Searching for new multiferroics: Ba12Fe28Ti15O84 intergrowth layered ferrite

The BaO-Fe2O3-TiO2 system has rich crystal chemistry and the existence of at least 16 different quaternary compounds was reported. These materials are nowadays attracting considerable interest for the production of permanent magnets and microwave devices, especially due to their high permittivity. Recently, ceramic quaternary ferrites Ba12Fe28Ti15O84 were prepared for the first time. Ba12Fe28Ti15O84 possesses a natural superlattice structure (monoclinic C2/m), constituted by perovskite/spinel-like layers. The aim of the present study is to investigate the magnetic properties, the possible ferroelectric behaviour and to explain theoretically the observed properties. The magnetic study determined a ferro/ferrimagnetic order similar to that of spinel ferrites with a main magnetic transition at ~415 K and the disappearance of magnetization above 500 K. The dielectric properties are influenced by polaron hopping conductivity associated to the Fe2+/Fe3+ valence state variations and to the electrical heterogeneity of the ceramics (semiconducting grains, insulating grain boundaries). The local ferroelectricity tested by AFM-piezoresponse experiments revealed local incomplete poling and switching with clockwise loops in various positions. On the basis of a Monte Carlo model built for a ferroelectric/nonferroelectric layered system, local incomplete poling and switching with abnormal clockwise loops were obtained in specific positions close to the interfaces. Although a typical ferroelectric behavior was not clearly demonstrated, the observed properties might result from a combination of BaTiO3-like ferroelectric order within perovskite layers with non-ferroelectric behavior of the spinel layers and charge defect associated effects.