Magnetoreflectivity study of excitons in molecular-beam epitaxially grown Zn, _ _xFexSe crystals

Single-crystal films of (001) Zn_1xFe_xSe(x = 0.017, 0.027, and 0.043) grown by molecular-beam epitaxy on (001) GaAs have been studied by magnetoreflectivity over the temperature range 4–77 K in magnetic fields up to 8 T in the spectral region near the band gap of 2.8 eV. The excitonic transitions were observed in both Faraday and Voigt geometries, permitting the determination of the Zeeman splitting of the exciton states in both the conduction and valence bands, and showed the Zeeman splitting precisely reflects the magnetization obtained earlier in bulk crystals. Combined with that magnetization data, these splittings yielded the electron/Fe²⁺ exchange energy for both bands. The Fe-concentration dependence of the data showed the Fe²⁺-Fe^{2 +} antiferromagnetic exchange clustering becomes evident for x>0.02. The temperature dependence of the spectra confirm that this system is a Van Vleck paramagnet and the spin-orbit splitting of the lowest Fe²⁺ crystal-field states is 2.2 meV.