Temperature dependent magnetic and structural properties of Al substituted nanostructured hexaferrites with large coercive fields

We report a comprehensive study of the temperature-dependent structural, magnetic, vibrational, and dielectric properties of Al-substituted M-type hexaferrites SrFe12-xAlxO19. Neutron powder diffraction and Mössbauer spectrometry show that Al3+ preferentially replaces Fe3+ at spin-up octahedral sites (2 a , 12 k ), disrupting the exchange coupling with the spin-down 4 f tetrahedral sites and leading to a progressive reduction of site-specific magnetic moments and a systematic decrease in the Curie temperature, supported by temperature-dependent susceptibility measurements. Raman spectroscopy reveals pronounced phonon anomalies near TC, particularly in modes associated with bipyramidal Fe–O vibrations, reflecting the weakening of both 4 e –12 k and 4 e –4 f exchange pathways. However, the coercive field exhibits a dramatic increase, reaching µ0HC ∼ 1.2 T for SrFe9.6Al2.4O19, among the largest values reported for this class. Susceptibility measurements suggest that Al substitution, while weakening the superexchange network, contributes to the stabilization of single-domain behavior.