new effective strategy to study magnetic disordered systems: field effect on spontaneous magnetization reversal of bulk BiFe0.5Mn0.5O3

We show a thorough study on the external field effects in the magnetic complex properties observed in the polycrystalline solid solution BiFe0.5Mn0.5O3, an intriguing compound synthesized in high pressure-high temperature conditions and candidate to high temperature multiferroism and magnetoelectricity. In analogy to what observed in other perovskites presenting cationic disorder on the B-site, the intrinsic inhomogeneity of BiFe0.5Mn0.5O3 [1][2] determines the presence of distinct mesoscopic regions (clusters) with different concentrations of iron or manganese. Their coexistence and interplay are likely responsible for a spontaneous magnetization reversal (MRV) occurring below the Neél temperature that is always observed in absence of applied magnetic field. SQuID magnetometry allowed to individuate two critical fields of the system (650 Oe and 2200 Oe), related to the presence of two independent magnetic phases. These phases result to be magnetically competitive for H < 650 Oe, while for an intermediate field regime the energy of this weak interaction becomes comparable to the energy of the system under field application. As a result, in the latter case, Zero Field Cooled magnetization thermal evolution depends on the sample degree of inhomogeneity. In the same conditions, applied field Mössbauer spectroscopy shows that the room temperature iron rich clusters are highly polarized by the field while the largest part of the material appears to consist of AFM clusters characterized by axial anisotropy and uncompensated moments. Moreover, anomalous T-dependence of the iron ions mean-square displacements suggests the presence of low temperature anhamornicities. Above the higher critical field the M(T) measurements denotes the trend expected for a typical antiferromagnetic material. The MRV phenomenon results to be highly sensitive on both the thermal and magnetic measurement conditions. Herein we propose a specific characterization strategy that takes into account this criticalities and in principle has a large applicability in the study of disordered perovskites showing similar phenomenology.