Creation and Magnetic Study of Ferrites with Magnetoplumbite Structure Multisubstituted by Al, Cr, Ga, and In Cations.

Multisubstituted barium ferrites with a magnetoplumbite structure were obtained by the method of solid-phase reactions with ferritization and pre-firing. Three-charged, mainly diamagnetic cations Al, Cr, Ga, and In were chosen as substituents for the Fe iron cations, the proportion of which in solid solutions did not exceed 50%. The values of the configurational mixing entropy were calculated for all the compositions. A slight deviation of the chemical composition of the obtained solid solutions from the nominal value was established by the energy-dispersive X-ray spectroscopy method. The phase purity and values of the unit cell parameters were refined from X-ray scattering data using full-profile analysis in the Rietveld method. A non-monotonic behavior of the unit cell parameters as a function of the B-sub-lattice average ionic radius of the magnetoplumbite structure was found. A minimum unit cell volume of ~667.15 Å was found for the composition BaFeAlCrGaO with a B-sub-lattice average ionic radius of ~7.449 Å. The average crystallite size varied within 5.5-6.5 μm. The temperature and field dependencies of the magnetization have been measured. The values of the saturation magnetization, residual magnetization, hysteresis loop squareness, and coercivity at 50 K and 300 K were extracted from the experimental data. Using the Law of Approach to Saturation, the magnetic crystallographic anisotropy coefficient and anisotropy field were calculated. Multisubstitution leads to a significant decrease in such magnetic parameters as the magnetic ordering temperature and spontaneous magnetization at both temperatures. The maximum magnetic ordering temperature of ~297.7 K was found for the composition BaFeGaO with a B-sub-lattice average ionic radius of ~7.586 Å in a field of 500 Oe. A maximum saturation magnetization of ~24.7 emu/g was found for the composition BaFeGaO with a B-sub-lattice average ionic radius of ~7.586 Å at 50 K. A maximum hysteresis loop squareness of ~0.72 was found for the composition BaFeAlCrGaO with an average ionic radius of ~7.449 Å at 50 K. A maximum magnetic crystallographic anisotropy coefficient of ~2.09 × 10 Erg/g was found for the composition BaFeAlCrGaInO with a B-sub-lattice average ionic radius of ~7.706 Å at 50 K. The frustrated magnetic state including the nano-sized clusters with an average diameter in the range of 50-200 nm was established from the results of measuring the ZFC and FC temperature magnetizations. The interpretation of the obtained experimental data is carried out taking into account the increased stability of high-entropy phases and regular changes in the intensity of the Fe(Al, Cr, Ga, In)-O-Fe(Al, Cr, Ga, In) indirect superexchange interactions as a result of magnetic dilution of the iron sub-lattice in the magnetoplumbite structure.