A theoretical model is developed for, and applied to, some coupled-film systems consisting of an underlying ferromagnetic thin film and a surface layer of antiferromagnetic material viewed as an assembly of uniaxial small particles. The magnetization of the film biases, and is in turn biased by, the particles through an interfacial exchange coupling. Above a blocking temperature, dependent on size, particles are able to reverse rapidly due to thermal fluctuation, thus exhibiting superparamagnetic response. By assuming a physically reasonable distribution of particle sizes, good agreement is obtained between computed curves for temperature and frequency dependence of hysteresis loop displacement and coercivity based on this model and corresponding experimental results for oxidized Permalloy films, reported in a companion paper. This thermal fluctuation model is also applied successfully to the case of oxidized cobalt films as studied by Schlenker. In this case it is necessary to include in the analysis the temperature variations of the antiferromagnetic anisotropy energy and of the interfacial exchange coupling.
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.
Displaced hysteresis loops, loop contraction, and increased coercivity are measured in oxidized 76–24 Ni–Fe film at low temperatures at frequencies of 0.05, 20, and 100 Hz using the Kerr effect. Switching in constant applied field by slow wall motion is also observed. A calculation of the observed temperature and frequency dependence of coercivity and loop displacement, based upon the magnetic aftereffect of fine antiferromagnetic oxide particles which are exchange coupled to the ferromagnetic film and have a distribution of anisotropy and coupling energies, is described.
By following the minimum-energy trajectory, stable and critical states can be calculated and M-H loops constructed for two coupled films in a more simple and direct manner than by the conventional methods. Furthermore, the method is shown to be applicable to a system of multiple films.
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