Variation in structural, optical and magnetic properties of Zn 1− xCr xO ( x = 0.0, 0.10, 0.15, and 0.20) nanoparticles: Role of dopant concentration on non-saturation of magnetization

Abstract

Cr-doped ZnO, i.e. Zn 1− x Cr x O ( x = 0.00, 0.05, 0.10, 0.15 and 0.20) nanoparticles were synthesized by sol–gel route. The structural and morphological properties of these nanoparticles were investigated by high resolution transmission electron microscope (HRTEM). The average particle size of Zn 1− x Cr x O nanoparticles decreases from 75 to 40 nm with the increase in x from 0.00 to 0.20. The rings observed in selected area diffraction pattern revealed that up to x = 0.10 these nanoparticles have single phase ZnO. However, a secondary spinel phase of ZnCr 2O 4 was observed for higher Cr doping ( x ≥ 0.15). The optical band gap calculated using UV–visible absorption was decreased from 3.27 to 2.27 eV with the increase in Cr-doping from 0.00 to 0.20 in ZnO nanoparticles. The undoped ZnO (Zn 1− x Cr x O; x = 0.00) nanoparticles did not show any hysteresis loop at room temperature, however, clear loops were obtained for x = 0.05–0.20. Additionally, magnetization ( M) vs. applied magnetic field ( H) loops for lower Cr-concentration ( x = 0.05) saturate at 5 kOe, and while those with higher Cr concentration ( x > 0.05) do not show saturation even at 10 kOe. This may be attributed to increase in the defects at higher Cr-doping into ZnO. The value of saturation magnetization was found to decrease from 4.24 emu g −1 to 1.96 emu g −1 with the increase in Cr doping from x = 0.05 to 0.20 in ZnO and may be due to the secondary ZnCr 2O 4 phase.