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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]

The field and temperature dependence of the attempt frequency is important for the analysis of dynamic-thermal reversal of magnetic grains, especially at relatively short times. Here we present a general approach by which the attempt frequency can be easily evaluated for interacting magnetic grains or subsystems. For the case of large energy barriers relative to thermal energy, a general asymptotic solution to the Fokker-Planck equation is developed. The stochastic equations are based upon a dynamic-thermal reversal model. Dynamic coercivity versus pulse time for the whole time range, from purely dynamic reversal to purely thermal reversal, is calculated. Our approach is illustrated by specific examples of an independent grain, two ferromagnetic coupled grains, and two antiferromagnetic coupled grains. The effect of the exchange interaction is emphasized. [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]

Intergranular exchange in CoCr-alloy polycrystalline thin films utilized for longitudinal recording media is analyzed in terms of details of their chemical composition. Two neighboring interacting grains are modeled by a linear "atomic" chain extending from the center of one grain to the center of another. The composition at each spin site determines the interspin exchange interaction. Thermal fluctuations are included via mean field theory and they yield spacial variations of the saturation magnetization and the crystal anisotropy that differ from the bulk properties. Equilibrium spin configurations for the linear chain are obtained for different Cr concentrations at the grain boundary and for different temperatures. Chromium segregation at the boundary is confirmed to be the mechanism for grain decoupling. An effective exchange parameter, assuming uniformly magnetized grains like those in large-scale micromagnetic simulations, is determined. This parameter decreases significantly with increasing temperature and increases slightly with relative anisotropy axis orientation. [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]

Presents a study that examined two-dimensional wall motion in submicron thin permalloy films. Diagram of dynamic hysteresis loops at various driving frequencies; Spatial and temporal profile of the films; Information on the hysteresis loss and coercivity of dynamic hysteresis loops versus frequency of driving field.

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.

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.

Studies localized surface nucleation of magnetization reversal. Problems caused by magnetization reversal; What was the main purpose of the study; Methods of the study; Results and conclusions of the study.

Analyzes magnetization patterns and remanence states of thin films with a surface anisotropy normal to the film plane. Total film energy integral; Critical conditions for both in-plane and normal applied field; Nucleation condition for formation of quasiwalls.

Presents a study which examined the micromagnetic modeling of magnetic anisotropy in textured thin-film media. Micromagnetic model; Results and discussion; Conclusion.

Discusses several implications of the theory that nonuniform thermal fluctuations are able to significantly reduce the coercivity of elongated ferromagnetic particles. Review of the Néel-Brown theory; Review of the theory of nonuniform magnetization reversal; Conditions for the occurrence of nonuniform saddle points for more general situations.

Presents a study which examined the energy barriers for thermal reversal of interacting single domain ferromagnetic particles. Energy surfaces for various interaction strengths; Analytical solutions; Calculation of an effective volume.

Recording in bit patterned media (BPM) requires strict synchronization of the signal misregistration time. A scheme is presented based on micromagnetic simulation of a single element which allows defining a writing window (WW) permitting for synchronized recording. The WW behavior for random distributions of the anisotropy field is studied. The width of WW is shown to be determined by the medium parameters deviations, the BPM element separation, and the head field strength. It is shown that significant limitations can be imposed on the BPM density and required head fields because of the random distributions of the BPM properties. [ABSTRACT FROM AUTHOR]

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]

A nonuniform grid (NG) algorithm for rapidly computing magnetostatic field for micromagnetic simulations is described. The algorithm relies on spatial NG representation of the potential from the source boxes and local interpolation. Multilevel implementations of the algorithm result in a linear computational complexity with respect to the number of effective magnetic charges and observers. The algorithm is highly adaptive with respect to structure’s geometry, i.e., it becomes automatically faster for low dimensional configurations such as quasiplanar bit patterned medium arrays. [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]

Anomalously large damping has been obtained for magnetic reversal based upon a model that adds damping and thermal fluctuations to normal modes around equilibrium. This accelerated reversal behavior is due to a large damping rate associated with damped nonlinear oscillation motion away from equilibrium. We suggest that this result may shed light on understanding the large damping rates observed in some magnetic reversal experiments. [ABSTRACT FROM AUTHOR]

A microtrack model approach is utilized to extend the simple William--Comstock (WC) formula to include finite grain size and intergranular exchange. The model assumes microtracks with finite size grains in each microtrack and random phase of grain centers between microtracks. Intergranular exchange is modeled as a mean field. For WC parameters much less than grain size, the model gives a finite transition parameter limit. The effect of exchange is to reduce the transition parameter. [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]

