Room-temperature ferromagnetism of Cu ion-implanted Ga-doped ZnO

► Cu ions were implanted into un-doped ZnO and Ga-doped ZnO films. ► Only Cu ion-implanted Ga-doped ZnO had ferromagnetism at room-temperature. ► A partial amount of implanted Cu existed as Cu2+ (d9) states in Ga-doped ZnO film. ► Almost all implanted Cu atoms existed as Cu1+ (d10) states in un-doped ZnO film.