Multi-ion-substituted single-domain particles of M-type hexaferrites: synthesis, crystal structure, magnetic and microwave absorption properties.

该项目的目的是开发具有最大磁参数的新型六角铁氧体材料的制备路线。钡和锶M型六角铁氧体具有高的磁各向异性和足够的磁化强度，现在可以成功地用于各种技术领域的硬磁体。它们的晶体结构采用一定的阳离子取代，这在原则上允许调整它们的磁性以适应特定的应用。磁性可以是多方面增加的，例如，通过用Al替代Fe，保持单畴尺度上的晶粒尺寸，并且最近，用Al替代Fe和用Ca替代Sr，该项目的俄罗斯申请人在所有磁性材料中实现了250 GHz的创纪录的自然铁磁共振频率，并且在铁氧体中实现了40 kOe的最高磁性。然而，阳离子取代通常会导致自发磁化强度的降低和磁各向异性只能适度增强。与此同时，准确的预测的磁性受到阻碍的Fe置换的复杂过程，其中可能包括所有五个晶体Fe-网站，以及由掺杂阳离子的局部晶格畸变。毫无疑问，通过改变化学成分和适当控制材料的微观结构，有可能进一步提高磁参数。在目前的项目中，我们计划使用溶胶-凝胶技术进行化学合成，并制备均匀的扩展阳离子取代的六角铁氧体亚微米颗粒的集合体，使得每个颗粒仅包含一个磁畴。后者将提供最大的磁性材料的磁性，并允许明确的磁参数分析。六角铁氧体将被两种或更多种元素掺杂，以取代Fe和/或碱土金属，从而更强地影响磁性能并稳定结构。首先考虑的是阳离子取代，这将提供自旋-轨道耦合贡献的磁各向异性。晶体结构的研究将揭示掺杂阳离子的位置和配位几何。在一定的磁场和温度范围内的直流磁化将被测量和微波吸收将被分析。将进行量子化学计算来模拟所观察到的磁性行为并预测磁性。揭示了“组成-制备条件-结构-形貌-磁性”之间的关系。结果表明，六角铁氧体相的化学组成和制备条件可用于制备具有增强的磁各向异性、磁化率和铁磁共振频率以及更好的磁参数热稳定性的磁化良好的六角铁氧体材料。

The aim of the project is to develop preparation routes of new hexaferrite materials with maximized magnetic parameters. Barium and strontium M-type hexaferrites possess high magnetic anisotropy and sufficient magnetization to be successfully implemented now for hard magnets in various technological fields. Their crystal structure adopts certain cation substitution, which in principle allows adjusting their magnetic properties to specific applications. The coercivity can be manifolds increased, for example, by Al for Fe substitution keeping the grain size on a monodomain scale, and recently, replacing both, Al for Fe and Ca for Sr, the Russian applicant of the project has achieved a record natural ferromagnetic resonance frequency of 250 GHz among all magnetic materials and a highest among ferrites coercivity of 40 kOe. However, cation substitution usually leads to decrease of the spontaneous magnetization and the magnetic anisotropy can be only moderately enhanced. At the same time exact predictions of the magnetic properties are hindered by a complex process of the Fe replacement, which may include all five crystallographic Fe-sites, as well as by a local crystal lattice distortion by the doping cation. Definitely, there is a potential to further increase magnetic parameters by varying the chemical composition and properly controlling the material’s microstructure. In the present project, we plan to elaborate chemical syntheses using sol-gel technique and prepare ensembles of uniform submicron particles of extended-cation-substituted hexaferrite so that every particle contained only one magnetic domain. The latter will provide maximal coercivity of the magnetic material and allow explicit analysis of magnetic parameters. The hexaferrite will be doped by two or more elements substituting Fe and/or alkaline earth metal to stronger affect magnetic properties and to stabilize the structure. The cation substitution will be priory considered, which will afford spin-orbital coupling contributing to magnetic anisotropy. The crystal structure study will be performed to reveal the doping cations’ location and coordination geometry. The dc magnetization in a range of magnetic fields and temperatures will be measured and the microwave absorption will be analyzed. Quantum chemical calculations will be performed to model the observed magnetic behavior and to predict the magnetic properties. The relations “composition – preparation conditions – structure – morphology – magnetism” will be revealed. As the result, the chemical compositions of hexaferrite phase and the preparation conditions will be proposed applicable to create well-magnetized hexaferrite materials with enhanced magnetic anisotropy, coercivity, and ferromagnetic resonance frequency as well as with better thermal stability of the magnetic parameters.