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Distortion of the playback pulse shape due to saturation is studied via a 3-D micromagnetic model for both MR/SAL and spin-valve heads. The deviation from linearity is examined as a function of both the Mr[Delta] of the recording medium and the off-track position of the device. Playback voltages for isolated transitions of both polarities are. calculated and their shapes are compared to linear references. The differences in structure between MR and GMR sensors result in very dissimilar distortions of the playback pulse shape under large applied fields.

An atomic-scale model is developed to analyze interlayer coupling in permalloy/silver/permalloy GMR spin valves. The coupling in a smooth interface spin valve is assumed to be anti-ferromagnetic due to interlayer electron scattering. With rough silver/permalloy interfaces, the coupling remains anti-ferromagnetic even with the enhanced magnetostatic energy.

Linear magnetization dynamics with relaxation and noise from a thermal bath is analyzed for two general classes of microscopic damping mechanisms. The resulting stochastic differential equations are always in the form of a damped harmonic oscillator driven by a thermal field. Backtransformation from the oscillator coordinates to the magnetization variables always results in a stochastic Bloch-Bloembergen equation for the transverse magnetization components. Detailed evaluation of relaxation rates for the magnon-electron confluence process and the slow-relaxing impurity process is given. Using experimentally observed parameters, a combination of these two processes show good agreement for a set of ferromagnetic resonance linewidth measurements versus temperature, frequency, and film thickness in NiFe. [ABSTRACT FROM AUTHOR]

The control of grain size statistics is essential in the manufacture of granular thin films used in high-density recording. In this article transmission electron microscope observations of perpendicular media are shown that indicate larger grain size dispersion than in typical longitudinal media. We explain this result by utilizing a simple Voronoi model that includes the crystallographic symmetry. The higher symmetry of the magnetic layer in perpendicular media increases the grain size dispersion. It is argued that out-of plane dispersion of the vertical axes of grains, important in longitudinal media, further enhances the differences between grain size distributions in these two types of media. The results are confirmed by experimental measurements. [ABSTRACT FROM AUTHOR]

A method is developed to analyze thermally agitated magnetization reversal in a single-domain ferromagnetic grain with a uniaxial anisotropy axis oriented at an arbitrary angle to a magnetic field. Random forces and phenomenological damping consistent with thermodynamics are introduced into the dynamic equations of the normal modes. This general approach reduces to the Landau-LifshitzGilbert formalism only when the magnetic energy is isotropic about the equilibrium position. A stochastic averaging method based upon a small damping parameter is used to transform the inherent two-dimensional problem into a one-dimensional form. The mean first exit time is calculated and the results for dynamic coercivity at different angles to the magnetic field are obtained. The method is ultilized to calculate the switching field versus pulse time for planar random media with noninteracting grains. The results fit well with direct Langevin simulation over short times and with Néel-Arrhenius models at long times. A general field and temperature dependent form of the attempt frequency, f[sub 0], is derived. [ABSTRACT FROM AUTHOR]

The effect of interactions on the blocking temperature of granular magnetic films is modeled using a variable time step Monte Carlo method. Both magnetostatic and exchange interactions are considered in the calculation of field cooled magnetization curves. Cubic grains with 28 Å side lengths are randomly packed within a larger cubic volume. Each grain has a random crystalline easy axis. For the parameters simulated, magnetostatic interactions do not have an effect on the blocking temperature, and exchange interactions cause average blocking temperatures to increase. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Noise in thin film media is concentrated at the transition center and must be characterized by a nonstationary autocorrelation function. The noise voltage spectrum is determined by the frequency response of this correlation function beyond the standard head field terms. In this work, the use of spectral measurement to determine the nonstationary noise is illustrated. Noise spectra of thin film media are experimentally studied in detail for various thin film disks using both inductive and MR heads. The results compare extremely well to the analytical expression of the spectrum derived utilizing a good approximation for the correlation function of the magnetization fluctuations. Both transition length and medium noise cross-track correlation width were determined from the measured spectra. The measured transition lengths agree with signal analysis, and the noise cross-track correlation widths range from 15 to 71 nm. Lowering the flying height reduces the transition widths, and the cross-track correlation width remains unchanged.

Analysis of medium noise mechanisms associated with micromagnetism revealed that low noise was observed in uniformly distributed crystalline anisotropy in the absence of exchange coupling. A micromagnetic model was developed to analyze four different types of media with various anisotropy distributions.

Shielded giant magnetoresistive heads are investigated experimentally to determine micromagnetic modelling. The magnetoresistive heads have spin-valve sensors. Partial unsaturation occurs when the pinning field is small and down-track output noise is produced. An increase in the pinning strength reduces playback instability.

