Origin of magnetization reversal and exchange bias phenomena in solid solutions of BiFeO3-BiMnO3: intrinsic or extrinsic?

Magnetic properties of BiFe(0.7)Mn(0.3)O(3) (with a Néel temperature (T(N)) of 425 K) and BiFe(0.6)Mn(0.4)O(3) (with T(N) = 350 K) were investigated by magnetic measurements between 5 and 400 K. They crystallize in space group Pnma with the √2a(p) × 4a(p) × 2√2a(p) superstructure (a(p) is the parameter of the cubic perovskite subcell) with a = 5.57800(9) Å, b = 15.7038(3) Å, and c = 11.22113(16) Å for BiFe(0.6)Mn(0.4)O(3). Both compounds show magnetization reversal or negative magnetization phenomena. However, it was found that the magnetization reversal is dependent on magnetic prehistory of a sample and measurement protocols. No magnetization reversal was observed when virgin samples were measured below T(N). Magnetization reversal effects appeared only when the samples were cooled in small magnetic fields from temperatures above T(N) or after the samples were magnetized. The exchange bias effect or a shift of isothermal magnetization curves, depending on the measurement conditions, was also observed. The exchange bias changes its sign as a function of temperature and cooling conditions. Our findings allowed us to propose the extrinsic origin (related to sample inhomogeneities) of the magnetization reversal effect in these two compounds.