Theory of magnetic enhancement in strontium hexaferrite through Zn-Sn pair substitution

TL;DR

This study uses first-principles calculations to identify the preferred site occupancy of Zn-Sn ions in Sr-hexaferrite, revealing a configuration that enhances saturation magnetization and aligns with experimental observations.

Contribution

The paper presents a new model for Zn-Sn site occupancy in Sr-hexaferrite that better explains magnetic enhancement, differing from previous models.

Findings

Zn-Sn ions prefer 4f1 and 4f2 sites in ground state

The new model has lower total energy than previous models

Predicts increased saturation magnetization and decreased anisotropy

Abstract

We study the site occupancy and magnetic properties of Zn-Sn substituted M-type Sr-hexaferrite SrFe$_{12-x}$(Zn$_{0.5}$Sn$_{0.5}$)$_x$O$_{19}$ with x = 1 using first-principles total-energy calculations. We find that in a ground-state configuration Zn-Sn ions preferentially occupy $4f_1$ and $4f_2$ sites unlike the model previously suggested by Ghasemi et al. [J. Appl. Phys, \textbf{107}, 09A734 (2010)], where Zn$^{2+}$ and Sn$^{4+}$ ions occupy the $2b$ and $4f_2$ sites. Density-functional theory calculations show that our model has a lower total energy by more than 0.2 eV per unit cell compared to Ghasemi's model. More importantly, the latter does not show an increase in saturation magnetization ($M_s$) compared to the pure $M$-type Sr-hexaferrite, in disagreement with the experiment. On the other hand, our model correctly predicts a rapid increase in $M_s$ as well as a decrease in magnetic anisotropy compared to the pure $M$-type Sr-hexaferrite, consistent with experimental measurements.