Track edge effects in tilted and conventional perpendicular recording are analyzed. The write fields are calculated utilizing a three dimensional micromagnetic simulation that includes both the write pole and the soft underlayer. The dynamic reversal time and thermal stability of the recording layer are analyzed using a recent model involving mode analysis and the optimal collective reversal barrier. For conventional perpendicular recording, the effect of the write field angle at the track edge is to lower the medium energy barrier, which increases the guard-band width. For tilted perpendicular recording, the guard band is decreased due to an energy barrier increase caused by the write field angle. [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]

Micromagnetic simulations have been performed to simulate dynamic hysteresis loops in perpendicular media. Thermal energy barrier distributions have been calculated. For a fixed percentage of magnetization switched, a linear variation between the scaled applied field H[sub a] (t) / H[sub K] and ln(ƒ[sub 0]t)[sup 1/2] is found. Without including intergranular magnetostatic interactions, intergranular exchange coupling reduces the thermal energy barrier distribution width, compared to the physical volume distribution width. However, the magnetostatic interactions increase the energy barrier width substantially. The increase is significantly reduced at smaller magnetostatic interactions. For the effective volume of 50% magnetization switched (V[sub 50]), the results show V[sub 50] = √&lang V² 〉 almost independent of intergranular magnetostatic and exchange interactions. [ABSTRACT FROM AUTHOR]

Low power, fast speed, small size, wide temperature range, and common-mode-noise-rejection capability are some of the attributes of magnetoresistive devices for galvanic isolation over conventional devices such as optical and capacitive devices. We have fabricated prototype galvanic isolators using spin-dependent tunneling materials. The tunnel junctions have been deposited by rf diode sputtering and the Al[sub 2]O[sub 3] barriers are formed by depositing a thin layer of Al, then oxidizing it with oxygen contained plasma. The junctions are then patterned using photolithography techniques to define the pinned and free layers separately. A series of tunnel junctions are connected in a Wheatstone bridge form and are fabricated directly on top of the integrated circuit (IC) electronics that are used to process the signals from the bridge. In its core magnetics design, this digital device simply employs a one-bit memory cell capable of operating at 5 V. The functions of the devices are tested using a function generator, an IC driver chip, a probe station, two dc power supplies, and a high-speed network analyzer. The power consumption for the isolator is < 1 mW when operating at 2 MHz, a factor of 10 lower than the giant magnetoresistance product of similar functions. The operating speed of this device reaches 60 MHz, limited by the design of the electronics. [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]

Micromagnetic simulations have been performed to compare the recording performance of oriented media (M[sub r]=1.50) with planar isotropic media in longitudinal recording. With no exchange, oriented medium exhibits substantially less dc noise power thus larger signal-to-noise ratio (SNR). But with increasing linear density, the oriented medium shows larger nonlinear amplitude reduction, larger nonlinear transition shift, and less SNR advantage. At finite exchange, there is no substantial SNR difference between oriented and isotropic medium. © 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]

Dynamic switching time in longitudinal thin film media is studied by numerical simulation. An exponential head field with various rise times is assumed. A linear relation (with slope S[sub w]) to correlate the inverse of switching time and the maximum reverse field is found to apply only for rise times larger than 0.3 ns. The switching constant S[sub w] increases with increasing rise time, decreasing damping constant, and increasing intergranular exchange. A wide range of S[sub w], from about 250 to about 3500 Oe ns, is found for rise times between 0.05 and 1 ns. © 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 examined the switching field dependence of magnetic tape particles using a three-dimensional micromagnetic model. Micromagnetic model; Results; Conclusions.

Presents information on a study which examined the feasibility of reversing single bit per particle media at high density and data rate in magnetic recording systems. Experimental results; Conclusion.

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 information on a study which obtained the scaling relations for hysteresis loops and single transitions for longitudinal thin film media with in-plane random uniaxial anisotropy. Model and scaling relations; Results; 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.

Deals with a study which examined domain-wall structures and dynamics in micron-thick Permalloy films with an in-plane anisotropy easy axis. Equilibrium domain-wall structure; Simplification of the effect of eddy current damping; Analysis of the dominant mechanism of the flux conduction.

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.

Focuses on a study regarding the dynamic evolution of a one-dimensional Bloch domain wall. Importance of studying the high-frequency effects in magnetic heads; Analysis of the computer simulation utilized used in the study; Motion of the Bloch wall.

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.