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 spon-taneous magnetic ordering and very fast and sensitive magneto-optical response. Furthermore, the col-loidal 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 [1-3]. 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 result-ing 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 na-noparticles to date. The colloids can be used in various fields, e.g., producing of durable magnetic re-cording media, electromagnetic wave shielding and magnetic force microscopy tips. Also, the nano-magnets 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. References 1. Stable colloidal solutions of strontium hexaferrite hard magnetic nanoparticles / L. A. Trusov, A. V. Vasiliev, M. R. Lukatskaya [et al.]. // Chemical Communications. – 2014. – 50. – С. 14581-14584. 2. Rotational dynamics of colloidal hexaferrite nanoplates / A. A. Eliseev, A. A. Eliseev, L. A. Trusov [et al.]. // Applied Physics Letters. – 2018. – 113. – 113106. 3. Tunable order in colloids of hard magnetic hexaferrite nanoplatelets / Ar. A. Eliseev, L. A. Trusov, E. O. Anokhin [et al.]. // Nano Research. – 2021. – URL: https://doi.org/10.1007/s12274-021-3572-z.