Wall Jumps Research Articles

Intermittent collective dynamics of domain walls in the creep regime

We study the ultra slow domain wall motion in ferromagnetic thin films driven by a weak magnetic field. Using time resolved magneto-optical Kerr effect microscopy, we access to the statistics of the intermittent thermally activated domain wall jumps between deep metastable states. Our observations are consistent with the existence of creep avalanches: roughly independent clusters with broad size and ignition waiting-time distributions, each one composed by a large number of spatio-temporally correlated thermally activated elementary events. Moreover, we evidence that the large scale geometry of domain walls is better described by depinning rather than equilibrium universal exponents.

Theory of Stable Multidomain Thermoviscous Remanence Based on Repeated Domain Wall Jumps

AbstractWe developed a theory of multidomain (MD) thermoviscous remanence that can be solved analytically to yield an expression relating the blocking temperature of a MD grain to its relaxation time. This expression is analogous to Néel's widely used theory of single‐domain (SD) thermoviscous remanence but yields a different time‐temperature relationship. The theory is based on a two‐domain model with a domain wall (DW) that can jump between different pinning sites. In contrast to previous theories of this kind, our theory considers repeated DW jumps rather than a single jump in isolation. It is shown that while SD remanence behavior is fully described by the two quantities (V,HK0), that is, volume and microscopic coercivity, MD particle remanence depends on three quantities: volume, Barkhausen volume, and DW pinning field, denoted (V,VBark,HK0). “Pullaiah” nomograms of this new theory show that while small MD grains behave almost identically to SD grains, larger grains show different slopes depending on the above quantities. The theory predicts that MD grains can be highly stable remanence carriers, in particular showing a high thermal stability. Grains with weak pinning fields, however, while thermally stable, are highly unstable under alternating field (AF) demagnetization, being demagnetized under fields of a few millitesla. Our theory also explains (1) why samples dominated by MD grains show curved vector demagnetization plots for both thermal and AF demagnetization, as well as (2) why MD grains affect thermal demagnetization plots even at high temperatures, while their remanence is completely removed within the first few AF demagnetization steps.

Ferroelectric domain wall dynamics characterized with X-ray photon correlation spectroscopy

Technologically important properties of ferroic materials are determined by their intricate response to external stimuli. This response is driven by distortions of the crystal structure and/or by domain wall motion. Experimental separation of these two mechanisms is a challenging problem which has not been solved so far. Here, we apply X-ray photon correlation spectroscopy (XPCS) to extract the contribution of domain wall dynamics to the overall response. Furthermore, we show how to distinguish the dynamics related to the passing of domain walls through the periodic (Peierls) potential of the crystal lattice and through the random potential caused by lattice defects (pinning centers). The approach involves the statistical analysis of correlations between X-ray speckle patterns produced by the interference of coherent synchrotron X-rays scattered from different nanosize volumes of the crystal and identification of Poisson-type contribution to the statistics. We find such a contribution in the thermally driven response of the monoclinic phase of a ferroelectric PbZr0.55Ti0.45O3 crystal and calculate the number of domain wall jumps in the studied microvolume.

Angular dependence of coercivity derived from alignment dependence of coercivity in Nd-Fe-B sintered magnets

Experimental results of the alignment dependence of the coercivity in Nd-Fe-B sintered magnets showed that the angle of magnetization reversal for anisotropically aligned magnets was bigger than that obtained from the theoretical results calculated using the postulation that every grain independently reverses its magnetization direction following the 1/cos θ law. The angles of reversed magnetization (θ1) for Nd13.48Co0.55B5.76Febal. with alignment α=0.95 and for Nd12.75Dy0.84B5.81Co0.55Febal. with α=0.96 were 30° and 36°, respectively, which were very similar to that of an ideal magnet with a Gaussian distribution (σ=31° and 44°, respectively) of the grain alignment. In this model, we postulated that every grain independently reversed according to the 1/cos θ law. The calculation results for the angular dependence of the coercivity using the values θ1=ω1(0°)=30°, σ=31° and θ1=ω1(0°)=36°, σ=44° could qualitatively and convincingly explain the observed angular dependence of the coercivity of Nd14.2B6.2Co1.0Febal. and Nd14.2Dy0.3B6.2Co1.0Febal.. It is speculated that the magnetic domain wall is pinned at grains tilted away from the easy magnetization direction, and when the magnetic domain wall de-pins from the tilted grains, the magnetic domain wall jumps through several grains. We suggest that the coercive force of the aligned magnet behaves like a low-aligned magnet owing to the magnetization reversal of the crust of the grains induced by the pinning and subsequent jumping of the magnetic domain wall.

