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We present a technique to determine the transition shape in digital magnetic recording. The method involves analyzing both temporal and spectral transition noise measurements. This combined method simultaneously determines the transition parameter and the cross-track correlation width along with the transition shape. Index Terms--Magnetic recording, transition noise, transition parameter, transition shape.

Magnetization dynamic equations with thermal fluctuations are derived fundamentally for small amplitude oscillations. A tensor form of phenomenological damping occurs whose components reflect a general elliptical magnetization motion. This analysis implies that thermal energy and associated damping must be applied to independent normal modes of the system. As an example, this formulism is applied to a thin film magnetoresistance sensor to find the eigenmodes for general nonuniform magnetization variations. Examples are given of both the various modes and the noise power spectra. Index Terms--Damping parameter, magnetoresistance noise, mode analysis, thermal fluctuations.

Experimental measurements and theoretical analysis of giant magnetoresistive (GMR) thermal magnetization fluctuation noise are presented. A new form of tensor damping, characteristic of the anisotropic thin-film geometry, is utilized. Rationalization and experimental confirmation for this expanded form are given as well. In general, this noise is inversely proportional to the sensor volume and, at frequencies less than the sensor ferromagnetic resonance (FMR), is proportional to the dynamic damping. Approximate crosstrack anisotropy and stabilization fields are determined from signal transfer curves and utilized to determine damping for each head sensor. Damping parameters do not vary appreciably, although variation is seen even within a given manufacturing batch. Index Terms--Dynamic damping constants, GMR heads, magnetization noise spectra, thermal fluctuations.

In this work, the effect of substrate texture on thin film disk noise and its impact on system performance are systematically examined. The surface structures of thin film disks with different substrate textures are characterized using Atomic Force Microscopy (AFM). Recording spectral analysis and AFM measurement yield disk surface parameters, surface roughness, correlation lengths of the texturing process both parallel and perpendicular to the texture line direction, and cross-hatch angle, all of which are used to characterize the texture noise. An error rate model that includes correlated texture noise is developed. Results are presented for various recorded patterns on disks with different textures. Error rate measurements are also performed. In general, it is shown that an improved system error rate results from a shorter perpendicular correlation length, smaller cross-hatch angle, and a smoother disk surface. The impact of texture on high density recording is also examined and discussed in detail.

Thermal stability is critical for magnetic grains in high density recording applications. Composite media grains consisting primarily of a hard and soft layer with intergrain exchange coupling are examined. Here it is shown, as in single domain particles, that power laws, to a good approximation, describe the field dependence of the energy barrier. In general, the power law exponent decreases with increasing relative moment and/or decreasing relative anisotropy of the soft/hard subgrain. Results for a variety of medium parameters as well as grain orientations are given. Index Terms--Coercivity time dependence, magnetic grains, magnetic reversal, multilayer media, perpendicular recording, thermal energy barriers.

Uniform magnetization (de) noise is studied using numerically generated arrays of grains. The effects of packing fraction, grain size distribution, and anisotropy orientation are examined. From electron microscopy results, in this initial study, the intergranular grain boundary separation is held fixed. In general, the de noise increases with increasing grain size distribution and decreases with increasing packing fraction. For a planar random anisotropy distribution, the noise is about an order of magnitude larger than that for well-oriented media. For typical parameters, the de noise can be less than the transition noise for oriented media and is always significantly larger for planar random media. Index Terms--Magnetic recording, magnetization noise, thin film granular media.

A three-dimensional micromagnetic perpendicular recording model has been developed to study the dynamics of perpendicular recording. The write field rise time, shape, and magnitude are investigated. At a data rate of 1 Gb/s, our simulation shows that after 1 ns, the dynamic write field, both the shape and the mgnitude, are about the same as compared to the static write field. The magnetostatic field of the medium causes a maximum of 10% write field mgnitude decrease in a hard transition. For a flux rise time of 0.2 ns, the head fields follow the reversal flux closely. For a flux rise time of 0.1 ns, both the head field rise time and the average magnetization at the air bearing surface (ABS) increase with decreasing damping constant. The results suggest that increasing the damping constant of both the write head and the medium and decreasing the throat height of the head is important for ultra-high-density recording. Index Terms--Dynamic field risetime, effect of damping, perpendicular recording, remanent field.

The effect of the magnetic properties and head geometry on the write head field is examined with a three-dimensional (3D) micromagnetic perpendicular recording model. The maximum write field is found to be 0.73 Bs for parameters characteristic of 200 Gbit/in(super 2).

The effects of finite flux rise time on the recording process is analyzed theoretically and experimentally. Results show that the finite flux rise decreases the field gradient during writing, leading to increased nonlinear transition shift, broader transition and noisier transition boundaries. The effects of finite rise time on recording phenomena is directly proportional to the disk velocity and rise time and inversely proportional to magnetic spacing.

A study was conducted to analyze a two-dimensional, dynamic, micromagnetic framework of a thin strip of permalloy. The model was utilized to examine the magnetization dynamics of the strip as it is subjected to a transverse step field. The damping was matched by varying the phenomenological damping parameter in the LLG equation. In addition, the strip was assumed to be infinitely long and thin that it is sufficient to discretize only across the transverse direction.

A study was conducted to analyze an expression for the remanent coercivity versus field time duration for polycrystalline thin film media supporting a two-dimensional random anisotropy axis orientation. Experimental results indicated that explicit averaging of th thermally induced reversal for random grains was well approximated by the thermal decay of one particle with anisotropy.

An MR sensor with patterned exchange-biasing layer at its end regions exhibits quiet and stable responses. A Mathematical model simulating an unshielded, boundary exchange-biased SAL/MR structure is developed. The modeled results are in good agreement with the experimental data. However, biasing characteristics are quite different from modeled results assuming infinitely long stripes. For the boundary exchange-coupled SAL/MR design, the stripe height has a significant impact on the biasing characteristics and the effect becomes even more pronounced as the track width is further reduced.

Magnetic recording transitions at very low flying heights were investigated using a micromagnetic model. The results showed that sharply written transitions undergo an irreversible demagnetization as they leave the recording head. In contact recording, the magnetic parameters should be sufficiently small in order to achieve magnetic pattern stability. Best overwrite was achieved when the writing field was between 2Hc and 3Hc.

A new method for analyzing noise and system linearity in a magnetic force microscope image was presented. The technique directly provides a test of system linearity for the improvement of image resolution. The results showed that nonlinearity in a given measurement could possibly be caused by magnetostatic interactions between the tip and the sample and rapid spatial change in the field gradients.

The phenomena of precessional reversal in exchanged-coupled dual-layer patterned media are examined by using micromagnetic simulations. The studies have shown that the reversal field and rise times needed for precessional reversal is controlled by varying the soft layer parameters and coupling strength between the layers.

The nucleation field for magnetic reversal is calculated for dual layer magnetic grains, which has allowed for nonuniform domain wall-like magnetization variations along the grain symmetry axis. The analysis has shown that a simple reaction has occurred when magnetostatic interactions are neglected and the reversal field is applied along the particle symmetry axis.

The temperature-dependent hysteresis of several samples of cylindrical nickel (Ni) particles is described and is related to the switching field derived from a micromagnetic model. It is found that there is a good agreement between the trends in switching field observed experimentally in Ni cylinder arrays and the trends predicted from the micromagnetic model, including a reasonable quantitative agreement.