Structural, optical, and magnetic properties of Ni doped ZnO nanoparticles: Correlation of magnetic moment with defect density

Abstract

We report on the room temperature ferromagnetism in the Zn1−xNixO (x=0, 0.03 and 0.05) nanoparticles (NPs) synthesized by a ball milling technique. X-ray diffraction analysis confirms the single crystalline, wurtzite ZnO structure for the 3% Ni doped ZnO NPs for higher milling time. HRTEM lattice image and SAED pattern show that the doped NPs are single crystalline with a d-spacing of 2.47Å corresponding to the (101) plane. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy confirm the presence of Ni ions inside the ZnO matrix with 2+ valance state. Room temperature magnetic measurements exhibit the hysteresis loop with saturation magnetization (Ms) of 1.6–2.56 (emu/g) and coercive field (Hc) of 260Oe. Micro-Raman studies illustrate doping/disorder induced additional Raman modes at ∼547, 574cm−1 in addition to 437cm−1 peak of pure ZnO. Photoluminescence (PL) spectra and UV–vis absorption measurements demonstrate some modification in the band edge emission and absorption characteristics, respectively. PL spectra also show defect related strong visible emission, which is believed to play a significant role in the FM ordering. These observations highlight the effect of changing defect density on the observed ferromagnetic moment values for the as synthesized Zn1−xNixO NPs. Magnetic interaction is quantitatively analyzed and explained using a bound magnetic polaron model and expected to arise from the intrinsic exchange interaction of Ni ions and OV, Zni defects. Systemic studies on the structural, magnetic, and optical properties reveal that both the nature of the defects as well as Ni2+ ions are significant ingredients behind attaining high moment as well as high ordering temperature in Ni doped ZnO NPs.