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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]

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

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]

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]

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]

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]

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 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.

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 a systematic study of media noise with different anisotropy distributions using a micromagnetic model. Details on model and media selection; Fluctuations in crystalline anisotropy amplitude; Discussion on perpendicular crystalline anisotropy.

A dual-layer patterned element magnetic recording medium design is presented that is expected to satisfy areal densities at 10 Tb/in². Each element consists of a small high anisotropy initial layer on top of which is deposited on an exchange-coupled soft layer of approximately equal thickness that extends in the recording direction almost to an adjacent element. This design has two distinct advantages. Reversal by precessional switching will occur for realistic record head field rise times (100-200 ps). Thermal stability for such small elements is maintained by hard section anisotropy fields that are an order of magnitude larger than the reversal field. Here, reversal mechanisms and system design are discussed in detail. [ABSTRACT FROM AUTHOR]

In this paper, we use micromagnetic simulation to study the switching field reduction and the switching field distribution in polycrystalline thin-film media. We first show that, for the coupled magnetic particles, the switching field of the high-anisotropy particle is reduced as a result of the exchange coupling with the low-anisotropy particle. The switching field reduction increases almost linearly with the anisotropy difference between the coupled particles. An optimal exchange exists that gives a maximum switching field reduction. This switching field reduction is more severe for a small applied field angle; therefore, the angular sensitivity of the switching field is reduced compared to the Stoner-Wohlfarth relation. Detailed micromagnetic simulations also show that in polycrystalline thin-film media, because of the variation and the reduction of the magnetic properties at the grain boundaries, the media coercivity can he reduced significantly. The primary reason for this switching field reduction is the exchange interaction between the grain boundary and the grain center region. [ABSTRACT FROM AUTHOR]

Presents a study that examined the effects of intergranular exchange and anisotropy distributions on signal-to-noise ratio. Details of the proposed tilted perpendicular recording as an effective solution for ultrahigh-density and high-data rate magnetic recording; Role of intergranular exchange and anisotropy distributions;Evaluation of thermal stability.

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]

We utilize an energy surface model and micromagnetic simulations to study fast switching in a single-domain particle under sub-Stoner–Wohlfarth switching fields. We show that for small damping and instantaneous field reversal, the switching field can be lower than the Stoner–Wohlfarth switching fields. For small field angles with respect to the anisotropy direction, the switching time decreases with decreasing field magnitude. The results are consistent with both micromagnetic simulations and experimental data. A simple physical explanation based on an energy surface model and the Landau–Lifshitz–Gilbert equation is given to explain these phenomena. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effect of quartic anisotropy (K[sub 2]) for nanograins with hexagonal crystalline structure on thermal stability has been studied. For reversal field parallel to the anisotropy axis, the presence of K[sub 2] increases the energy barrier in small fields but does not change the nucleation field. The mean first passage time was calculated from the stochastic differential equation. Reversal field at a finite temperature versus field pulse time showed very little effect at short times but thermal stability at long times was significantly extended. This effect was shown to be appreciable only for fields oriented within about 15° to the anisotropy axis. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Deals with a study that investigated the effects of three anisotropy sources on the orientation ratio in circumferentially-textured longitudinal recording media. Background on the magnetic anisotropy introduced by the circumferential texturing in cobalt-alloy longitudinal recording media; Methodology; Results and discussion.