Temperature Dependent Magnetic and Structural Properties of Al Substituted Nanostructured Ferrites with Large Coercive Fields

TL;DR

This study investigates how aluminum substitution affects the structural, magnetic, vibrational, and dielectric properties of SrFe12-xAlxO19 ferrites across temperatures, revealing increased coercivity and site-specific magnetic changes.

Contribution

It provides new insights into the temperature-dependent magnetic and structural behavior of Al-substituted ferrites, highlighting the enhancement of coercive fields and site-specific magnetic modifications.

Findings

Al substitution reduces Curie temperature and magnetic moments.

Pronounced phonon anomalies occur near the Curie temperature.

Coercive field reaches up to 1.2 T, among the highest for this class.

Abstract

We report a comprehensive study of the temperature-dependent structural, magnetic, vibrational, and dielectric properties of Al-substituted M-type hexaferrites SrFe$_{12-x}$Al$_x$O$_{19}$. Neutron powder diffraction and M\"ossbauer spectrometry show that Al$^{3+}$ preferentially replaces Fe$^{3+}$ 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 $T_C$, 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 $\mu_0H_C$ $\sim$ 1.2 T for SrFe$_{9.6}$Al$_{2.4}$O$_{19}$, 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.