Phase-dependent and defect-driven d(0) ferromagnetism in undoped ZrO2 thin films

Undoped ZrO2 thin films are prepared on < 100 > Si substrates by reactive DC magnetron sputtering using a Zr target. By controlling the oxygen partial pressure during the deposition process, we can successfully control the phase structure of the as-deposited film, which can be tetragonal, monoclinic or a mixture of them. A magnetic property measurement reveals that phase-dependent d(0) ferromagnetism exists in ZrO2 thin films. Specifically, only tetragonal ZrO2 thin films can be room-temperature ferromagnetic. Photoluminescence measurements, X-ray photoelectron spectroscopy analyses and post thermal annealing experiments suggest the d(0) ferromagnetism in undoped tetragonal ZrO2 films is mainly driven by oxygen vacancies.