Electron paramagnetic resonance study of ZnAl2S4 spinel

Abstract

Single crystals of ZnAl2S4 spinel doped by paramagnetic Cr3+ and Mn2+ ions have been studied by the electron paramagnetic resonance.(EPR) technique. The crystal field symmetry around the impurity ions has been determined from the angular behaviour of X-band EPR spectra. The anisotropic EPR signal of the Cr3+ ions shows splitting into 31 narrow lines due to the super-hyperfine interaction. between unpaired electron spins of the chromium centres and nuclear spins of six neighbouring Al-27 (I = 5/2) ions. It has been established that the Cr3+, ions are located at the octahedral sites in the spinel structure, and the super-hyperfine interaction results from a weak covalent bounding with the Al atoms. The EPR signals of the Cr3+ paramagnetic centres show no fine-structure splitting due to a perfectly cubic symmetry of the local crystal field in the octahedral sites of the ZnAl2S4 spinel structure. A weak EPR signal consisting of six components has been ascribed to the transitions between hyperfine levels of the Mn2+, (1 = 5/2, S = 5/2) ions located at tetrahedral sites, while the fine-structure splitting of each component could be resolved only for special orientations of the sample in the external magnetic field. The parameters of the EPR signal of both chromium and manganese centres indicate that there is an essential covalence in the ZnAl2S4 spinel crystal. Very narrow linewidths (similar to 2 Oe) of the Cr3+ EPR signal components point to very high homogeneity and quality of the ZnAl2S4 crystals.