Towards room temperature ferromagnetism in multiferroic Bi-based double perovskites

Bismuth-based perovskites (as for example BiFeO3 and BiMnO3) are known to be magnetoelectric multiferroics. The term multiferroism indicates the coexistence of two or more primary ferroic orders (ferroelettricity, magnetism, ferroelasticty and ferrotoroidicity) in the matter. The possible fields for the application of this class of compounds include spintronics, data storage and micromechanics, with the intriguing possibility of exploiting the different degrees of freedom simultaneously. However, low magnetic ordering temperatures and low magnetoelectric coupling are still the principal limitations to technological applications for most of magnetoelectric multiferroics. Bi-based double perovskites are known to overcome some of these drawbacks: BiFeO3, for example, is a room temperature magnetoelectric multiferroic. As a consequence we have studied the double perovskites Bi2MMnO6 system (M= Fe, Co, Cu) in order to explore the effects of the partial substitution of the perovskitic B cation over the ferroic ordering temperatures and magnetoelectric coupling strength. The samples were synthesized in high pressure and high temperature conditions using a multianvil press, starting from the binary oxides. Powder X-Ray Diffraction and Transmission electron microscopy analysis reveal, for all the studied compounds, a distorted structure compared to the ideal simple perovskite, probably due to the stereochemical effect of the Bi3+ 6s2 lone pair. Bi2FeMnO6 and Bi2CoMnO6 structures crystallize with orthorhombic cell, the first with asymmetrical coordination and the latter with a I-centered symmetry, while Bi2CuMnO6 exhibits a monoclinic lattice, in close analogy with BiMnO3. Magnetic characterizations show in all cases a ferromagnetic nature of interactions, with large variations of the Curie Temperatures (275 K, 69 K and 323 K, respectively). Furthermore the M(H) curves confirm for Bi2CoMnO6 and Bi2FeMnO6 a ferromagnetic behavior, with coercive fields equal to 0.45 T and 0.1 T. In addition, measurements of the dielectric constant as a function of temperature reveal, for Bi2CuMnO6 and Bi2CoMnO6, the presence of anomalies matching the magnetic transition temperatures, suggesting that these compounds may be promising multiferroic materials, with the possible presence of room temperature magnetoelectric coupling.