An empirical eigenfunction expansion, investigating the noise correlation in dibit recording patterns, identifies dibit in-phase and out-of-phase jitters to be the dominant noise modes. A unified effect of both parameters results in noise amplitude increase and bit separation decrease, simultaneously causing supralinear noise improvement in the data.

A pseudo-random sequence method to identify nonlinear effects has been utilized at high recording densities. In this paper, this method is extended to distinguish two different nonlinear effects, nonlinear bit shift and partial erasure. Following the nonlinear bit shift echo derivation in the pseudo-random sequence method, it is shown here that partial erasure also produces an echo that is only 1/2 bit cell apart from the echo for nonlinear bit-shift. A numerical simulation of writing 31 bit long pseudo-random sequences is utilized to study this phenomenon. The magnetization instead of the waveform is analyzed to obtain higher spatial numerical resolution. The two nonlinear effects are identified from the echoes and their response to the writing pre-compensation is studied. It is found that partial erasure is not reduced efficiently by precompensation.

Application of PRML sequence detection in magnetic recording has been demonstrated with substantial increase in the linear density. This method achieves optimal performance when the noise is white and Gaussian. In a recording channel dominated by medium noise, the noise is correlated due to inter-granular interactions. The characteristics of the noise strongly affect the PRML performance. In this paper, the signal dependent medium noise is characterized approximately by the cross track average magnetization in the presence of transitions. Error rates for detecting a single bit, a dibit and a tribit with medium noise correlations included are calculated. It is found that errors in detecting a dibit or a tribit are mainly induced by the magnetic noise addition of adjacent transitions.

In this paper, nonlinearities and medium noise in perpendicular recording are studied by simulating the dibit recording process. Strong nonlinear bit shift is observed: the second transition shifts away from the first transition. Transition noise on the second transition decreases with a reduction of the bit separation due to a shortening of the second transition. To a very good approximation, even when the bit separation is very small, medium fluctuations are determined only by the cross track average magnetization. The cross track correlation of non-exchange coupled granular media with sufficiently low magnetization is roughly one grain diameter.

Noise voltage autocorrelation in thin film media has been experimentally studied in the time domain. Unlike a spectral measurement, the time domain noise autocorrelation function provides a good characterization of non-stationary noise that is particularly useful for thin film media. A new measurement technique is developed to eliminate the noise due to the lack of noise-free timing reference. An empirical eigenfunction expansion is applied to the measured noise autocorrelation matrix to identify possible physical mechanisms for the noise modes and eliminate alignment jitter. The noise autocorrelation function is reconstructed by using only the leading noise modes.

A self-consistent two-dimensional numerical model is developed to study eddy current damped domain wall motion in conductive, soft thin films. The micromagnetic Landau-Lifshitz equation and Maxwell's equation are solved simultaneously to provide a rigorous description of magnetization dynamics interacting with eddy current, which is relevant to the dynamic response of thin film inductive heads. Simulation results indicate that for permalloy films thinner than 3000 angstroms, eddy current damping can be neglected in wall dynamics; whereas for film thickness greater than 1 micron, this damping effect could be the dominating loss mechanism.

The micromagnetic characteristics of small unshielded magnetoresistive elements were investigated using a two-dimensional model to determine the response of the magnetization to uniform cycling of a transverse external field. The results showed that in the case of longitudinal patterned exchange bias, the pinning width needed to be greater than a critical value to attain a noise free response.

Domain wall dynamics in micron-thick permalloy thin films with an in-plane anisotropy easy axis were investigated using micromagnetic numerical simulations. The results showed that large damping had a stabilizing effect on the steady-state wall structure under applied fields. Hard-axis fields induced wall structure transitions under small damping and low-frequency fields. The transformations were more suppressed when large damping and high-frequency fields were present.

Numerical micromagnetic simulation was used to investigate and characterize magnetization correlations in thin films in the presence of a written recorded transition. The results showed that the variance relative to average magnetization was not dependent on magnetic interaction parameters and head medium spacing. Cross track correlation was not influenced by head-medium spacing.

Improvements to a method of calibrating a high-density tape recording system are suggested. The small gap approximation is not made, and allowances for the effects of demagnetization are included. A better and smaller estimate of the effective head-media spacing is obtained.

