Tuning the microstructure, magnetostatic and magnetodynamic properties of highly Al-substituted M-type Sr/Ca hexaferrites prepared by citrate-nitrate auto-combustion method

Double substitution of strontium hexaferrite by calcium and aluminum leads to a tremendous rise of hard magnetic properties, such as coercivity and natural ferromagnetic resonance frequency (NFMR). However, the properties are also inextricably linked to the material microstructure (especially, particle size), to the solid solution inhomogeneity as well as aluminum ions distribution among iron sites in crystal structure. In this work, we obtained M-type hexaferrite particles Sr(1-x/12)Ca(x/12)Fe(12-x)AlxO(19) (x = 4-6) via a facile citrate-nitrate auto-combustion method and studied the influence of the annealing temperature in a broad range on the microstructure, features of crystal structure and hard magnetic properties. At low annealing temperatures (900-1000 degrees C) hexaferrite nanoparticles with 90% of nominal Al content and a wide chemical distribution are formed. Next, with an increase in the annealing temperature the distribution significantly narrows, chemical composition becomes close to the nominal one and particles size transfer firstly to submicron, then to micron range. The aluminum distribution over iron sites is independent distinctly on the annealing temperature. For all the compositions single domain particles with the maximum coercivity values between 22.8 and 36 kOe are obtained at 1200 degrees C. At 900-1000 degrees C the samples demonstrate coercivities up to 25 kOe, while above 1300 degrees C, the crystallites begin to pass into a polydomain state with a reduced coercivity. The hexaferrites with narrow chemical distribution reveal resonance absorption in sub-terahertz band. The highest NFMR frequency of 270 GHz was observed for x = 5.5 sample annealed at 1400 degrees C.

Automated summary

Double substitution of strontium hexaferrite by calcium and aluminum greatly enhances its hard magnetic properties, but these properties are also closely related to the material microstructure, solid solution inhomogeneity, and aluminum ions distribution. In this study, M-type hexaferrite particles with varying aluminum content were synthesized through a simple method and the effects of annealing temperature on microstructure, crystal structure features, and hard magnetic properties were investigated. The annealing temperature influenced the particle size, chemical composition, and coercivity. The hexaferrites with narrow chemical distribution exhibited resonance absorption in the sub-terahertz band, with the highest NFMR frequency observed at x = 5.5 sample annealed at 1400°C.