Effects of high-temperature annealing on magnetic properties of (Zn _0.91Mn_0.09)O thin films grown by RF magnetron sputtering

The (Zn_0.91Mn_0.09)O thin film annealed at 1000 °C for 60 min in N₂ atmosphere showed two kinds of Curie temperature (T_C) values of T_C1 around 130 K and T _C2 above 300 K while the as-grown (Zn_0.91Mn _0.09)O thin film showed typical ferromagnetism with the T _C of 108 K. It is expected that the increase of T_C up to T_C1 might be attributed to the enhancement of crystal magnetic anisotropy because the increases of (0 0 0 l) peak for X-ray diffraction patterns and Mn²⁺-related emission for photoluminescence spectra were observed for annealed (Zn_0.91Mn_0.09)O thin films. The increase of T_C above T_C2 might be originated from the formation of nano-sized (Zn_0.91Mn_0.09)O islands because it was confirmed that the nano-sized (Zn_0.91Mn_0.09)O islands were formed after annealing treatment and that they revealed clearly the magnetic domains at 300 K for the measurements of atomic force microscopy and magnetic force microscopy, respectively. The room temperature ferromagnetism in the nano-sized (Zn_0.91Mn_0.09)O islands is considered to be originated from the inhomogeneous distribution of Mn ions.