Direct measurement of the magnetic anisotropy field in Mn--Ga and Mn--Co--Ga Heusler films

TL;DR

This study measures the magnetic anisotropy field in Mn--Ga and Mn--Co--Ga Heusler films using various techniques, revealing high-frequency dynamics and large anisotropy energies relevant for magnetic applications.

Contribution

It provides the first direct measurement of the magnetic anisotropy field in Mn--Ga and Mn--Co--Ga Heusler films using SQUID, TR-MOKE, and pulsed magnetic fields.

Findings

Anisotropy field of 4.5 T in Mn--Ga films.

High ferromagnetic resonance frequency of 125 GHz.

Effective anisotropy energy density of 1.23 MJ/m^3 in Mn--Co--Ga films.

Abstract

The static and dynamic magnetic properties of tetragonally distorted Mn--Ga based alloys were investigated. Static properties are determined in magnetic fields up to 6.5~T using SQUID magnetometry. For the pure Mn$_{1.6}$Ga film, the saturation magnetisation is 0.36~MA/m and the coercivity is 0.29~T. Partial substitution of Mn by Co results in Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$. The saturation magnetisation of those films drops to 0.2~MA/m and the coercivity is increased to 1~T. Time-resolved magneto-optical Kerr effect (TR-MOKE) is used to probe the high-frequency dynamics of Mn--Ga. The ferromagnetic resonance frequency extrapolated to zero-field is found to be 125~GHz with a Gilbert damping, $\alpha$, of 0.019. The anisotropy field is determined from both SQUID and TR-MOKE to be 4.5~T, corresponding to an effective anisotropy density of 0.81~MJ/m$^3$. Given the large anisotropy field of the Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$ film, pulsed magnetic fields up to 60~T are used to determine the field strength required to saturate the film in the plane. For this, the extraordinary Hall effect was employed as a probe of the local magnetisation. By integrating the reconstructed in--plane magnetisation curve, the effective anisotropy energy density for Mn$_{2.6}$Co$_{0.3}$Ga$_{1.1}$ is determined to be 1.23~MJ/m$^3$.