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

The static and dynamic properties of tetragonally distorted Mn–Ga based alloys were investigated. Static magnetic properties are determined in magnetic fields up to 6.5 T using SQUID magnetometry. For the pure Mn1.6Ga film, the saturation magnetisation is 0.36 MA/m and the coercivity is 0.29 T. Partial substitution of Mn by Co results in Mn2.6Co0.3Ga1.1. 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/m3. Given the large anisotropy field of the Mn2.6Co0.3Ga1.1 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 Mn2.6Co0.3Ga1.1 is determined to be 1.23 MJ/m3.