The exchange parameter $J_{\mathrm{NN}}$ for nearest-neighbor (NN) ${\mathrm{Co}}^{2+}$ ions in a new II-VI diluted magnetic semiconductor system ${\mathrm{Zn}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Co}}_{\mathrm{x}}$S has been directly determined using inelastic-neutron-scattering techniques. The ${\mathrm{Co}}^{2+}$ ions in the system have a magnetic ground state with quenched orbital moment, so that the Co-Co interaction can be described, to a very good approximation, as a Heisenberg exchange between two S=(3/2 spins. Inelastic-scattering spectra obtained from polycrystalline samples with x=0.01–0.06 reveal distinct maxima corresponding to transitions between the energy levels of isolated NN Co-Co pairs. The energy of the first excited level E=2$J_{\mathrm{NN}}$ has been found to be E=8.19±0.05 meV, which yields $J_{\mathrm{NN}/k_B=47.5\mathrm{}\pm 0.6}$ K, in good agreement with the result obtained from recent magnetic-susceptibility studies. The observed sequence of the excitation energies shows a slight deviation from the Landé interval rule, indicating biquadratic effects in the Co-Co exchange.

• Received 31 July 1989

DOI:https://doi.org/10.1103/PhysRevB.41.504