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

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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.