

The magnetic phase variations under hydrostatic pressure on multiferroic BiMnO3 have been examined by the dc magnetization [M-g(T)], magnetic hysteresis [mu(eff)(H)], and ac susceptibility [chi(')(g)(T)]. Three magnetic transitions, manifested as kinks I, II, and III on the M-g(T) curves, were identified at 8.7 and 9.4 kbar. With increasing pressure, transition temperatures of kink I and kink II (T-kI and T-kII) tend to decrease, but the temperature of kink III (T-kIII) showed more complex variation. Under increasing magnetic field, T-kI and T-kII increase; however, T-kIII decreases. Combining M-g(T) curves with mu(eff)(H) and chi(')(g)(T), more detailed properties of these three kinks would be shown as follows. Kink I is a long-range soft ferromagnetic transition which occurs at T-kI similar to 100 K under ambient pressure but is suppressed completely at 11.9 kbar. Kink II emerges at 8.7 kbar along with T-kII similar to 93 K which is also long-range soft ferromagnetic but canted in nature. Kink III, a canted antiferromagnetic transition, appears at T-kIII similar to 72.5 K along with kink II also at 8.7 kbar. The proposed phase diagrams at ambient pressure, 9.4 and 11.9 kbar show the different magnetic features of BiMnO3. These findings are believed to result from the variations in crystal structure influenced by the external pressure. These results also indicate the common complicated correlation between the lattice distortion and the spin configuration in multiferroic system.