Room temperature ferromagnetism of ZnO nanocrystals in amorphous ZnO-Al2O3 matrix

In this work, we demonstrate room temperature ferromagnetism of ZnO nanocrystals (NCs) embedded in alumina-based amorphous matrix. Our study has shown that solid solution occurred for lower Al concentration [< x=0.09 for (Zn1-x,Al-x)O]. No ferromagnetism appears in Al-doped ZnO with x < 0.09. With higher Al concentration, alumina-based amorphous structure appeared and fully amorphous was found in the composition of (Zn-0.30,Al-0.70)O. Ferromagnetism was found in (Zn1-x,Al-x)O with a threshold of 16 mol % of Al. The room temperature ferromagnetism was found to be attributed to ZnO NCs in an alumina-based amorphous ZnO-Al2O3 matrix. Ferromagnetism reached a maximum with a volume percentage of ZnO NCs to be similar to 27 vol %. The detailed investigations (transmission electron microscopy, x-ray photoelectron spectroscopy, and superconducting quantum interference device) show that the optimized size of these NCs is in the range of 4-5 nm. No ferromagnetism appears if the sample contains a fully amorphous structure.