Influence of oxygen partial pressure and annealing on magnetic properties of Al -doped ZnO thin films

Al doped ZnO films have been prepared using different Ar:O-2 ratios and annealing conditions on glass substrates by dc reactive magnetron co-sputtering. The results of magnetic measurements show that different oxygen partial pressures and annealing temperatures have great influence on the magnetism of doped ZnO thin films. The fact that only the films prepared at an Ar:02 ratio of 1:1 and annealed at 200 C in vacuum show clear room temperature ferromagnetism which disappears after annealed at 500 C in vacuum indicates that the ferromagnetism may be related to internal lattice stress. These ferromagnetic films have been annealed at 200 C in air subsequently and show lower coercivity and enhanced saturation magnetization, which may be attributed to the annealing in air causing more interstitial Al atoms to convert into substitutional Al ions, and consequently increasing the charge transfer between the Al and the ZnO matrix, leading to increased magnetism.