Optical and magnetic properties of Co-doped ZnO synthesized by magnetic assisted hydrothermal method

Cobalt(Co) doped ZnO nanoparticles were synthesized by a hydrothermal method with a magnetic field pretreatment(in the hydrolysis reaction). The pretreatment time(t) was set as 0 h(no pretreatment), 1 h, 2 h and 4 h. The crystallite structure, the lattice constant have been estimated by X-ray diffraction (XRD) with Rietveld refinement, which shows slight lattice expansion due to Co doping. Scanning electron microscopy (SEM) reveals flower-like microstructure and transmission electron microscopy (TEM) indicates the single crystalline in nature. The defect, the optical and magnetic properties of the four obtained samples were systematically investigated by Raman spectroscopy, photoluminescence (PL) spectra and vibrating sample magnetometer (VSM) respectively. The results show that the pretreatment time of magnetic field has a significant influence on optical and magnetic properties of Co-doped ZnO, and the effective time of magnetic field in the hydrolysis reaction on the magnetic property is between 1 h and 2 h. When 1 h ≤ t ≤ 2 h, more Co atoms are incorporated into ZnO and the saturation magnetization (M_S) increases by nearly three times, comparing with the sample without pretreatment. Samples of different pretreatment time show changes in the emission band of PL spectra and defect related bands of Raman spectra. The characterization of Raman scattering combined with the analysis of PL spectra suggests that the original of room temperature ferromagnetism(RTFM) may arise from the Co²⁺–V_O⁺–Co²⁺ bound magnetic polaron(BMP).