Magnetic exchange interactions in Co-based II-VI diluted magnetic semiconductors: Zn1-xCoxS.

Abstract

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{\ensuremath{-}}\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\char21{}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\ifmmode\pm\else\textpm\fi{}0.05 meV, which yields ${J}_{\mathrm{NN}/{k}_{B}=47.5\ifmmode\pm\else\textpm\fi{}0.6}$ 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 Land\'e interval rule, indicating biquadratic effects in the Co-Co exchange.