UV irradiations induce variations of ferromagnetism and resistivity in nitrogen doped ZnO films

The saturated room-temperature ferromagnetism observed from nitrogen-doped zinc oxide (ZnO:N) increases significantly after ultraviolet light (UV) irradiation. During the period of irradiation, the resistivity as a function of time decreases dramatically and reaches saturation eventually. The log scale of the resistivity as a function of irradiation time reveals two sequential linear lines with different slops. Both of linear lines correspond to different dominant transports which result from some electrons recombination captured by the impurity band induced from the N doping. Similarly, the reverse properties of resistivity change appear at the samples after turning off the UV irradiation. The experiments convince us of the magnetic mechanism arising from the Coulomb excitation.