Site occupancy and magnetic properties of Al-substituted M-type strontium hexaferrite

TL;DR

This study uses first-principles calculations to analyze how aluminum substitution affects the site occupancy and magnetic properties of strontium hexaferrite, revealing site preferences, effects on magnetization, and anisotropy.

Contribution

It provides detailed insights into Al site preferences and their impact on magnetic properties, which was not previously well understood.

Findings

Al$^{3+}$ ions replace Fe$^{3+}$ at specific sites reducing magnetization.

Increased Al raises magnetic anisotropy field.

Temperature influences Al site occupancy, affecting magnetic behavior.

Abstract

We use first-principles total-energy calculations based on density functional theory to study the site occupancy and magnetic properties of Al-substituted $M$-type strontium hexaferrite SrFe$_{12-x}$Al$_{x}$O$_{19}$ with $x=0.5$ and $x=1.0$. We find that the non-magnetic Al$^{3+}$ ions preferentially replace Fe$^{3+}$ ions at two of the majority spin sites, $2a$ and $12k$, eliminating their positive contribution to the total magnetization causing the saturation magnetization $M_s$ to be reduced as Al concentration $x$ is increased. Our formation probability analysis further provides the explanation for increased magnetic anisotropy field when the fraction of Al is increased. Although Al$^{3+}$ ions preferentially occupy the $2a$ sites at a low temperature, the occupation probability of the $12k$ site increases with the rise of the temperature. At a typical annealing temperature ($> 700\,^{\circ}{\rm C}$) Al$^{3+}$ ions are much more likely to occupy the $12k$ site than the $2a$ site. Although this causes the magnetocrystalline anisotropy $K_1$ to be reduced slightly, the reduction in $M_s$ is much more significant. Their combined effect causes the anisotropy field $H_a$ to increase as the fraction of Al is increased, consistent with recent experimental measurements.