Anisotropic paramagnetism of Co-doped ZnO epitaxial films

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∥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 is discussed in terms of magnetic interactions between the Co dopant atoms.