S. H. Charap

Thermal stability of recorded information at high densities

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.

Thermal fluctuation aftereffect model for some systems with ferromagnetic‐antiferromagnetic coupling

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...

Thermal instability at 10 Gbit/in/sup 2/ magnetic recording

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. >

High density magnetic recording media design and identification: susceptibility to thermal decay

Systematic simulation studies of magnetic and recording properties of thin film media at ultrahigh density have been performed with thermal effects taken into account. In the bulk magnetic property study, the focus is on coercivity. A formula for the ratio of the writing coercivity to that from the vibrating sample magnetometer (VSM) is given. The writing of di-bits and their subsequent thermal decay are simulated for a wide range of medium parameter values. From these simulations, the medium performance is mapped out on an anisotropy-constant-saturation-magnetization plane and the range of proper medium parameters suitable for stable storage determined. Ways to predict the thermal decay of written information by bulk measurements are discussed.

Vector Preisach modeling

A vector hysteresis model for particulate media has been developed and applied to a two‐dimensional, self‐consistent simulation of the magnetic recording process. Vector modeling is accomplished using a scalar Preisach model recording process. Vector modeling is accomplished using a scalar Preisach model for each of the two perpendicular directions, with each model responding to the corresponding component of the applied field. The Preisach density function is factored to permit description of hysteretic behavior based solely on the major loop. Each scalar Preisach model is partially demagnetized by the corresponding perpendicular field component. This feature couples the models and provides a substantial degree of reversibility with respect to rotating applied fields. Vector model performance was tested by the degradation of the longitudinal remanent moment as a function of vertical applied field. This model has been successfully used in a recording simulation.

Magnetic viscosity in high‐density recording

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.

Magnetic viscosity in recording media

Assuming a continuous distribution of relaxation times or frequencies responsible for magnetic aftereffect, it is shown that the inverse Laplace transform of the relaxing magnetization M(t) yields the function g(f)/f, where g(f) is the distribution of relaxation frequencies. Published data for carbonyl iron is fitted by the form M(T)/M0=(1+t/t0)−n, with n=1.5 and t0 =26 ms. The corresponding distribution of relaxation frequencies is g(f)∼(ft0)n  exp(−ft0). Using a simple, noninteracting particle model of a recording medium based upon the switching field distribution of the material, we have calculated the change in magnetization (decay) between 100 and 1000 s after applying a reversing field. A Lorentzian switching field distribution 20 Oe wide and centered at 200 Oe was assumed. The decay over this time span versus applied field is determined for various temperatures between 6 and 300 K. As the temperature is reduced, the field for peak decay increases, approaching the assumed value of Hc (200 Oe) as the...

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.

Simulation of magnetic aftereffect in particulate recording media

Employing a numerical integration of the Landau-lifshitz-Gilbert equation for phenomena with nanosecond characteristic times and a Monte Carlo method for much longer time periods, the authors have developed a method that in practice allows efficient simulation of magnetic aftereffect in an assembly of interacting particles for any time length of practical interest. Each particle is assumed to behave according to the Stoner-Wohlfarth model with its easy axis pointing in an arbitrary direction; thus the reversible component of magnetization change is incorporated in magnetic aftereffect studies. This simulation method was implemented on a workstation, and the hysteresis loops of the granular Fe(SiO/sub 2/) thin film material with a sweep time of 10 minutes were calculated with magnetic aftereffect. The thermal broadening of a demagnetization-limited written transition was simulated for the same material. After a sharp transition was created in the center of an assembly of particles, the change in the transition width over 100 seconds was studied. >

Vector Preisach modeling (invited)

While the scalar Preisach model has long served as a description of hysteretic processes, the response of ferromagnetic materials is essentially vector in character and scalar models are likely to fail in important respects for many advanced applications. A 2D vector Preisach model has been developed for the simulation of magnetic recording and playback for oriented media. The objectives of the model are (i) faithful reproduction of both scalar and rotational measurements made on bulk material and (ii) calculational efficiency for use in the simulations. Past work on vector hysteresis models is reviewed. The present model is a superposition of Preisach models with different orientation directions. On the basis of the properties of the single‐domain uniaxial particle, the effect of the transverse field is included as the boundary between ‘‘up’’ and ‘‘down’’ magnetization on the Preisach plane is calculated; then the reversible rotation is calculated as well. The performance of the model in describing measured bulk properties is presented.