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A general approach by which the attempt frequency can be easily evaluated for interacting magnetic grains or subsystems is presented. Comparison to an independent grain shows the possible recording advantage of the antiferromagnetic media.

The compositional and temperature dependency of the intergranular exchange in CoCr-alloy polycrystalline thin films is determined using a one-dimensional quasiatomic model. Details of the magnetic grain structure in CoCr-alloy thin films and experimental data that characterize single crystal materials are discussed.

An analytic model for surface nucleation is described to illustrate the phenomenon of magnetization reversal that begins with nucleation of a nonuniform or uniform energy barrier state. It is shown that inhomogeneous boundary conditions will lead to barrier states of the magnetization field near the surface of ferromagnetic samples. Consequently, activation energies for switching are only minimally dependent on sample volume.

An analysis of magnetization patterns and remanence states of thin films with a surface anisotropy normal to the film plane was conducted. The total film energy integral is expanded to second order to find both nucleation and equilibrium. Results show that for a given film and applied field a critical surface anisotropy is required for the magnetizations to deviate from initial saturation. Analytic expressions for the critical conditions for in-plane and normal applied field are provided. Remanent state magnetization distributions are observed.

Thermally activated reversal of elongated particles having high aspect ratio through non-uniform structures produces weaker coercive forces than estimated by Neel-Brown's uniform magnetization reversal theory. The coercive forces are angular dependent and consistent with isolated single-domain particles. The energy barriers in the elongated particles are less than those caused by the uniform reversal process.

Magnetic thin film domain wall motion under dynamic fields was analyzed using computer simulation models. The results showed that two-dimensional domain walls exhibited enhanced dynamical behavior under ac driving fields. At higher amplitude driving fields, wall structure transitions were observed. Transition threshold increased with the external frequency. Film thickness direction was dominant for low drives.

An analytical solution was used to solve the energy barrier problem in the thermal reversal of two interacting single domain particles. The results showed that the lowest energy barriers were saddle points of the energy surfaceand they corresponded to reversal modes during thermal agitation. Thermal switching was either asymmetric fanning or asymmetric coherent rotation, depending on the bond angle, when there was weak dipole coupling.

A micromagnetic cellular automaton model was used to analyze magnetization correlations and noise in polycrystalline thin-film recording media. The results showed that self-organized criticality with domain patterns characterized by the intergranular exchange and magnetostatic interactions was exhibited by the magnetization reversal process. The state of magnetization didnot entirely determine the domain width. Noise was observed to be related to this domain width.

The single-domain ferromagnetic particle with uniaxial anisotropy coupled to a single-domain superparamagnetic particle to enhance thermal stability of recording media is analyzed. The thermally agitated magnetization reversal can be reduced to the random magnetization dynamics of the first particle with an effectively larger anisotropy field and the magnetic external field is altered in a manner that depends on the sign of the coupling and can be either enhanced or suppressed.

The transition parameter and cross-track correlation length are obtained by mode analysis of measured noise. The amplitude mode is the second most dominant noise made in the total nonstationary transition noise and it emerges due to the decline in the sensing area of the giant magnetoresistance (GMR) head with an increase in density recording.

The effect of intergranular interactions on thermal energy barrier distribution in perpendicular media is examined. The width of the energy barrier distribution declines due to intergranular exchange whereas the barrier width increases substantially as a result of magnetostatic interactions.

An analytic model for reversal field versus pulse time in a fine magnetic grain is developed for the case when the external magnetic field H(sub 0) is parallel to the axis of uniaxial anisotropy. The expressions include both thermal fluctuations and magnetization dynamics.

Micromagnetic simulations of planar isotopic magnetic thin film recording media are performed to study initial magnetization reversal processes in response to pulses with linear and exponential ramps. In addition, the effects of exchange coupling and gyromagnetic damping are examined.

The process of magnetization reversal in a fine ferromagnetic grain with a strong uniaxial anisotropy is simulated for the case of an instantaneously applied reversal magnetic field. This nonlinear diffusion is forbidden for either a strong reversal field and/or a small grain size.

The dynamic magnetization process in thin film platelets is studied by numerical micromagnetics. Initial work including a magnetostrictive energy dissipation mechanism results in complete magnetization reversal.

A three-dimensional micromagnetic model with cubic discretization was used to study the magnetization reversal in elongated particles for a variety of aspect ratios, shapes and particle sizes. Findings showed that cubic discretization need to be as fine as the exchange length to simulate a rectangular particle. Roughness arising from cubic discretization is well if the discretization size is on the order of the exchange length for curved shapes.

A study was conducted to analyze the mechanism of nonlinear damping during large angle magnetization reversal. A single grain was subdivided and solved using the gyromagnetic term. The magnetic moment was then reassembled in a novel equilibrium direction. Experimental results indicated irregular oscillations at a well defined mean mean value. Findings also showed that the average magnetization decreases.

A study was conducted to analyze the scaling of hysteresis and transition parameter with grain size in longitudinal thin film media. Closely packed hexagonal grains were used in the simulation. It was assumed that each grain was uniformly magnetized. Single transitions were then written using the Karlqvist head field assuming perfect imaging. Experimental results indicated that the smallest obtainable transition parameter was half the grain size.

A study was conducted to determine the feasibility of reversing single bit per particle media at a 1 GHz data rate with a density of 100 Gb/in squared in magnetic recordings. The media particles were cubic with 40 nm side length, 80 n center to center spacing and perpendicular uniaxial anisotropy. The reversal of individual elongated particles was also examined with fields from a moving probe. The minimum bit cell time needed to achieve reversal was much longer than the 1 ns corresponding to a 1 GHz data rate.

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]

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.

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.

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.