Effects of annealing temperature on the structure, photoluminescence and ferromagnetism properties of Cr-implanted ZnO nanowires

Room temperature ferromagnetism was observed in Cr-implanted ZnO nanowires annealed at 500, 600, and 700 °C. The implantation dose for Cr ions was 1×1016 cm−2, while the implantation energies were 100 keV. Except for ZnO (100), (002), and (200) orientations, no extra diffraction peaks from Cr-related secondary phase or impurities were observed. With the increasing of annealing temperatures, the intensity of the peaks increased while the FWHM values decreased. The Cr 2p1/2 and 2p3/2 peaks, with a binding energy difference of 10.6 eV, appear at 586.3 and 575.7 eV, can be attributed to Cr3+ in ZnO nanowires. For the Cr-implanted ZnO nanowires without annealing, the band energy emission disappears and the defect related emission with wavelength of 500–700 nm dominates, which can be attributed to defects introduced by implantation. Cr-implanted ZnO nanowires annealed at 500 °C show a saturation magnetization value of over 11.4×10−5 emu and a positive coercive field of 67 Oe. The origin of ferromagnetism behavior can be explained on the basis of electrons and defects that form bound magnetic polarons, which overlap to create a spin-split impurity band.