Gallium-substituted X-type hexagonal ferrites Sr2Co2GaxFe28−xO46: effect of substitution and heating temperature on phase formation and magnetic and dielectric properties

Abstract

Polycrystalline X-type hexagonal ferrites are investigated regarding phase formation at different temperatures and the effect of partial substitution of Ga³⁺ for Fe³⁺ ions in the octahedral and tetrahedral sites of the unit cell. The system of Sr₂Co₂Ga_xFe_28−xO₄₆ (x ranges from 0.0 to 2.0 in steps of 0.4) was synthesised by the sol–gel auto-combustion technique and calcined between 1200–1300 °C with a temperature of 1300 °C required to be able to obtain a pure X-phase. Lattice parameter a did not alter with partial substitution, suggesting minimum crystal distortion, while the unit cell volume shrank from 2488 to 2465 ų due to smaller size of Ga³⁺. Denser and more agglomerated grains were observed in the samples heated to 1300 °C. Soft magnetic behaviour was seen in all samples and the saturation magnetisation was 63–69 A m² kg⁻¹ for pure X-ferrites, with low coercivity between 8.5–9.5 kA m⁻¹ (106–119 Oe). Reasonably high H_a and K₁ values were observed due to a cone of magnetisation partially aligned along the c-axis, and lower H_C, H_a and K₁ values were seen for pure X-ferrites, decreasing monotonously with increasing gallium substitution. dM/dH curves for pure X-ferrites indicated one magnetic domain type with restricted domain dispersion, and superior exchange coupling with more gallium substitution. Mössbauer spectroscopy of samples heated to 1300 °C reveals Fe³⁺ ions in high spin state and probable occupancy of Ga³⁺ in the octahedral site a = a₁+a₂. Room temperature (RT) dielectric studies show a significant rise in dielectric constant at low frequencies <50 Hz and in ac conductivity with increasing gallium.