Origin of Room Temperature Ferromagnetism in ZnO Nanoparticles

Four independent sets of Zn _{1- x} M _x O (M=Co or Fe) nanoparticle were produced using chemical hydrolysis methods; one in diethylene glycol (ZnO-1), the other in denatured ethanol (ZnO-2), with x ranging from 0-10%. The pure samples show a small amount of ferromagnetism in agreement with [1]. The magnetic moment per dopant ion decreases rapidly with increasing x (Figure 1). Since doping effects each sample set differently and x-ray photoelectron spectroscopy does not show any change in the oxidation state of dopant ions with x , the observed magnetic behavior of Zn _{1- x} M _x O cannot be explained using previous the dilute magnetic semiconductor models. With increasing x , x-ray diffraction data showed systematic changes in the lattice volume, and the photoluminescence data showed an increase in bandgap (Figure 2,3). Since the most extensive changes in lattice volume and bandgap of Zn _{1- x} M _x O occur at the very low doping levels where the ferromagnetic behavior is also strongest, the observed ferromagnetism is attributed to changes in the electronic structure of ZnO due to the dopant incorporation.

1. Sundaresan, A., et al., Ferromagnetism as a universal feature of nanoparticles of the otherwise nonmagnetic oxides. Physical Review B, 2006. 74 : p. 161306.