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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]

Examines magnetization correlations in polycrystalline thin-film recording media numerically via a micromagnetic cellular automaton model. Determination of effective correlation widths and lengths from measurements of power spectra; Results of simulations of the reversal process; Presentation of correlation coefficients and cross-track correlations or domain widths for varying magnetic interaction strengths.

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.

A capped composite patterned medium design is described which comprises an array of hard elements exchange coupled to a continuous cap layer. The role of the cap layer is to lower the write field of the individual hard element and introduce ferromagnetic exchange interactions between hard elements to compensate the magnetostatic interactions. Modeling results show significant reduction in the reversal field distributions caused by the magnetization states in the array which is important to prevent bit errors and increase achievable recording densities. [ABSTRACT FROM AUTHOR]

Microwave-assisted magnetic reversal (MAMR) is studied for media comprising exchange-coupled composite elements comprising soft and hard layers. Reversal in such elements occurs under substantially reduced reversal fields, microwave fields, and microwave resonant frequencies as compared to those for homogeneous elements. Reversal can occur in uniform modes as well as nonuniform domain wall assisted modes depending on the soft layer thickness. In addition, a multilevel MAMR scheme is suggested where the recording media comprise multiple levels of elements, with each level having a distinct resonant frequency. These levels are addressed individually by tuning the frequency of the microwave field. [ABSTRACT FROM AUTHOR]

The single-particle energy surface model is utilized to study the switching field limit and the magnetization ringing in the dynamic reversal process. A simple analysis of magnetization ringing is shown and a method to reduce magnetization ringing in dynamic switching is suggested. We show that for the applied field smaller than the grain anisotropy field, the magnetization ringing is reduced with decreasing applied field magnitude. The energy surface model also predicts that for high data rate magnetic recording, the dynamic switching field may be well below the Stoner–Wohlfarth value. The magnetic write width can be much larger than the physical write pole width in perpendicular magnetic recording systems. [ABSTRACT FROM AUTHOR]

Dynamic relaxation for nonlinear magnetization excitation is analyzed. For direct processes, such as magnon-electron scattering and two-magnon scattering, the relaxation rate is determined from the linear case simply by utilizing the magnetization oscillation frequency for nonlinear excitation. For an indirect process, such as slow-relaxing impurities, the analysis gives an additional relaxation term proportional to the excitation level. In all cases the effective magnetization damping is increased compared to Landau-Lifshitz-Gilbert damping. [ABSTRACT FROM AUTHOR]

We consider a single-domain ferromagnetic particle with uniaxial anisotropy coupled to a single-domain soft ferromagnetic particle (superparamagnetic particle). The problem of thermally agitated magnetization reversal in this case can be reduced to the random magnetization dynamics of the first particle with an effectively larger anisotropy field. The magnetic external field is also altered in a manner that depends on the sign of the coupling and can be either enhanced or suppressed. [ABSTRACT FROM AUTHOR]

A simple energy surface model is introduced to explain single domain particle switching under sub-Stoner-Wohlfarth fields. The criterion is that if the initial energy exceeds the subsequent energy barrier maximum, then the particle can reverse. Reversal will occur only for small damping constants and short field rise times, so the magnetization dynamics explores a sufficiently large portion of the energy surface. The results are confirmed by micromagnetic analyses and are consistent with both large scale micromagnetic simulation and experimental data. This effect may be utilized for ultrahigh density (1 Tbit/in[SUP2]) and ultrahigh data rate magnetic recording (>2 Gbit/s). [ABSTRACT FROM AUTHOR]

Poisson's equation is solved exactly to obtain the medium demagnetization field for both a single pole head and a single pole with a trailing shield (shielded single pole) head designs in perpendicular recording. To write an isolated transition, for the semi-infinite single pole head, head imaging can be neglected when calculating the demagnetization field. To the semi-infinite shielded single pole head, when the pole separation is reduced, perfect imaging from both the head and the soft magnetic underlayer is a good approximation. [ABSTRACT FROM AUTHOR]

A method has been developed to analyze thermal fluctuations in systems with a nonuniform magnetization reversal. The application is to either interacting single domain particles or elongated grains where the magnetization can reverse nonuniformly. In our approach, the stochastic differential equations are obtained by adding damping and thermal fluctuations consistent with the fluctuation-dissipation theorem to each independent mode of the interacting system around equilibrium. The collective thermal reversal mode is obtained from the optimal path of a weakly thermal agitated system. Asymptotic analysis gives the first mean exit time formula for the long time thermal excitation region. Examples of 2:1 and 5:1 aspect ratio particles are given. [ABSTRACT FROM AUTHOR]

A general scheme is developed to analyze the effect of nonuniform thermal magnetization fluctuations in a thin film. The normal mode formalism is utilized to calculate random magnetization fluctuations. The magnetization noise is proportional to the temperature and inversely proportional to the film volume. The total noise power is the sum of normal mode spectral noises and mainly determined by spin-wave standing modes with an odd number of oscillations. The effect rapidly decreases with increasing mode number. An exact analytical calculation is presented for a two-cell model. [ABSTRACT FROM AUTHOR]

