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.

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The variation with temperature of the magnetizations of single crystals of Ni, Fe, and Fe+3 wt % Si are studied. New data for Fe and Fe(Si) is presented along with previously reported measurements for Ni. These data were obtained by means of the pyromagnetic effect at various applied fields and in the temperature range 4.2-140, 30, and 120°K for the Fe, Fe(Si), and Ni crystals, respectively. The observed departures from T 3/2 behavior are well described by spin-wave theory. Attempts to ascribe some of the measured variation of the magnetization to Stoner-type excitations or to variation of the moment per atom due to lattice expansion are mainly unsuccessful. The coefficients of the JT 3/2 term appropriate for zero spin-wave energy gap are C=7.5=t=0.2, 3.4=1=0.2, and 4.4±0.2X10-6 deg~3/ 2 for Ni, Fe, and Fe(Si), respectively. The coefficients of the T 6/2 term for zero gap are determined only for the Ni and Fe crystals as Z>= (1.5=1=0.2) X10~8 deg~6 /2 and (1±1)X10~9 deg~6 /2 , respectively. The measured variation of the spin-wave energy gap with applied field is consistent with the known g values of 2.19 and 2.09 for Ni and Fe. The magnitude of the gap at zero field is fully explained by the effects of magnetocrystalline anisotropy and magnetic-dipolar coupling. The values of the C and D coefficients are compared with results from independent experiments and are discussed in relation to theories of ferromagnetism in metals. OF FERROMETALS:Ni, Fe, AND Fe + 3% Si 2053 ARGYLE, CHARAP, AND PUGH

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.

For future ultrahigh-density magnetic recording, the magnetic viscosity in thin-film media will become an issue due to the drastic reduction in grain size. An algorithm combining a Monte Carlo method and molecular dynamics was employed to study the thermal effects in thin-film media. The component of the field perpendicular to the plane defined by the axes of shape anisotropy and uniaxial crystalline anisotropy makes it necessary to use the three-dimensional energy surface to find the minimum energy barrier. This barrier is used to sample the reversal rate and the elapsed time. Hysteresis loops for various KuV/kT ratios and sweep times are simulated. Isolated and di-bit transitions are written, taking into account thermally assisted switching. After the head field is turned off, the subsequent thermal decay is computed for time spans as long as 6 months. Significant aftereffect is found for grain volumes about twice that for ordinary superparamagnetism.