Effect of coercive aluminium on the structural and magnetic characteristics of calcium and magnesium based hexaferrites

Aluminum substituted Calcium-Magnesium (Ca-Mg) hexaferrites having general formula Ca_0.5Mg_0.5Fe_12-xAl_xO₁₉ (where x is increased with increment of 0.5 from 0.0 to 2.0) are prepared by sol–gel auto combustion technique. X-ray diffraction patterns confirm the hexagonal structure with no evidence of secondary phases. The reduction in lattice parameters is observed as the Al⁺³ doping level increases. This can be attributed to the short ionic radii of Al⁺³ ions (0.535 Å) compared to Fe⁺³ ions (0.645 Å). The obtained c/a ratio falls within the acceptable range of 3.98. Consequently, the formation of magnetoplumbite structure is also confirmed. The SEM image reveals the uniform distribution of particles. The exchange of Fe⁺³ ions with coercive Al⁺³ ions led to a decline in saturation magnetization Ms and a significant increase in the coercivity Hc. The magnetic moment m_B and Mr of hexaferrites exhibit a decline when Al ions are introduced. This reduction can be ascribed to several factors: the substitution of non-magnetic Al³⁺ ions in place of Fe³⁺ ions, the spin-canted ordering of Fe³⁺ ions, and the valency of Fe ions from 3+ to 2+.