Destruction of ferromagnetism in Cu-doped ZnO upon thermal annealing: role of oxygen vacancy

We report on the role that oxygen vacancies have in determining the magnetic properties of Cu-doped ZnO. The observed magnetic hysteresis and the temperature dependence (irreversibility) of magnetization behave like supermoments blocked along random directions for a low-temperature annealed sample. The magnetic hysteresis (finite coercivity and low field saturation) and the irreversibility of temperature-dependent magnetization are destroyed upon thermal annealing. We propose that oxygen vacancies play an important role in inducing supermoments to explain the observations.