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

该项目旨在开发可在室温以上使用的新型多铁材料。第一个目标材料是铁电立方- batio_3 （c-BTO）和六边形- batio_3 （h-BTO）。在传统的浮区炉上成功地生长了这些材料的单晶。生长的晶体通常直径为5毫米，长度为50毫米。这是首次证明h-BTO和c-BTO可以获得如此大尺寸的晶体。在生长过程中，通过调整生长速率、生长速度、生长气氛等生长条件，我们也可以得到与原始成分几乎相同尺寸的掺铁晶体。对得到的单晶进行了磁性和介电测量，c-BTO-Fe在室温下表现出铁磁性和铁电性。仔细分析M-H环在不同温度下，Fe离子可以呈现Fe_<3+>和Fe_<4+>的混合价态。热容测量结果表明，Fe_< 3+>和Fe_<4+>离子分别处于高自旋和低自旋状态。然而，铁磁性演化的起源目前尚不清楚。相反，h-BaTiO_3-Fe体系给出了一致的结果。Fe_<3+>和Fe_<4+>离子分别在ti1位处于高自旋态和Ti2位处于低自旋态。铁磁性来源于面共享八面体中Ti2位的Fe_<4+>离子相互连接，而ti1位的Fe_<3+>离子起孤立自旋作用。对M-H环、磁化率和低温热容量的详细测量表明，铁磁性来自于低自旋态的成对Fe_<4+>离子。这些结果为研究c-BTO中铁磁性的演化机制提供了线索。即使在c-BTO中，Fe离子也倾向于呈现Fe_<3+>和Fe_<4+>的混合价态。晶体场可以使Fe_<4+>离子在立方相中也处于低自旋态。这些结果将有助于对无法获得单晶的所谓稀磁体系的完全理解。真正的多铁性晶体c-BTO的发现将增强对这一领域的全面认识。少

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