Magnetic resonance studies of mixed chalcospinel CuCr2SxSe4-x (x=0; 2) and CoxCu1-xCr2S4 (x=0.1; 0.2) nanocrystals with strong interparticle interactions

Magnetic resonance characteristics of mixed chalcospinel nanocrystals CuCr₂S_xSe_4−x (x = 0 and 2) and Co_xCu_1−xCr₂S₄ (x = 0.1 and 0.2) have been investigated. It has been established based on TEM, SEM and resonance data that all the samples contain both blocks with sizes from 1 to 50 m of compacted nanosized crystallites and individual nanoparticles with sizes from 10 to 30 nm. The studies provide evidence of strong interparticle interaction in all the samples leading to high values of the blocking temperature. Magnetic dipolar field arise in the boundary regions of interacting adjacent nanocrystals below the blocking temperature. This results in inhomogeneous broadening of the magnetic resonance spectrum along with appearance of additional absorption lines. With increase in magnetic anisotropy at low temperatures, a shift of the resonance field along with line broadening are observed for all the studied compounds due to freezing of the moments in the nanoparticles, both in the individual and compacted ones. A gapped characteristic of the resonance spectrum is established below the freezing temperature T_fr, with the energy gap defined by the averaged magnetic anisotropy <H_A>. Anionic substitution of sulfur by selenium results in a decrease in the magnetic anisotropy. In contrast, cationic substitution of copper by cobalt increases the magnetic anisotropy due to a strong contribution from the latter ion.