Direct measurement of the anisotropy field in CoMnGa thin films

High anisotropy MnGa-based ferrimagnetic thin films have received heavy attention recently as they possess high uniaxial anisotropy combined with low saturation magnetisation tuneable properties and low Gilbert damping [1-3]. These materials are therefore useful as both magnetic recording elements (due to their high anisotropy) as well as active elements in next generation spin-transfer torque devices (due to their high ferromagnetic resonance frequencies and low Gilbert damping).

Unfortunately, due to their rather high intrinsic anisotropies (up to several tens of Teslas) common measurement methods such as SQUID magnetometers and conventional ferromagnetic resonance techniques are in the worst case unsuitable for characterisation. The anisotropy field is usually characterised by extrapolation of the in-plane magnetisation curve beyond the limit of the measurement device, e.g. SQUID [2].

Here we use the techniques of high-field extraordinary Hall effect (EHE) up to 60 T, conventional SQUID (up to 7 T) and time-resolved magneto-optical Kerr effect (TR-MOKE) to characterise MnGa and CoMnGa thin films grown by sputtering on MgO (001) substrates. While conventional SQUID is used to extract the saturation magnetisation, EHE is employed in order to directly measure the magnitude of the in-plane anisotropy field and reconstruct the tilt angle of magnetisation with respect to the film normal. TR-MOKE is used to probe the high frequency response of these materials.

MnGa films (figure 1) are found to have a saturation magnetisation of 0.36 MA/m, coercivity of 0.25 T, an anisotropy field of 4.5 T, and anisotropy energy of +0.8 MJ/m³. From TR-MOKE, the precession frequency is determined to be of order 125-150 GHz and a Gilbert damping factor of 0.015.

CoMnGa (figure 2) films were found to have a saturation magnetisation of 0.2 MA/m, a coercivity of 1 T (figure 2 (a)), an anisotropy field of 18 T (figure 2 (b)) and an anisotropy energy of +1.8 MJ/m³. From the anisotropy field the estimated precession frequency is of order 500 GHz.

References:

[1] Balke B., et al., Appl. Phys. Lett, 90, 152504 (2007);
[2] Kurt H., et al., Phys. Rev. B, 83, 020405 (2011);
[3] Ma Q.L., et al., Appl. Phys. Lett. 101, 032402 (2012).