Anisotropic paramagnetism of Co-doped ZnO epitaxial films

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 $\text{Co}\text{ }K$ edge is isotropic. Electron paramagnetic resonance shows that the $g$ factors are not significantly altered compared to ${\text{Co}}^{2+}$ 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 $H\ensuremath{\perp}c$ data, for $H\ensuremath{\parallel}c$ the effective spin model indicates that $D$ is reduced by 75% compared to ${\text{Co}}^{2+}$ 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.