Effect of crystallite size on electrical and magnetic properties of CuCr2S4 nanoparticles obtained by mechanical alloying from sulphides

The CuCr2S4 nanoparticles obtained by high-energy ball milling of CuS and Cr2S3 sulphides were about 3.3 nm in size. SEM images showed formation of smaller or larger agglomerates and DFT based ab-initio calculations exhibited metallic ground state with band gap of 0.8 eV width just above (similar to 0.5 eV) the Fermi level and magnetic moments located mostly at Cr sites. Measurements of magnetic susceptibility, magnetic hysteresis, electrical conductivity and thermoelectric power showed antiferromagnetic order with the Neel temperature of 40 K, superparamagnetic-like behaviour with the blocking temperature of 90 K, n-type conductivity, and a metal-insulator transition at 375 K and below this temperature the variable-range hopping in the temperature range of 108 K < T < 256 K. These results are due to the reduction of crystallite sizes to the nanometre scale, which causes narrowing of the 3d t(2g) mixed-valence band of chromium ions.