Studies on the mechanism of the evolution of multiferroicity for the practical application

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This project has aimed at developing new multiferroics which can be used above room temperature. First target materials were ferroelectric cubic-BaTiO_3 (c-BTO) and hexagonal-BaTiO_3 (h-BTO). Single crystals of these materials were successfully grown by the conventional type floating zone furnace. The size of grown crystals was typically 5 mm in diameter and 50 mm in length. This is the first demonstration that such large sized crystals were obtained for h-BTO and c-BTO. Tuning the growth conditions including growth rate, growing speed, and atmosphere during the growth, we could also obtain the Fe-doped crystal with almost the same size with the pristine composition.Magnetic and dielectric measurements were carried out for the obtained single crystals, c-BTO-Fe showed a ferromagnetism and a ferroelectricity at room temperature. Careful analysis for M-H loop at various temperatures, Fe ion can take the mixed valence state of Fe_<3+> and Fe_<4+>. Heat capacity measurement revealed that F … More e_<3+> and Fe_<4+> ions take high spin and low spin state, respectively. However, the origin of the evolution of the ferromagnetism is not clear at the present time. On the contrary, h-BaTiO_3-Fe system gave consistent results. Fe_<3+> and Fe_<4+> ions take high spin state in Ti l site and low spin state in Ti2 site, respectively. Ferromagnetism originates from Fe_<4+> ions in Ti2 sites in the face-shared octahedral which adjoin each other, and other Fe_<3+> ion in Ti l site act as an isolated spin. Detailed measurements on M-H loop, magnetic susceptibility, and low temperature heat capacity revealed that ferromagnetism is coming from the paired Fe_<4+> ions in the low spin state. These results may give a hint to consider the mechanism of the evolution of ferromagnetism in c-BTO. Even in c-BTO, Fe ion tends to take mixed valence state of Fe_<3+> and Fe_<4+>. The crystal field may render the Fe_<4+> ions to take the low spin state even in the cubic phase. These results will serve as the complete understanding for so-called dilute magnet system, for which single crystal can not be obtained. The discovery of the real multeferroic crystal c-BTO will enhance the comprehensive understanding this field. Less