Electronic band structure and exchange coupling constants in ACr2X4 spinels (A = Zn, Cd, Hg; X = O, S, Se)

We present the results of band structure calculations for ACr(2)X(4) (A= Zn, Cd, Hg and X= O, S, Se) spinels. Effective exchange coupling constants between Cr spins are determined by fitting the energy of spin spirals to a classical Heisenberg model. The calculations reproduce the change of the sign of the dominant nearest-neighbor exchange interaction J(1) from antiferromagnetic in oxides to ferromagnetic in sulfides and selenides. It is verified that the ferromagnetic contribution to J(1) is due to indirect hopping between Cr t(2g) and e(g) states via X p states. Antiferromagnetic coupling between third Cr neighbors is found to be important in all the ACr(2)X(4) spinels studied, whereas other interactions are much weaker. The results are compared to predictions based on the Goodenough-Kanamori rules of superexchange.