Improving the enormous coercivity in Ba0.54Ca0.46Fe6.5Al5.5O19 M-type hexaferrite via annealing treatment, prepared by sol-gel auto-combustion method

Abstract

Improved rare-earth-free M-type hexaferrite powders with the formula ${\text{Ba}}_{0.54}{\text{Ca}}_{0.46}{\text{Fe}}_{6.5}{\text{Al}}_{5.5}{O}_{19}$ were synthesized via the sol-gel auto-combustion method. The samples were annealed between 1100 °C and 1250 °C with a 50 °C step. The effects of annealing treatment on the structural, microstructural and magnetic properties were studied. X-ray diffraction analysis show ${\text{Ba}}_{0.54}{\text{Ca}}_{0.46}{\text{Fe}}_{6.5}{\text{Al}}_{5.5}{O}_{19}$ as major phase with impurity for the calcination temperatures below 1250 °C and single phase at 1250 °C. The average crystal size increases (50.23–74.14 nm) with the increase in the annealing temperature. The surface morphology was examined using field emission scanning electron microscopy, an average grain size was found to increase from 0.58 to 0.97 μm. ${\text{Ba}}_{0.54}{\text{Ca}}_{0.46}{\text{Fe}}_{6.5}{\text{Al}}_{5.5}{O}_{19}$ powders showed an enormous coercivity ($H_c$) that increased from a value of 21.16 kOe to 24.18 kOe with increasing annealing temperature from 1100 °C to 1250 °C. Various magnetic parameters were estimated including the anisotropy field ($H_a$) and the effective magnetic anisotropy constant ($K_{eff}$) which permitted us to understand the effect of annealing treatment on the values of $H_c$. The obtained $H_c$ values in ${\text{Ba}}_{0.54}{\text{Ca}}_{0.46}{\text{Fe}}_{6.5}{\text{Al}}_{5.5}{O}_{19}$ powders demonstrate a significant potential for the high-density recording applications, thereby presenting a cost-effective alternative to expensive rare-earth materials.