Positive temperature dependence of magnetic anisotropy in NiFeW thin films fabricated by gradient composition sputtering

Thin films of NiFeW were fabricated by gradient composition deposition technique and their static and dynamic magnetic properties were characterized at various temperatures. The doping composition of W was controlled by varying the deposition power of W. It was found that when the doping composition of W is low, the temperature dependence of the magnetic anisotropy is negative, i.e. it is decreased with temperature. This result can be interpreted in terms of the dominant contribution of the pair-ordering magnetic anisotropy. However, when the doping composition of W is high enough, it becomes positive with magnetic anisotropy being significantly increased with the increasing of temperature. The peculiar observation of the increasing of magnetic anisotropy with temperature is due to the fact that the contribution of the stress-induced magnetic anisotropy becomes dominant when the W composition is increased. Because of the increasing of the stress upon the heating of the films, this stress-induced magnetic anisotropy increases with temperature.