Y-型和Z-型六角铁氧体中反常磁电效应机制的研究

Hexaferrites have been considered as one of the most promising multiferroic materials，because of its giant magnetoelectric coupling coefficients and the room temperature magnetoelectric effects. In previous research references, it has been found that the polarizations of Y-type hexaferrite can be reversed by magnetic field while the direction of polarizations in Z-type hexaferrite cannot be reversed. The physical mechanism behind those phenomena have been well explained and understood. However, anomalous magnetoelectric effects have recently been reported in some Y-type hexaferrites and Z-type hexaferrites: polarization in the Y-type hexaferrite cannot be reversed while that in the Z-type hexaferrite can be reversed by magnetic field. The physical mechanism behind these anomalous magnetoelectric effects have not been clarified so far. This project intends to study such anomalous magnetoelectric effects in Y-type and Z-type hexaferrite. By systematically measuring M-H, magnetodielectric, magnetocurrent behaviors and neutron diffractions for Y-type and Z-type hexaferrite single crystals, the magnetoelectric phase diagrams for Y-type and Z-type hexaferrite can be clearly determined. According to the magnetoelectric phase diagrams, the detailed magnetic structure evolution during the anomalous magnetoelectric behavior processes can be inferred. Combining with the origin of electric polarizations, the mechanisms for anomalous magnetoelectric effects in Y-type and Z-type hexaferrites can be revealed，which will improve our understandings of the magnetoelectric effects in hexaferrites.

六角铁氧体因具有巨大的室温磁电效应而被认为是最有潜力的单相多铁材料之一。人们发现，大多数Y-型六角铁氧体中的电极化可以被磁场翻转，而Z-型六角铁氧体中电极化方向却不能被翻转。该现象背后的物理机制也已被人们所理解。然而，在极少数的六角铁氧体中人们发现了反常的磁电效应： Y-型六角铁氧体中的电极化不能被磁场翻转而Z-型中的电极化可被磁场翻转。这种反常现象到目前为止并不能被人们理解。本项目以具有反常磁电效应的Y-型和Z型六角铁氧体为研究对象，通过系统地研究单晶样品的磁性、磁介电、磁释电以及单晶中子衍射试验绘制出两种样品的磁电相图。根据磁电相图，推测出两种六角铁氧体在表现反常磁电效应时磁结构的具体演变过程，并结合其电极化产生的机制分别给出Y型和Z-型六角铁氧体反常磁电效应的物理原因，从而完善人们对六角铁氧体体系磁电效应的理解。

本项目主要是Y-型和Z-型六角铁氧体的反常磁电耦合行为研究。六角铁氧体是目前为止发现具有最大磁电耦合效应的单相多铁性材料，并且在室温下仍具有磁电耦合效应，研究其背后的物理机制的意义不仅限于学术本身，对发现具有应用潜力的单相多铁材料同样具有重要的指导意义。我们生长了多种具有磁电耦合效应的Y-型和Z-型六角铁氧体。在Y-型六角铁氧体中，如Co2-Y型六角铁氧体中，发现当Sr2+的含量增加时，样品的磁基态会从正常的纵向锥型磁有序演变为c分量具有反铁磁排列的纵向锥型磁有序。随着温度的升高，当面内磁场降为零时，样品的横向锥型磁有序变为反常纵向锥型磁有序再变为横向锥型磁有序，随之，电极化的方向将不再翻转，即反铁磁的纵向锥型磁有序在反常磁电行为中扮演着重要的作用。.其次，我们发展了高压下的磁电耦合测试技术，制备了Mg2-Y六角铁氧体单晶，描绘出了Mg2-Y在压力-温度-磁场下的磁电耦合相图。.最后我们生长了多种高质量的Z-型六角铁氧体，在Z-型六角铁氧体中，发现Sr-Z型六角铁氧体在轴向压力下电极化被磁场翻转的反常现象。