ANTIFERROMAGNETIC EXCHANGE CONTRIBUTION TO VANVLECK PARAMAGNETISM IN FE-BASED MAGNETIC SEMICONDUCTORS

Magnetization measurements up to 20 T have been performed in Cd1-xFexTe (x < 4%, 1.6 less-than-or-equal-to T less-than-or-equal-to 20 K) and Cd1-xFexSe (x = 1.8%, T = 1.7 K) for different crystallographic directions. A theoretical computation involving the diagonalization of the Hamiltonian of the isolated Fe2+ ion subjected to crystal-field, spin-orbit, and Zeeman terms within the lowest 5D manifold, provides the gross features of the magnetization curves, in particular their high field anisotropy. However, we argue that a quantitative agreement can be achieved only if the Fe-Fe spin interaction is taken into account. It includes not only the contribution of nearest-neighbor (nn) pairs or larger clusters, but also a sizeable Heisenberg interaction between more distant ions, treated in the mean-field approximation. Assuming a random distribution of Fe2+ ions, a quantitative agreement with experiments is achieved in the present domain of measurements. Exchange constants are also given.