Evidence of oxygen defect induced ferromagnetism in heat treated electrospun ZnO nanowires

Aqueous solution containing polyvinyl alcohol and zinc acetate was electrospun into composite nanofibers. Upon heat treatment between TA=500 and 600 °C under ambient as well as nitrogen atmospheres, the as-spun composite nanofibers transformed into ZnO nanowires and exhibited room temperature ferromagnetism (RTFM). Powder X-ray diffraction and high resolution transmission electron microscopy confirmed that the heat treated nanowires consisted of single phase ZnO in hexagonal wurtzite structure. Photoluminescence spectra showed green emission at ~505 nm implying the presence of oxygen vacancy defects in ZnO lattice. Micro-Raman spectra revealed a defect related band at ~579 cm−1 as well as the wurtzite band at ~438 cm−1. Presence of singly ionized oxygen (Vo+) vacancy in the nanowires was confirmed by resonance absorption at g≈1.99 in electron paramagnetic resonance spectra of the samples. Magnetization measurements showed ferromagnetic behavior in the samples with high Curie temperature. Strong dependency of the properties on TA provides insights on the mechanism behind RTFM and the way to enhance it in undoped ZnO nanowires.