Intrinsic versus extrinsic ferromagnetism in HfO2−x and Ni:HfO2−x thin films

We have investigated the possible evolution of an intrinsic stable ferromagnetic moment in oxygen deficient undoped and magnetically doped HfO2−x thin films grown by reactive molecular beam epitaxy. Neither oxygen vacancies nor substituted Ni in combination with such vacancies results in an observable magnetic moment for a broad range of oxygen vacancy concentrations. By combining integral and element specific magnetization measurements, we show that a fluctuating deposition rate of the magnetic dopant induces extrinsic ferromagnetism by promoting the formation of metallic clusters. We suggest the element specific measurement of an induced magnetic moment at the nonmagnetic site as a proof of intrinsic ferromagnetism in diluted magnetic semiconductors.