Single-domain particles of manganese-for-iron substituted M-type barium hexaferrite: synthesis, crystal structure, and magnetic properties

Single-phase barium hexaferrite powders with crystallite sizes in a single-domain region and with the general composition BaFe$_{12-x}$Mn$_x$O$_{19}$, where $x$ = 0, 2, 4, 6, were synthesized applying a citric sol-gel auto-combustion technique with final annealing temperatures of 900 – 1200$^\circ$C. The crystal structures were refined, and the magnetic properties were studied. The observed variations in atomic positions with the Mn-for-Fe substitution revealed presence of Mn in three oxidation state +2, +3, and +4, with a preference of Mn$^{2+}$ to the tetrahedral 4f$_1$ site and Mn$^{4+}$ to the octahedral 2a and 12k sites. With the Mn-doping, the samples’ magnetization decreased, while coercivity increased and reached 8.4 kOe for $x$ = 6. The rise of the annealing temperature resulted in a slight growth of magnetization with a general tendency of the coercivity to decrease. A Curie temperature decreased with the Mn-doping remaining above room temperature for the maximal doping.