Nonlinear amplitude reduction in high density disk recording has been studied. Dibit responses were measured at two flying heights and disks with varying coercivity, magnetic moment, and substantially different noise level were used. It is shown that the nonlinear amplitude loss is reduced substantially at lower flying heights. Increasing coercivity, or decreasing magnetic moment similarly reduces this loss. Measurements of disks with different noise levels indicate more reduction for the low noise medium only at the lower flying height. By deconvolving isolated pulses, transition lengths for single transitions were determined at each flying height. It is also found that the nonlinear amplitude reduction curves scale with transition length at a fixed flying height, indicating a universal curve.

Measurements of hysteresis loops were performed on thin permalloy rectangular platelets with various dimensions prepared by nanolithography. The particle coercivity, Hc, exhibits a sensitive size dependence. The measured Hc are compared with micromagnetic predictions utilizing two-dimensional models without magnetization variation through the thickness. Excellent overall agreement is found, indicating that a simplified two-dimensional approach is sufficient for characterizing reversal mechanisms for particles with thicknesses smaller than or comparable to the exchange length.

Coercivity mechanisms in thin 1000 angstrom permalloy films due to inhomogeneous distributions of in-plane uniaxial anisotropy are investigated numerically, utilizing a two-dimensional domain wall model. The distribution of anisotropy could be attributed to the polycrystalline grain structure of thin films, as well as magnetostrictive stress. Typical anisotropy orientation dispersion induced by polycrystalline grains does not appear to account for the wall coercive force observed in soft permalloy films. Other mechanisms such as stress induced anisotropy may be dominant.

A nonuniform grid (NG) algorithm is described for rapidly computing magnetostatic field for micromagnetic simulations. Multilevel implementations of the algorithm have resulted in a linear computational complexity with respect to the number of effective magnetic charges and observers and the algorithm is highly adaptive with respect to structure's geometry.

A capped composite patterned medium design is described that comprises an array of hard elements exchange coupled to a continuous cap layer. Modeling results have shown major reduction in the reversal field distributions caused by the magnetization states in the array, which is important for preventing bit errors and has increased achievable recording densities.

Microwave-assisted magnetic reversal (MAMR) is analyzed for media comprising exchange-coupled composite elements comprising soft and hard layers. Reversal has occurred in uniform modes as well as nonuniform domain wall assisted modes depending on the soft layer thickness and a multilevel MAMR scheme is described where the recording media has multiple levels of elements.

A scheme is described based on micromagnetic simulation of a single element that has allowed defining a writing window (WW) permitting for synchronized recording. The studies have shown that significant limitations are imposed on the bit patterned media (BPM) density and have required head fields because of the random distributions of the BPM properties.

A technique to determine the head-medium magnetic spacing is presented utilizing square wave recording spectral measurements. The transition parameter is first determined at low densities using time domain transition noise analysis, as spectral analysis does not give a unique separation of the spacing and the transition parameter.

The basic design of a patterned media recording system is studied. A nonreturn to zero inverted (NRZI) coding scheme is recommended that allows for a wider shield-to-shield giant magnetoresistance (GMR) reader gap compared to single dot bits.

The effect of a correlation between the grain switching field and the grain size is examined by Monte Carlo simulation of granular media. The noise power for the case of grain-sized-correlated switching is approximately twice that for the case of uncorrelated switching, in reasonable agreement with the experiment.

The track edge effects in tilted and conventional perpendicular recording are analyzed utilizing micromagnetic simulations and theoretical analysis. For conventional perpendicular recording, the effect of the write field angle is to lower the medium energy barrier, which increases both the magnetic write width and the guard-band width and for tilted perpendicular recording, the guard-band is decreased due to the increased energy barrier caused by the write field angle and a large switching field H(sub SW).

A generalized transition parameter formula including the finite grain size and intergranular exchange has been obtained based upon a microtrack approach. The finite grain size is modeled by introducing finite grains to each microtrack and a random shift of grain center location between microtracks.

A simple energy surface model along with micromagnetic simulations of a single domain particle under a sub-Stoner-Wohlfarth field is discussed. It shows that for small damping and short applied field rise time, the reversal field can be well below the Stone-Wohlfarth value.

Presents a study that examined the magnetization reversal in longitudinal and perpendicular thin films via computer simulation. Analysis of the collective micromagnetic processes; Cause of the reversal; Effect of the collective formation of vortices.

Presents a study which investigated the magnetization reversal processes in CoCr perpendicular thin films. Theoretical model and calculation method; Results and discussion; Conclusions.

Presents a study which numerically investigated the magnetization process in ferromagnetic cubes. Cubic discretization procedure of the micromagnetic energy functional; Mathematical framework for describing the ferromagnetic particle; Computation of the complete hysteresis loops for a wide range of particle sizes.