The dynamic magnetization process in thin film platelets is studied by numerical micromagnetics. Simulations are conducted without the addition of the phenomenological damping term in the Landau–Lifshitz equation. Excess Zeeman energy is transferred to magnetostatic-exchange coupled spin waves. This allows the average magnetization to relax towards the equilibrium configuration. The strength of the applied field and the size of the platelet both affect how much energy can be transferred to the spin wave modes. For large enough platelets and fields that are not too large, nearly complete reversal can be obtained without a mechanism to dissipate magnetic energy. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The process of magnetization reversal in a fine ferromagnetic grain with a strong uniaxial anisotropy has been simulated for the case of an instantaneously applied reversal magnetic field. The “quasi” single-domain grain was considered as a system of 64 or 1000 subcubes coupled by exchange and dipole–dipole interactions. The system of Landau–Lifshitz equations was numerically solved without any phenomenological damping terms. The dynamic process is characterized by a rapid magnetization reversal accompanied by a decrease in the average magnetization. The system exhibits a nonlinear excitation of nonuniform magnetic oscillations (spin waves) driven by the uniform mode. We found limitations on magnetization reversal for small grain size and large applied field magnitude. The damping parameter has been obtained for small magnetization oscillations. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An analytic model for reversal field versus pulse time in a fine magnetic grain has been 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. Therefore application is for fields both less than and greater than the intrinsic uniaxial anisotropy field H[sub K]. For the case of -H[sub 0]/H[sub K]>1 the mean first passage time from the initial saturated state to the thermodynamically equilibrium reversed state is calculated. For the case of -H[sub 0]/H[sub K]<1 we calculate the mean first passage time from the thermodynamically equilibrium state of the metastable well to the thermodynamically equilibrium stable reversed state. It is shown that simple Ne´el–Arrhenius analysis is applicable for times greater than at least 100/αγH[sub K]. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Micromagnetic simulations of planar isotropic magnetic thin film recording media are performed to study initial magnetization reversal processes in response to pulses with linear and exponential ramps. Linear ramps lead to smaller zero-crossing times T[sub 0] than exponential ramps due to the more rapid field increase during the pulse ramps. It is shown that T[sub 0] increases substantially when the pulse amplitude is less than about half the anisotropy field, in which case pulse overshoots are effective to reduce T[sub 0]. In addition, the effects of exchange coupling and gyromagnetic damping are examined. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents information on a study which simulated dynamic magnetization reversal in a single grain for an instantaneously applied reversal magnetic field. Methodology; Results and discussion; Conclusions.

Presents a study that carried out a two-dimensional micromagnetic simulation for small unshielded magnetoresistive elements. Response of the magnetization to uniform cycling of a transverse external field; Magnetic response for an isolated element without any bias field; Effect of increasing the edge biasing angle beyond 45 degrees.

Examines magnetization correlations in thin film media in the presence of a recorded transition utilizing numerical micromagnetic simulation. Determination of the stationary cross track correlation and the nonstationary down track correlation functions; Overview of the micromagnetic modeling of thin film media; Equation for the magnetization fluctuations.

Presents information on a study which modeled the magnetization domain structures in thin metallic films utilized as recording media using cellular automation on a two-dimensional triangular lattice. Computer simulation modeling of planar isotropic films; Results and discussion; Conclusions.

Presents information on a study which measured the magnetization fluctuations of uniformly magnetized media in sputtered CoP thin films as a function of Cr underlayer thickness. Experimental procedure; Results and discussion; Conclusions.

Presents information on a study which examined the effect of cobalt modification on the magnetization reversal of elongated γ-Fe[sub2]O[sub3] particles. Description of the micromagnetic model; Results of the equilibrium and transient states of the particles; Conclusions.

We present a semi-analytical approach for the analysis of synchronization in bit patterned media (BPM) recording. The analysis is based on full numerical micromagnetic calculations of the reversal surfaces of separate BPM element. These reversal surfaces arc used to define writing window zones for correct recording in BPM element arrays. The simulations show that staged media lead to improved performance and larger writing window zones (WWZs). The presented model and results are important for estimating BPM designs in terms of density, bit-error rates (BERs), and performance. [ABSTRACT FROM AUTHOR]

Precessional reversal in composite patterned media elements has been studied for various applied field parameters. The decrease of the percentage of the elements covered by the applied field leads to a rapid increase of the reversal field and decrease of the critical rise time required for precessional reversal. For weak and strong applied fields, the reversal time respectively was found to decrease and increase with an increase of the applied field strength and decrease of the rise time. For certain (large) values of the applied field strength and pulse duration bands of non-reversal can be obtained. This behavior is associated with multiple precessional oscillations between two equilibrium states. Such oscillations can be eliminated by properly choosing the field pulse duration and element damping constant. [ABSTRACT FROM AUTHOR]

