Field dependent giant reversal of magnetization in Bi2FeMnO6, an RT multiferroic double Perovskite

We have recently synthesized Bi2FeMnO6, 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, suggesting the presence of antiferroelectricity. The magnetic characterization indicates an unconventional magnetic behavior producing a complex ferrimagnetic transition around 500 K followed, at lower temperatures, by a field-dependent giant reversal of the magnetization. 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 2700 Oe. The reversal of magnetization is usually interpreted as the competition of DM interactions with single-ion anisotropy [1], however our studies show a completely different nature of this complex phenomenon. Indeed Fe-57 Mossbauer spectroscopy revealed the presence of iron ions with dramatically different behavior: a little part of them being ordered parallel to the applied field at RT, and a large population of paramagnetic ones that gradually align antiferromagnetically with respect to the first class of ions as the temperature is decreased. As a result, in absence of a sufficiently large applied field, the magnetization goes down to negative values below 250 K. Moreover, the study of the electric permittivity as a function of temperature reveals the presence of anomalies in concomitance with the start of the magnetization reversal process, suggesting the possibility of magnetoelectricity in Bi2FeMnO6. [1] P.Mandal et al., Phys. Rev. B 82, 100416(R) (2010)