Presents information on a study which developed a computer simulation model to conduct micromagnetic analysis of thin magnetic films. Methodology of the study; Results and discussion; Conclusion.

A method to analyze thermally agitated magnetization reversal in a single-domain ferromagnetic grain with a uniaxial anisotropy axis oriented at an arbitrary angle to a magnetic field is presented. Random forces and phenomenological damping consistent with thermodynamics are introduced into the dynamic equations of the normal modes.

The effect of nonuniform thermal magnetization fluctuations in an ultrathin film is studied. The normal mode formalism is utilized to calculate random magnetization fluctuations.

Tape medium noise is extremely important in high-density tape recording systems. Here, AC and DC noise spectra are measured in MP tapes and analyzed assuming tape particle clustering. Measured DC noise of a saturated tape is larger than AC bulk erase noise. At long wavelengths, both spectra are larger than simple theoretical estimates that assume independent particles. At short wavelengths, the measurements are below the theory. Data fitting yields mean cluster lengths in the range of two to four particles. A small percentage of the tape is shown to be in 'pole-free' pairs for the AC noise. Index Terms--Particle clusters, tape medium noise, tape particles.

Micromagnetic simulations have been performed to examine thermal energy barrier distributions in longitudinal recording. Results show that intergranular exchange reduces the energy barrier distribution width compared to the width of the physical volume distribution, which agrees well with experimental measurements. The rms average energy barrier is almost unchanged by the interactions. Intergranular magnetostatic interactions also decrease the thermal energy barrier width but only for either small exchange or small volume distributions. These results indicate that a measurement of energy barrier width reduction can be used to estimate the intergranular exchange of typical longitudinal medium. Index Terms--Energy barrier distribution, intergranular exchange, micromagnetic simulation, thermal stability.

Switching speed in perpendicular recording media has been studied from micromagnetic simulations, by applying a uniform reversal field to an array of magnetic grains. Increasing the damping constant reduces the switching time, but a weak exchange interaction leads to an increase in the switching time. A linear relationship is found between the switching time and the head field rise time for different field strengths, damping constants and weak exchanges. A 20 [degrees] field angle to the perpendicular direction substantially improves the switching speed. Index Terms--Micromagnetic simulation, perpendicular recording, switching speed.

We present initial results of a study of a system of fine magnetic grains with resonance interactions. For the case of magnetostatic and exchange interactions a general criterion for collective magnetization motion of a pair of grains is derived. Simulations demonstrate a dynamic cluster structure in both longitudinal and perperndicular recording media. These clusters may significantly reduce thermal degradation of recorded patterns. An indirect interaction between grains through an antiferromagnetic substrate is also discussed. Index Terms--Collective magnetization dynamics, resonance interaction, thermal fluctuations.

Thermal dynamic effects are discussed for both short write times and long storage times in thin film media. A new analytic model for coercivity versus pulse time for the entire time range is presented for planar random media. Neel-Arrhenius analysis is shown to be accurate only for times greater than about 100-1000 nsec. The effective volume in the long time Neel-Arrhenius regime is the rms volume for a distribution of grain sizes. For planar random media the volume is increased about 7% and is independent of intergranular interactions. The presence of a small [K.sub.2] hexagonal anisotropy in typical Co based alloys increases the stability, but has less effect on the intrinsic reversal field and the short time dynamic writing field. Increasing the damping constant may significantly reduce the write field, but, for both longitudinal and perpendicular media, yields only a very small increase in the thermal decay. Analysis of short time write fields in terms of a 'switching constant' is shown not to be fundamental to these hard materials. Index Terms--Anisotropy, coercivity, damping, dynamics, thermal activation.

Transmission electron microscopy experiments have shown that CoCrTa films for hard disk drives experience chromium segregation at grain boundaries. The resultant changes in the ferromagnetic parameters of the material together with the variation in the anisotropy direction from grain to grain are determining factors governing its magnetic behavior. In this paper, the case of two grains, differing in the direction of their magnetic anisotropy and possessing a change in their material content at the interface which we model by a Gaussian distribution, is solved analytically. Effects due to the diffuse interface such as localized regional changes in the anisotropy, exchange energy, and magnetic moment are included using perturbation theory. One of the outputs is the profile of the magnetization orientation in the inhomogeneous medium--thus yielding the smoothness and extent of the change. Such knowledge, in addition to its intrinsic scientific interest, points the direction, both for large scale computer calculations and for media improvement. Index Terms--Magnetic orientation, micromagnetics.