PNNL

Abstract

We have measured the full temperature dependence of M(H) curves for 5%, 10%, and 15% Co-doped ZnO epitaxial films with high crystalline perfection. Bulk magnetometry reveals pure paramagnetism with anisotropic M(H) curves at low temperatures, whereas the x-ray magnetic circular dichroism measured at the Co K-edge is isotropic. Electron paramagnetic resonance shows that the g-factors are not significantly altered compared to Co2+ impurities in ZnO. The M(H) measurements are compared to simulations using either an effective spin model with zero field splitting D or Brillouin functions with an effective temperature ansatz. Whereas both models reproduce well the H ? c data, for H k c the effective spin model indicates that D is reduced by 75% compared to Co2+ impurities in ZnO. The dependence of the M(H) curves and D on the Co concentration are discussed in terms of magnetic interactions between the Co dopant atoms.