Effect of titanium substitution and temperature variation on structure and magnetic state of barium hexaferrites

Highlights: • The range of solid solutions of Ti-doped Ba-hexaferrites was expanded to 2.00. • Ti-doped Ba-hexaferrites BaFe_12−xTi_xO₁₉ magnetic state interpretation was given. • Mechanism of occupation nonequivalent crystallographic positions by Ti was determined. • The spin-glass component of the magnetic phase state is fixed. • The critical magnetic field of the spin-glass component disappearance was found. -- Abstract: A number of solid solutions based on BaFe_12−xTi_xO₁₉ M-type barium hexaferrite doped with titanium cations up to x = 2.00 were obtained using conventional ceramic technology. The phase composition, crystal structure and unit cell parameters were refined by the Rietveld method using powder X-ray diffraction data up to T = 900 K. It was found that all the compositions have a magnetoplumbite structure satisfactorily described by P6₃/mmc space group (No. 194). With increasing temperature and doping concentration, the unit cell parameters increase almost monotonically. The minimum volume of V ~ 696.72 ų was determined for the composition with x = 1.00 at T = 100 K, while the maximum value of V ~ 714.00 ų is observed for the composition with x = 2.00 at T = 900 K. The mechanism of occupation nonequivalent crystallographic positions with titanium cations is established. The spin-glass component of the magnetic phase state is fixed. The T_dif temperature of the difference between the ZFC-FC curves decreases with an increase in the concentration of titanium cations and the magnetic field from ~237.2 K to ~ 44.5 K, while the T_inf inflection temperature of the ZFC curve increases from ~21.0 K to ~23.8 K. With an increase in the doping concentration, both the D_av average and D_max maximum clusters grow up to ~ 100 nm. As the magnetic field increases above the critical value, the spin-glass component disappears. For compositions with x > 1.00, the magnetization is not saturated in fields up to 6 T. Along with the formation of the spin-glass component, doping with titanium cations for barium hexaferrite lowers the T_C Curie temperature down to T ~600 K. The M_s spontaneous and M_r remanent magnetizations, as well as the B_c coercivity, decrease with increasing doping concentration almost monotonically, while the latter has an inflection point at x = 1.00. The minimum values of spontaneous and remanent magnetization, as well as coercivity, are observed for the composition with x = 2.00 and amount to M_s ~17.7 emu/g, M_r ~1.9 emu/g, and B_c ~3.9 × 10⁻³ T, respectively. An interpretation of the magnetic state of the doped BaFe_12−xTi_xO₁₉ barium hexaferrite is given taking into account the mechanism of occupation nonequivalent crystallographic positions with titanium cations.