Structural, Optical and Room Temperature Magnetic Study of Mn-Doped ZnO Nanoparticles

Undoped and Mn-doped ZnO nanoparticles (Zn1-xMnxO), with nominal weight percentages (0.00 <= x <= 0.10), have been synthesized by co-precipitation technique. The synthesized nanoparticles are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV) and Fourier transform infrared spectroscopy (FTIR). From XRD analysis, the compound ZnMnO3 is formed for x >= 0.05 with cubic structure (a = 8.3694 angstrom) and its concentration increases with x. Moreover, XRD analysis reveals the wurtzite hexagonal crystal structure for ZnO. The lattice parameters (a and c) of Zn1-xMnxO are calculated and they increase with the doping concentration of Mn as a consequence of the larger ionic size of Mn2+ ions compared to Zn2+ ions. The crystallite size is calculated for all the samples using Debye-Scherrer's method (SSM), Williamson-Hall methods (UDM, USDM and UDEDM) and Size-Strain Plot method (SSP), and the results are in good agreement with TEM. The presence of functional groups and the chemical bonding is confirmed by FTIR spectra that shows a peak shift between undoped and doped ZnO. The energy bandgap (E-g) is calculated for different concentrations of Mn (0.00 <= x <= 0.10) by using the UV-visible optical spectroscopy, between 300 nm and 800 nm, showing a noticeable drop in E-g with x. At room temperature, the magnetization of the samples reveals the intrinsic ferromagnetic (FM) behavior of undoped ZnO, ferromagnetic behavior of ZnxMn1-xO (0.01 <= x <= 0.03) and the co-existence of ferromagnetic and paramagnetic behavior for ZnxMn1-xO (0.05 <= x <= 0.10). This ferromagnetism is decreased for the doped samples as a consequence of antiferromagnetic coupling between Mn ions. The two samples correspond to x = 0.01 and x = 0.10, tend to be superparamagnetic because of the formation of single domain particles as a consequence of small particle size. x = 0.03 shows an optimum value of Mn concentration for maximum saturation magnetization and the best ferromagnetic nature.