ИСТИНА

Exchange-coupled hard/soft ferrite nanoparticles are prospective to squeeze out a part of expensive magnets based on rare-earth elements. However, known exchange-coupled composite ferrite nanoparticles often suffer from a lack of a powerful enough hard-magnetic core, a high defectivity of magnetic phases, and a poor interface between them. Herein we demonstrate the first efficient synthesis of sandwiched nanomagnets, which exhibit the pronounced exchange-coupling effect. This work is featured by using individual high-coercive strontium hexaferrite nanoplates prepared by a borate glass crystallization method as cores for the composite particles. High crystal quality of the hexaferrite cores as a substrate provides an epitaxial growth of CoFe2O4 layers on the 001 facets from the organic high-boiling solvent and results in enhancing the remanence magnetization and the maximum energy product of the composite material. The obtained composite nanoparticles are characterized by well-defined epitaxial interfaces between the constituent phases. The exchange coupling results in strongly different magnetic properties compared to ones expected for a mechanical mixture of hexaferrite and cobalt ferrite. At room temperature, the magnetic properties are determined by the uniaxial magnetocrystalline anisotropy of the hexaferrite, while upon cooling the situation changes to the triaxial magnetocrystalline anisotropy typical for the spinel cobalt oxide. In the temperature range 5 – 300 K the composite shows significant improvement of the (BH)max values of both initial phases. The results of this work open new prospects for fabrication of multilayer oxide heterostructures with a synergetic performance which expands applications of exchange-coupled composites. The research was supported by Russian Science Foundation grant no 20-73-10129