Study of complex thermally induced field dependent magnetization reversal in BiFe0.5Mn0.5O3, a potentially RT multiferroic type-I perovskite

We have recently synthesized BiFe0.5Mn0.5O3, a novel multiferroic compound belonging to the class of bismuth-based double perovskites, in HP/HT conditions. In this system the electric properties are ascribed to the stereochemical effect induced by the 6s2 lone pair of the Bi ion, while magnetism depends on the complex pattern of interactions involving iron and manganese. Single-crystal X-ray diffraction allowed us to determine the crystallographic structure of the system as a B-site disordered perovskite and suggests the presence of antiferroelectricity. The magnetic characterization indicates an unconventional magnetic behavior producing a weak magnetic transition around 420 K followed, below 288 K, by a field-dependent reversal of the magnetization process. We registered the higher negative value of the magnetization -0.75emu/g, for an applied field of 70 Oe. Larger fields gradually inhibit the magnetization reversal process, which is completely suppressed at 1500 Oe. Despite the actual interpretation of the magnetization reversal process [1], our studies give a different explanation of this complex phenomenon. By performing temperature dependent neutron diffraction experiments, we could determine the spin ordering scheme of the system as a G-type structure, involving the sole presence of AFM interactions. On the other hand, Fe-57 Mossbauer spectroscopy revealed the presence of iron ions with dramatically different behaviour: a little part of them being ordered at RT, due to the presence of iron-rich clusters, and a large population of paramagnetic ones that gradually gets ordered with a continuous mechanism taking place at 288 K, when the long range antiferromagnetism occurs leaded by the manganese ions ordering. The current results suggest that the weak ferromagnetic effects observed can be due to uncompensated Dzyaloshinkii-Moriya (DM) interactions characterized by different D vectors and thermal dependencies as could be for the Fe-O-Fe, Mn-O-Mn and Fe-O-Mn interactions. This mechanism is responsible for the magnetization reversal and allows to justify its strong dependence on the applied magnetic field according also to the low magnitude of the magnetic response. As a consequence, in absence of a sufficiently large field, the magnetization goes down to negative values below 250 K but can be easily reversed up only increasing the external field[2]. [1] P.Mandal et al., Phys. Rev. B 82, 100416(R) (2010) [2] D.Delmonte et al., Phys. Rev. B, submitted (2012)