ENERGY-LEVEL SPECTRA OF CO-2+ AND FE-2+ IN DILUTED MAGNETIC SEMICONDUCTORS

In this paper we study the low-lying energy levels of Co2 + and Fe2+ in diluted magnetic semiconductors, such as Cd1-xCoxTe and Cd1- xCoxSe, and their iron-based counterparts. In the first of these compounds, the magnetic ion sits on a site of symmetry Td, while in the second the symmetry is trigonal (C3v). We develop a formulation that permits a continuous variation from Td to C3v symmetry. Comparison with experimental data in Cd 1-xCoxSe shows that the C 3v distortion amounts to about 10% of the crystal potential at the Co2+ site. Our study of the energy spectra of Fe2+ in Td and C3 v crystal potentials reveals that, even in the cubic field, the levels exhibit an anisotropy which manifests itself in an anisotropy of the magnetization M in the regime in which M is not a linear function of the magnetic field B. The study includes all the levels in the lowest terms of the (3d)7 and (3d)6 configurations of Co2 + and Fe2+, thus considerably extending work by previous authors. The calculations are carried out to second order in the spin-orbit interaction and in B for the lowest orbital states, and to first order in B for the excited states. The g factors of all the levels are obtained including their anisotropy for the Co2+ Γ8 states.