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 (2a, 12k), disrupting the exchange coupling with the spin-down 4f 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 4e–12k and 4e–4f 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.