ИСТИНА

Stable colloids of magnetically hard hexaferrite nanoparticles attract a lot of attention today. In contrast to conventional ferrofluids of superparamagnetic nanoparticles each hexaferrite particle carries a high permanent magnetic moment and acts as a tiny magnet whose position and orientation can be controlled by external magnetic fields. This results in a plenty of distinctive properties including spontaneous magnetic ordering and very fast and sensitive magneto-optical response. Furthermore, the colloidal particles are a good basis for creation of various magnetic composites, e.g., exchange-coupled ones, and they are convenient building blocks for creating of various magnetic nanostructures, films, and coatings. In this work, we describe a strategy for producing the colloidal hexaferrite nanoparticles with record-high coercivity. The method is based on the glass-crystallization route, in which the magnetic particles are formed within the borate glass matrix during the devitrification. We describe the way to obtain chromium-substituted nanoparticles, which can be further extracted and stabilized in aqueous colloids. The diameter of the colloidal particles can be tuned in the range of 20 – 80 nm, and the resulting coercivity varies from 4200 to 9800 Oe, respectively. The saturation magnetization of the particles is 30 – 40 emu/g. The reported values correspond to the highest coercivity/size ratio for hexaferrite nanoparticles to date. The colloids can be used in various fields, e.g., producing of durable magnetic recording media, electromagnetic wave shielding and magnetic force microscopy tips. Also, the nanomagnets are required for magneto-mechanical microsystems and magnetically self-assembled nanostructures. This work was funded by the Russian Science Foundation, Grant No. 20-73-10129.