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Exchange interactions and Curie temperatures in Cr-based alloys in Zinc Blende structure: volume- and composition-dependence from first-principles calculations

We present calculations of the exchange interactions and Curie temperatures in Cr-based pnictides and chalcogenides of the form CrX with X=As, Sb, S, Se and Te, and the mixed alloys CrAs$_{50}$X$_{50}$ with X=Sb, S, Se, and Te. The calculations are performed for Zinc Blende (ZB) structure for 12 values of the lattice parameter between 5.44 and 6.62 Å, appropriate for some typical II-VI and III-V semiconducting substrates. Electronic structure is calculated via the linear muffin-tin-orbitals (LMTO) method in the atomic sphere approximation (ASA), using empty spheres to optimize ASA-related errors. Whenever necessary, the results have been verified using the full-potential version of the method, FP-LMTO. The disorder effect in the As-sublattice for CrAs$_{50}$X$_{50}$ (X=Sb, S, Se, Te) alloys is taken into account via the coherent potential approximation (CPA). Exchange interactions are calculated using the linear response method for the ferromagnetic (FM) reference states of the alloys, as well as the disordered local moments (DLM) states. These results are then used to estimate the Curie temperature from the low and high temperature side of the ferromagnetic/paramagnetic transition. Estimates of the Curie temperature are provided, based on the mean field and the more accurate random phase approximations. Dominant antiferromagnetic exchange interactions for some low values of the lattice parameter for the FM reference states in CrS, CrSe and CrTe prompted us to look for antiferromagnetic (AFM) configurations for these systems with energies lower than the corresponding FM and DLM values. Results for a limited number of such AFM calculations are discussed, identifying the AFM[111] state as a likely candidate for the ground state for these cases.