Magnetocrystalline anisotropy of the easy-plane metallic antiferromagnet Fe$_2$As

TL;DR

This study combines experimental torque magnetometry and density functional theory to quantify the magnetocrystalline anisotropy of Fe$_2$As, revealing extremely small in-plane anisotropy and temperature dependence relevant for spintronic applications.

Contribution

It provides the first combined experimental and theoretical analysis of Fe$_2$As's magnetocrystalline anisotropy, highlighting the small in-plane anisotropy and its temperature dependence.

Findings

In-plane anisotropy ${K_{22}}$ is very small and temperature-dependent.

The out-of-plane anisotropy ${K_1}$ is large and calculated via DFT.

The antiferromagnetic resonance mode is predicted at 670 GHz with in-plane polarization.

Abstract

Magnetocrystalline anisotropy is a fundamental property of magnetic materials that determines the dynamics of magnetic precession, the frequency of spin waves, the thermal stability of magnetic domains, and the efficiency of spintronic devices. We combine torque magnetometry and density functional theory calculations to determine the magnetocrystalline anisotropy of the metallic antiferromagnet Fe$_2$As. Fe$_2$As has a tetragonal crystal structure with the N\'eel vector lying in the (001) plane. We report that the four-fold magnetocrystalline anisotropy in the (001)-plane of Fe$_2$As is extremely small, ${K_{22}} = - 150~{\rm{ J/}}{{\rm{m}}^{\rm{3}}}$ at T = 4 K, much smaller than perpendicular magnetic anisotropy of ferromagnetic structure widely used in spintronics device. ${K_{22}}$ is strongly temperature dependent and close to zero at T > 150 K. The anisotropy ${K_1}$ in the (010) plane is too large to be measured by torque magnetometry and we determine ${K_1} = -830~{\rm{ kJ/}}{{\rm{m}}^{\rm{3}}}$ using first-principles density functional theory. Our simulations show that the contribution to the anisotropy from classical magnetic dipole-dipole interactions is comparable to the contribution from spin-orbit coupling. The calculated four-fold anisotropy in the (001) plane ${K_{22}}$ ranges from $- 292~{\rm{ J/}}{{\rm{m}}^{\rm{3}}}$ to $280~{\rm{ J/}}{{\rm{m}}^{\rm{3}}}$, the same order of magnitude as the measured value. We use ${K_1}$ from theory to predict the frequency and polarization of the lowest frequency antiferromagnetic resonance mode and find that the mode is linearly polarized in the (001)-plane with $f = $ 670 GHz.