Temperature and frequency dependence of exchange anisotropy effects in oxidized NiFe films

Hysteresis loops have been measured for thin NiFe films between 5 and 300 K using the magneto-optic Kerr effect. Films that are free of any surface oxide do not show a displacement of the easy-axis loop along the field axis, and the coercive force and anisotropy field are very weak functions of the temperature. Oxidized films show striking exchange anisotropy effects; displaced loops appear below a transition temperature ranging from 30 to 90 K as the film is more heavily oxidized. Loop width and anisotropy field increase with decreasing temperature, exhibiting a change in slope in the neighborhood of the transition temperature. The steady-state parameters have characteristic frequency dependences; with increasing frequency the loop displacement and anisotropy field increase and the loop width decreases in the range 0.05–100 Hz. The interpretation of these results in terms of thermal aftereffect taking place in the antiferromagnetic oxide is presented in a companion paper.