It is conceivable that early generations of patterned media will utilize read heads whose dimensions are several times larger than an ‘island’ of magnetization. For such a scenario, we propose a ‘multiple islands per read head’ model where the output from the read head is a function of the magnetization from several independently written tracks of islands. In particular, we focus on a ‘3 islands per read head’ model where the output from the read head is a function of the magnetization from three independently written tracks of islands. The readback signal is determined from a finite track-width magnetoresistive (MR) head model using reciprocity calculations, and two noise sources—island position jitter and AWGN electronics noise—are considered. By sampling the noisy signal at intervals corresponding to the down-track island separation, we obtain a discrete-time readback channel model whose performance is obtained under maximum-likelihood sequence detection. [ABSTRACT FROM AUTHOR]

Noise caused by thermal magnetization fluctuations in giant magnetoresistive sensors, mainly in the free layer, limits the achievable signal-to-noise ratio and thus the density of magnetic recording systems. Here, we describe a quasi-analytical method to study this noise. First, by discretizing the free Layer into sufficiently small rectangular cells, an equilibrium state of the free layer is obtained using a quasi-static energy minimization technique. A linear tensor relation between the magnetization spectra and the thermal fluctuations is found for the two approaches: 1) Landau-Lifshitz-Gilbert-Brown and 2) collective spin wave excitation. In the former, damping and thermal fields are added to each cell and then a Fourier transform is taken to obtain the spectra. For the eigenmode approach, the lossless dynamic equations are diagonalized, yielding the system's intrinsic eigenmodes. Then damping and thermal fluctuations are added to each eigenmode, independently. The sensor playback noise power spectrum density, a measurable quantity, is calculated by sununing up the contributions from each eigenmode or cell magnetization. This computationally light treatment yields a clear physical picture of the phenomenon, relating damping, thermal fluctuation, and noise. We describe each step of the calculation procedure in detail and give examples. [ABSTRACT FROM AUTHOR]

Damping and thermal fluctuations have been introduced to collective normal modes of a magnetic system in recent modeling of dynamic thermal magnetization processes. The connection between this collective stochastic dynamics and physical local relaxation processes is investigated here. A system of two coupled magnetic grains embedded in two separate oscillating thermal baths is analyzed with no a priori assumptions. It is shown explicitly that by eliminating the oscillating thermal bath variables and using a Markovian approximation, stochastic dynamics occur in the normal modes of the magnetic system in the form of independent damped harmonic oscillators driven by thermal fields. The grain interactions cause the local coupling to the thermal bath to be felt by the collective system and the resulting dynamic damping to reflect the system symmetry. This form of stochastic dynamics is in contrast to a common phenomenological approach where a thermal field is added independently to the dynamic equations of each discretized cell or interacting grain. The dependence of this collective stochastic dynamics on the coupling strength of the magnetic grains and the relative local coupling to thermal bath is discussed. [Copyright &y& Elsevier]

Presents information on a study which developed a three-dimensional micromagnetic perpendicular recording model to analyze high-density perpendicular write fields. Description of the simulation model; Discussion of tapered-neck fields; Dependence of the perpendicular write field on the soft-underlayer thickness.

Presents a study that theoretically examined the playback characteristics of a giant magnetoresistive head for perpendicular recorded magnetization transitions with a soft underlayer. Shape of the isolated signal for different permeabilities of the underlayer; Formulation of an analytical expression for the pulsewidth Tsub[50] and Dsub[50]; Determination of a formula for the transition-noise-limited signal-to-noise ratio.

Presents information on a study which proposed a model of signal-to-noise ratio (SNR) that depends on the thermal decay of recorded magnetization in a longitudinal thin-film disk media. Description of the experimental measurement technique of noise decay; Review of the model for calculating magnetization decay; Model for medium noise and SNR; Calculation results of noise power and SNR decay; Experimental comparisons; Conclusions.

Proposes a Guzik 1701 MP Spinstand, an experimental technique for the measurement of time dependence of playback voltage due to the decay of recorded magnetization patterns. Descriptions of the technique; Parameters of the thin film disks; Methods used to conduct the study; Conclusions.

Magnetic reversal in exchange-coupled composite elements under microwave fields is characterized by several unique properties including reduced reversal fields, microwave fields, microwave resonant frequencies, and reduced sensitivity to anisotropy distributions as compared to homogeneous elements. We find that reversal can occur in uniform and nonuniform regimes. The uniform regime is characterized by coherent spin precession enhancement by the microwave field. In the nonuniform regime domain walls in the soft layer mediate reversal and under linearly polarized microwave fields, can lead to a formation of localized reversal/nonreversal areas in the “applied field-frequency” phase plane. [ABSTRACT FROM AUTHOR]

A micromagnetic model for the writing process in perpendicular magnetic recording is developed to study the effect of the soft underlayer on the dynamic reversal of the recording layer. The upper part of the soft material is discretized micromagnetically, each spin following LLG dynamics; the remaining part has infinite permeability. The validity of the model is tested in the static case. Dynamically, high-frequency spin motion is observed in the soft layer. Nevertheless, the field inside the data layer follows the time dependence of the input waveform. [ABSTRACT FROM AUTHOR]

Provides information on a study which performed micromagnetic simulations to compare longitudinal thin film media with and without a grain size distribution. Effect of grain size distribution on the effect of the exchange interaction; Use of various techniques to generate a non-uniform grain size distribution; Why the same micromagnetic simulation algorithm for the uniform grains can be applied without any modification.