Ferromagnetism in Zn1−xMnxO nanoparticles prepared by mechanical milling

In this paper, we have prepared a Zn_0.98Mn_0.02O paramagnet by solid-state reaction, and then created defects by mechanical milling. Changing the milling time (t_m) from 0.5 to 20 h (used the grinding zirconia medium), we obtained nanoparticles (NPs) with different crystallite sizes (d) of 30-157 nm (calculated from XRD patterns and the Williamson-Hall method). A decrease of d increases the lattice strain (ε), and density of lattice defects. This broadens and blurs the spectral lines of Raman scattering and electron spin resonance (ESR), not shown. Interestingly, magnetization studies versus magnetic field at 300 K reveal the samples d <; 150 nm exhibiting FM order. The FM order becomes largest for d = 72 nm, corresponding to saturation magnetization M_s ≈ 6×10^-3 emu/g. Apart from this d value, M_s will be gradually decreased. To find out the origin/nature of the observed phenomenon, we recorded X-ray absorption fine structure (XAFS) spectra. It appears that there is the coexistence of Mn²⁺ and Mn³⁺ ions in NPs. Their concentration ratio is slightly changed with decreasing d, due to a slight shift of absorption edge. Based on the features of Fourier-transformed XAFS and ESR spectra, we suggest that ferromagnetism in NPs is mainly related to oxygen vacancies. Lattice distortions can lead to the presence of zinc interstitials for the samples d <; 72 nm, which decrease M_s.