Hydrothermal synthesis and magneto-optical properties of Ni-doped ZnO hexagonal columns

Single crystal Zn_1-xNi_xO (x=0, 0.02, 0.04, 0.06) hexagonal columns have been synthesized by a simple hydrothermal route. The hexagonal columns of the products are about 3 μm in diameter and about 2 μm in thickness. X-ray diffraction (XRD), Ni K-edge XANES spectra and TEM indicate that the as-prepared samples are single-crystalline wurtzite structure and no metallic Ni or other secondary phases are found in the hexagonal columns. Optical absorption and Raman results further confirm the incorporation of Ni²⁺ ions in the ZnO lattice. Magnetic measurements indicate that the Zn_1-xNi_xO hexagonal columns exhibited obvious ferromagnetic characteristic at room temperature. The coercive fields (H_c) were obtained to be 135.3, 327.79 and 127.29 Oe for x=0.02, 0.04 and 0.06, respectively. The ferromagnetism was assumed to originate from the exchange interaction between free carriers (holes or electrons) from the valence band and the localized d spins on the Ni ions.