Containerless processing of metastable multiferroic composite in Ln-(Mn,Fe)-O system (Ln: Lanthanide)

It has been known that the hexagonal structure with a space group of P6₃cm is stable in the LnMnO₃ system (Ln: Lanthanoid) having rare-earth elements with smaller ionic radii than that of Gd³⁺. The hexagonal LnMnO₃ (h-LnMnO₃) that shows multiferroic properties such as ferroelectricity and ferromagnetism in one phase has attracted great interest in the field of electronic industry. Nevertheless, since h-LnMnO₃ shows antiferromagnetism as well as low magnetic transition temperature below 100 K, no materials for practical applications have been developed. As one of the solutions to this problem, a composite material composed of nano-sized h-LnFeO₃ and ferromagnetic phases is expected. Based on this idea, we tried to synthesize multiferroic composites consisting of h-LuFeO₃ and Fe₃O₄ by containerless processing technique. However, the results showed that not only the composite structure of h-LuFeO₃ and Fe₃O₄ but also orthorhombic phase (o-LuFeO₃) and Fe₂O₃ were identified. These results suggest that it is necessary to improve the stability of h-LnFeO₃ and suppress the chemical reaction from Fe₃O₄ to Fe₂O₃. From this point, we tested samples in which one-third of Fe ions was substituted by Mn ions. As a result, a fine composite structure composed of LuMn_1/3Fe_2/3O₃ and MnFe₂O₄ was obtained.