Scaling of quadratic and linear magneto-optic Kerr effect spectra with L2₁ ordering of Co₂MnSi Heusler compound

Spectral ellipsometry is a useful technique allowing fast, nondestructive, and contactless characterization of thin films and constituent materials. In this Letter, we show that both the linear magneto-optic Kerr effect (LinMOKE, proportional to the magnetization M) and the quadratic MOKE (QMOKE, proportional to $M2$⁠) can be a useful extension of spectral ellipsometry and are able to sense the crystallographic ordering of Heusler compounds. This is demonstrated for the Heusler compound Co₂MnSi, which has a crystallographic transition from a B2 to an L2₁ structure with increasing annealing temperature T_a. We investigated a set of Co₂MnSi thin films deposited on MgO(001) substrates and annealed from 300 to 500 °C. The amplitude of LinMOKE and QMOKE spectra, detected in the extended visible spectral range of 0.8–5.5 eV, scales linearly with T_a, and this effect is pronounced at the resonant peaks of the QMOKE spectra below 2.0 eV. Furthermore, the spectra of the magneto-optic (MO) parameters, which fully describe the MO response of Co₂MnSi up to the second order in M, are obtained depending on T_a. Finally, the spectra are compared with ab initio calculations of a purely L2₁-ordered Co₂MnSi Heusler compound.