W型六角铁氧体的可控制备与磁电特性研究

Owing to their high magnetization, large magnetocrystalline anisotropy, excellent chemical stability, W-type haxeferrites (BaMe2Fe16O27, Me2W) have received considerable interests in the field of permanent magnet, magnetic refrigeration as well as microwave devices. Compared with the most studied M-type hexaferrites (BaFe12O19, BaM), the magnetic properties of W-type ferrites can be highly improved and widely changed by replacing the trivalent or the divalent cation sites. However, successful fabrication of W-type hexaferrites has always been a challenge, owing to their complex crystalline structure. Specifically, there have been much fewer studies on Fe2W, due to the instability of the Fe2+ ions, which tend to be oxidized to Fe3+ during the high-temperature fabrication process. In recent years, a few studies have adopted the method of calculation to predict the physical properties of Fe2W, but little experimental work has been conducted to verify these predictions. In addition, some exotic physical phenomena observed on W-type hexaferrites remain elusive. For instance, the nature and the origin of the composition variation induced spin reorientation transition in Fe2W are yet to be resolved. Therefore, this project focuses on Fe2W-related hexaferrite magnetic materials. We will synthesize polycrystalline and single crystal W-type hexaferrite samples, and reveal their electrical transport properties by the measurements of the electrical parameters, direct and alternating current resistivities as an example. Substituting part of divalent iron ions by cobalt, spin reorientation transition will take place in Fe2-xCoxW. And by integrating various strategies involving microstructure characterization, magnetic structure analysis, and magnetic property measurements, it is expected that the relationship between magnetic properties and structures will be established.

W型六角铁氧体具有较大的饱和磁化强度、磁晶各向异性，以及稳定的化学性能，且相比主流的M型铁氧体能进行多种阳离子替换进而广泛地调控磁性，因此在永磁体、磁制冷以及微波器件等多个领域具有广阔的应用前景，引起了人们极大的研究兴趣。然而，W型铁氧体结构相对复杂，较难合成，尤其是最早被发现却较少被研究的BaFe2Fe16O27（Fe2W），目前多是采用计算的方法对其磁电性能进行预测，而鲜有实验上的验证。此外，W型铁氧体所表现出的成分诱导的自旋重取向等有趣的物理现象也尚未得到深入的理解。因此，本项目拟以Fe2W基六角铁氧体为研究对象，制备高质量样品；通过交直流电阻等电学参数的测试，提出不同温度下的导电模型，并初步表征材料的电输运特性；通过Co元素掺杂，诱导其自旋重取向相变，结合对Fe2-xCoxW体系的晶体结构/磁结构的精细表征和磁学性质测试，系统研究成分诱导的磁相变机理，建立磁性能与结构的构效关系。

六角铁氧体是一种传统的磁性材料，价格低廉，应用广泛。近些年来，随着螺旋磁有序诱导的多铁性等新物理现象在该体系的发现，该体系引起了新一轮的研究兴趣。然而，六角铁氧体的晶体结构非常复杂，较难合成。尤其是对于Fe2W（W型六角铁氧体BaFe2Fe16O27）等材料，二价、三价铁元素的同时存在更是进一步增大了制备难度。因而，目前关于它们的研究非常有限，尚缺乏对其磁学等基本物理性能的系统表征和分析。针对这一问题，本项目系统研究了Fe2W的可控制备以及Co元素掺杂对其磁、电性能的影响及作用机理，并在此基础上成功合成并表征了Fe2Y（Ba2Fe2Fe12O22）Y型六角铁氧体陶瓷材料。取得的主要研究成果包括：采用通入氮气气氛高温烧结的办法，制备了传统工艺难以获得的二价和三价铁元素共存的Fe2Y和Fe2W六角铁氧体陶瓷材料；分析了Fe2Y在50K等温度附近交、直流磁特性的异常变化，通过交、直流磁化以及热剩磁曲线的测量和分析，给出了低温区自旋玻璃态相变的存在证据；系统研究了Co元素掺杂对Fe2W磁、电性能的影响及作用机理，分析了Fe2-xCoxW零场冷ZFC磁化曲线在20K附近的陡变现象，阐明了该低温突变并非Verwey转变，而属于自旋玻璃态转变；给出了Fe2-xCoxW在40—400K温度范围变程跃迁的导电性规律，分析了Co元素掺杂对Fe2-xCoxW陶瓷导电性能的抑制作用。本项目的研究工作和成果对于六角铁氧体的基础研究方面具有一定的参考意义和价值，也为进一步探索Fe2W等材料体系新奇的物理性能提供了实验基础。