Surface ferromagnetism in hydrogenated-ZnO film

Nonmagnetic ZnO films were found to exhibit room temperature ferromagnetism after hydrogen annealing at elevated temperatures (100-500°C), accompanied by (OH) bonds detection. The areal saturation magnetization M_s (∼1.1× 10-5 emu cm-2) was insensitive to film thickness, suggesting surface magnetism. The attribution to OH bonds on surface was further supported when the ferromagnetism disappeared after a short immersion for 1 s in acid solution while ferromagnetism was relatively stable in basic environment. The alternative H₂ - and Ar-annealing can switch ferromagnetic "on" and "off" state, as the annealing under Ar atmosphere can reduce OH bond quantity significantly. First-principles calculations have further confirmed that OH-terminated ZnO surface belonging to the p31m two-dimensional space group has the lowest formation energy of -2.97 eV and a magnetic moment of 0.30 μB per OH due to unpaired magnetic moment of electrons occupying O 2p orbital. Insufficient surface OH concentration may result in antiferromagnetism and/or paramagnetism.