Magnetic state of a system of barium hexaferrite nanocrystals in the vicinity of the curie temperature

In order to study the magnetic state of a system of nanocrystals of highly anisotropic hexagonal barium ferrite in the vicinity of the Curie temperature, a macrocrystal surface layer comparable in thickness to the particles of the system is chosen as a model. As revealed by simultaneous application of gamma-, x-ray, and electron Mössbauer spectroscopy, the transition of a ～200-nm-thick surface layer to the paramagnetic state starts 55 K and is complete 3 K below the Curie temperature of the bulk of the crystal. This temperature range overlaps the range where the particles transfer from the magnetically stable to the superparamagnetic state. The data obtained made it possible to refine the high-temperature part of the H-T diagram for particles with a close-to-critical volume. The regions of the diagram where mixed magnetic states originating simultaneously from the size and surface factors coexist are identified.