Interpretation of Magnetization Reversal and evidences of magnetoelectricity in bulk BiFe0.5Mn0.5O3 double perovskite

We have recently synthesized BiFe0.5Mn0.5O3, a novel multiferroic compound belonging to the class of bismuth-based double perovskites, in high pressure/high temperature 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 ferromagnetic 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. However the low-temperature magnetism of BiFe0.5Mn0.5O3 shows intricate mechanisms. Such mechanisms evidently confirmed a different origin, that is weaker in terms of energy with respect to the AFM leading contribution. Thanks to Fe-57 Mossbauer spectroscopy we have been able to discriminate the presence of iron ions with a 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 driven by the manganese ions ordering. The current results suggest that the weak ferromagnetic effects observed can be due to uncompensated Dzyaloshinkii-Moriya 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]. Furthermore we measured a giant magnetodielectric effect that can partially controlled by an external magnetic field and other interesting evidence of the possible presence of magnetoelectricity in this intriguing compound. [1] P.Mandal et al., Phys. Rev. B 82, 100416(R) (2010) [2] D.Delmonte et al., submitted (2013)