Antiferromagnetic exchange contribution to Van Vleck paramagnetism in Fe-based magnetic semiconductors

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Abstract
Magnetization measurements up to 20 T have been performed in Cd1−xFexTe (xT≤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.