Electron paramagnetic resonance study ofZnAl2S4 spinel

Abstract

Single crystals of ZnAl2S4 spineldoped by paramagnetic Cr3+ and Mn2+ ions have been studied by the electron paramagnetic resonance (EPR) technique. Thecrystal field symmetry around the impurity ions has been determined from theangular behaviour of X-band EPR spectra. The anisotropic EPR signal of theCr3+ ions shows splitting into 31 narrow lines due to the super-hyperfine interaction betweenunpaired electron spins of the chromium centres and nuclear spins of six neighbouringAl27 (I = 5/2) ions. It has beenestablished that the Cr3+ ions are located at the octahedral sites in the spinel structure, and the super-hyperfineinteraction results from a weak covalent bounding with the Al atoms. The EPR signals of theCr3+ paramagnetic centres show no fine-structure splitting due to a perfectlycubic symmetry of the local crystal field in the octahedral sites of theZnAl2S4 spinel structure. A weak EPR signal consisting of six componentshas been ascribed to the transitions between hyperfine levels of theMn2+ (I = 5/2,S = 5/2) ions located at tetrahedral sites, while the fine-structure splitting of eachcomponent could be resolved only for special orientations of the sample in theexternal magnetic field. The parameters of the EPR signal of both chromiumand manganese centres indicate that there is an essential covalence in theZnAl2S4 spinel crystal. Verynarrow linewidths (∼2 Oe) of the Cr3+ EPR signal components point to very high homogeneity and quality of theZnAl2S4 crystals.