Quantification of the magnetic exchange via element-selective high-field magnetometry: Co-doped ZnO epitaxial films

We have measured the element specific $M(H)$ curves for 5$%$, 10$%$, and 15$%$ Co-doped ZnO epitaxial films with high crystalline perfection using x-ray magnetic circular dichroism (XMCD). The XMCD$(H)$ curves do not saturate up to 17 T directly evidencing antiferromagnetic exchange between neighboring Co dopant atoms. Angular-dependent XMCD$(H)$ measurements in combination with theoretical calculations based on a well-established model Hamiltonian allow to determine both the next-neighbor exchange J as well as the single ion anisotropy D quantitatively. While for 5$%$ and 10$%$ Co doping, $J=15$ K and $D=3$ K, for 15$%$ Co doping, they are reduced to 10 and 2 K, respectively. High-field, element selective magnetometry based on XMCD is thus shown to be a suitable technique to determine microscopic magnetic parameters even in thin epitaxial films of dilute magnetic systems.