Evidence by EPR of ferromagnetic phase in Mn-doped ZnO nanoparticles annealed at different temperatures

Abstract

This paper reports the influence of the temperature annealing (Ta) on the structural, morphological, Raman and EPR characteristics of Mn-doped ZnO nanoparticles. XRD studies reveal a wurzite-type structure, while the formation of ZnMnO3 secondary phase was evidenced only for the nanoparticles annealed at Ta=700°C. This impurity phase was also identified by Raman scattering in the samples thermally treated at 600°C and higher temperatures. TEM investigations reveal that the average particle size of samples starts from 13nm. With the increase of synthesis temperature, the average particle size reaches a value of 55nm for 700°C. From the XPS spectrum of Mn 2p core-level doublet only the 2+ valence state for manganese ions was evidenced. EPR investigations show that depending on the annealing temperature, Mn ions are incorporated either in the interior of ZnO nanoparticles (Ta=425 and 500°C) or at their surfaces (Ta>500°C). For the sample annealed at Ta=425°C, a new broad resonance line arises, which was attributed to a ferromagnetic phase. We assume that this ferromagnetic phase could be due to the interaction between the Mn2+ ions and uncompensated acceptor defects incorporated into the ZnO lattice during the thermal treatment of the samples. Our investigations show that the ferromagnetism in Mn-doped ZnO nanoparticles could appear in the low-temperature annealed samples and disappears in the samples thermally treated at high temperatures.