Exchange interactions and magnetism of ${\mathrm{Co}}^{2+}$ in ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$Te

A study of cobalt-carrier exchange interactions is carried out in very dilute ${\mathrm{Zn}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Co}}_{\mathit{x}}$Te (x≤1%) from correlated magneto-optical and magnetization measurements at T=1.8 K. The resonant magneto-optic Kerr effect is used to complement magnetoreflectance experiments, and determine precisely the exciton Zeeman splittings on single crystals of various Co compositions. Correlated measurements of magnetization performed on the same samples provide a determination of exchange integrals for ${\mathrm{\Gamma }}_6$ and ${\mathrm{\Gamma }}_8$ bands within the framework of the exchange model based on the mean-field approximation. In the investigated field and composition range, we find ${\mathit{N}}_0$α=0.31±0.03 eV; ${\mathit{N}}_0$β=-3.03±0.15 eV. This abnormally large value of the p-d exchange constant does not vary with Co concentration in the investigated region. Low-temperature magnetization data obtained in fields up to 15 T are consistently interpreted in a model including the contributions of isolated ions and ${\mathrm{Co}}^{2+}$-${\mathrm{Co}}^{2+}$ pairs of second and third neighbors for the values of exchange constants previously determined. Antiferromagnetic interactions between more distant ions than third neighbors are also taken into account. The long-range exchange parameters obtained by using either a modified Brillouin function or a mean-field approach are in good agreement. © 1996 The American Physical Society.