S. H. Charap

Simulations have been carried out with the purpose of identifying the thermal stability limits on data storage density in longitudinal recording on thin film media. The simulations use a combination of molecular dynamics based upon the Landau-Lifshitz-Gilbert equation of motion and a Monte Carlo method for dealing with magnetic viscosity. Based upon the limits on media coercivity imposed by available heads and SNR considerations, but assuming that sufficient head resolution can be achieved, an upper bound of about 36 Gbit/in./sup 2/ is projected.

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.

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\ifmmode^\circ\else\textdegree\fi{}K for the Fe, Fe(Si), and Ni crystals, respectively. The observed departures from ${T}^{\frac{3}{2}}$ 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 ${T}^{\frac{3}{2}}$ term appropriate for zero spin-wave energy gap are $C=7.5\ifmmode\pm\else\textpm\fi{}0.2, 3.4\ifmmode\pm\else\textpm\fi{}0.2, \mathrm{and} 4.4\ifmmode\pm\else\textpm\fi{}0.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6} {\mathrm{deg}}^{\ensuremath{-}\frac{3}{2}}$ for Ni, Fe, and Fe(Si), respectively. The coefficients of the ${T}^{\frac{5}{2}}$ term for zero gap are determined only for the Ni and Fe crystals as $D=(1.5\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8} {\mathrm{deg}}^{\ensuremath{-}\frac{5}{2}} \mathrm{and} (1\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9} {\mathrm{deg}}^{\ensuremath{-}\frac{5}{2}}$, 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.

The limitation on recording density imposed by thermal stability is systematically studied by a method combining molecular dynamics and Monte Carlo computer simulations. The thermal decay for as long as 6 months has been simulated for written di-bits at the projected anisotropy, grain size, and bit length for 10 Gbit/in/sup 2/ magnetic recording. In the presence of demagnetizing field, a single layer film has little thermal effect at the upper limit of the projected grain sizes, while thermal decay is obvious when grain sizes are at the lower limit. The magnitude of the noise peak does not change significantly while the noisy region becomes wider after thermal decay. Compared with a single layer medium of the same total thickness, a double layer film has much more serious thermal decay and the negative interaction between layers tends to demagnetize the film, therefore making the thermal effect worse. The thermal decay in multilayer media tends to cancel the gain in noise reduction obtained by dividing the film layer at ultrahigh recording density. The effects of magnetostatic and exchange interaction, anisotropy, and grain volume on thermal stability are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>