Ferromagnetism in Zn1–xMnxO Nanoparticles Prepared by Ball Milling

Previous studies pointed out that ferromagnetism in Mn-doped ZnO was related to exchange interactions between Mn ions mediated by lattice defects. This means that it is possible to modify a Mn-doped ZnO paramagnet to a ferromagnet by creating lattice defects in it. The present work starts from a paramagnetic Zn_0.98Mn_0.02O sample prepared by solid-state reaction and then creates more defects upon mechanical milling. By changing the milling time (t_m) from 0.5 to 20 h, we produced nanocrystalline (NC) samples with average crystallite sizes (d) ranging from 30 to 157 nm. The d decrease generated lattice strain and defects. This broadens and blurs the lines of Raman scattering and electron spin resonance (ESR) spectra. Interestingly, magnetization studies versus magnetic field revealed the samples with d ≤ 150 nm exhibiting room-temperature ferromagnetic (FM) order. The FM order became largest as d = 72 nm, corresponding to a saturation magnetization of M_s ≈ 0.006 emu/g. Apart from this d value, Ms would be gradually decreased. X-ray absorption fine structure (XAFS) spectra revealed a coexistence of Mn²⁺ and Mn³⁺ ions in the samples. Their concentration ratio was slightly changed with decreasing d, due to the slight shift of the absorption edge. With the features of Fourier-transformed XAFS and ESR spectra, we believe that ferromagnetism in the NC samples is related to oxygen vacancies residing on the surface of nanoparticles. Local lattice distortions can lead to zinc interstitials for the samples d <; 72 nm, which decreases M_s.