Structural and magnetic properties of Sr0.8La0.2Co0.2Fe11.8−xAlxO19 hexaferrite particles prepared via sol-gel auto-combustion method

M-type hexaferrite particles with chemical composition of Sr0.8La0.2Co0.2Fe11.8−xAlxO19 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared using sol-gel auto-combustion technique followed by a heat treatment. X-ray diffraction patterns showed the presence of single hexaferrite phase with no detection of other phases for all samples. The atomic ratios obtained from energy dispersive X-ray spectroscopy were close to the initial stoichiometric ratios. The observation of the particles achieved using a field emission scanning electron microscope depicted a transition from multi-domain particles to single domain particles due to Al substitution. The room temperature Mössbauer spectral analysis demonstrated that Al3+ ions preferred 12k and 2a sites. DSC analysis showed that the Curie temperature (TC) increased firstly to a maximum value of 707 K at x = 0.2, and then decreased with further increasing Al content. The room temperature MS and Mr values decreased linearly with Al substitution while HC increased rapidly to a maximum value of 9390.2 Oe at x = 1.0, which was ∼47% increase in coercivity as compared to that of Sr0.8La0.2Co0.2Fe11.8O19. The magnetic results indicated that a small amount of Al substitution in La-Co co-substituted Sr-ferrites can obviously improve coercivity with relatively high saturation magnetization.