Effects of high-temperature annealing on magnetic properties of (Zn0.91Mn0.09)O thin films grown by RF magnetron sputtering

The (Zn0.91Mn0.09)O thin film annealed at 1000 °C for 60 min in N2 atmosphere showed two kinds of Curie temperature (TC) values of TC1 around 130 K and TC2 above 300 K while the as-grown (Zn0.91Mn0.09)O thin film showed typical ferromagnetism with the TC of 108 K. It is expected that the increase of TC up to TC1 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 Mn^2+-related emission for photoluminescence spectra were observed for annealed (Zn0.91Mn0.09)O thin films. The increase of TC above TC2 might be originated from the formation of nano-sized (Zn0.91Mn0.09)O islands because it was confirmed that the nano-sized (Zn0.91Mn0.09)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 (Zn0.91Mn0.09)O islands is considered to be originated from the inhomogeneous distribution of Mn ions.