Plyometric Exercises from a Knee Anterior Cruciate Ligament Prevention Program on the Rate of Isometric Torque Development and Vertical Jump Height

Purpose: Plyometric exercises are incorporated in anterior cruciate ligament injures of the knee (ACL) injury prevention programs that have effectively decreased risk factors for anterior cruciate ligament injures of the knee injuries, but the impact on the rate of isometric torque development of the musculature of the knee is unclear. The purpose of this study was to determine the effects of a 6-week plyometric program pulled from a commonly utilized anterior cruciate ligament injures of the knee injury prevention program on rate of isometric torque development (RTD) in healthy active females. Methods: College-aged participants were randomly split into two different groups (16-plyometric training and 15-control). Plyometric exercises included 180° jumps, bounding for distance, bounding in place, broad jump-stick, cone jumps, hop - hop stick, jump into bounding, jump - jump - jump vertical jump, mattress jumps, scissors jump, single-legged jumps for distance, squat jumps, step-jump up-down-vertical jump, tuck jumps, and wall jumps. Participants in the training group performed the exercises three times a week on alternating days, for 30-minute sessions. All participants performed a vertical jump measurement and completed maximum voluntary isometric contractions (MVICs) of ankle plantar flexion, knee extension, and knee flexion rate of torque development. Results: There was a main effect of session for plantar flexion rate of torque development time windows and vertical jump. The analysis revealed no significant differences for group by session interactions for any other rate of torque development measurement or the vertical jump. Conclusions: Plyometric exercises utilized from an anterior cruciate ligament injures of the knee injury prevention program did not change lower body isometric torque development or vertical jump height of active females. Since the plyometric training did not improve vertical jump, the training may not have been performed vigorous enough for the participants to cause a neuromuscular adaptation. Based on findings, 6-weeks of plyometric training from an isolated ACL injury prevention program should not be used to increase the vertical jump of healthy, active females.

The effect of ankle Kinesio taping on range of motion and agility during exercise in university students

Objective: The purpose of this study was to examine the effects of ankle Kinesio taping on range of motion and agility during exercise in university students. Design: Cross-sectional study. Methods: Thirty subjects were randomly allocated to two groups: taping group (n=15) and non-taping group (n=15). All groups underwent the same exercise program including stretching for 30 minutes. The exercise program proceeded in the following order: five minutes of stretching, a 20-minutes exercise program, and additional five minutes of stretching. Of the eight exercise methods suggested by Purcell et al, seven were chosen (lateral shuffle, forward and backward running, agility ladder, figure-of-8, forward jogging while jumping over cones, wall jumps and zigzags); 90o cuts with lateral shuffle were omitted. The range of motion of ankle dorsiflexion and plantarflexion was measured using the goniometer. Agility was measured using the side hop test. Results: For ankle range of motion, the taping group showed significant differences in dorsiflexion and plantarflexion on both sides (p<0.05). The non-taping group showed significant differences only in left plantarflexion (p<0.05). There was a significant difference in dorsiflexion on both sides between the taping group and the non-taping group (p<0.05). All groups showed significant differences in agility on the left and right ankle (p<0.05). There was a significant difference in left ankles between the taping group and the non-taping group (p<0.05). Conclusions: Kinesio taping increased range of motion and agility during exercise in university students. Additional research on Kinesio taping for improving range of motion and agility is needed.

Electrically controlled reversible and hysteretic magnetic domain evolution in nickel film/Pb(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.32 (011) heterostructure

We report direct Lorentz microscopy observations of electrically induced magnetic domain motion in a nickel film/Pb(Mg1/3Nb2/3)O3]0.68-[PbTiO3]0.32 (PMN-PT (011)) heterostructure. The 0.5 mm-thick PMN-PT substrate contains a 10 μm-wide, 60 nm-thick Ni/Pt electron-permeable observation region. Stress from the substrate creates magnetoelastic anisotropy of up to 4 kJ m−3 in the nickel film resulting in reversible magnetization rotation as well as non-reversible domain wall jumps (i.e., Barkhausen jumps). The observed magnetization of the film is directly related to the local strain gradient as computed by the finite element method, providing strong evidence of the effectiveness of the strain-mediated magnetoelectric approach for device applications.

Polarization reversal and jump-like domain wall motion in stoichiometric LiTaO3 produced by vapor transport equilibration

The polarization reversal and domain structure evolution has been studied in stoichiometric lithium tantalate prepared by vapor transport equilibration process. The first in situ visualization of domain kinetics has demonstrated the jump-like motion of few strictly oriented plane domain walls, which leads to short isolated current pulses in the switching current data. The proposed model of jump-like domain wall motion caused by interaction with pinning centers representing the areas with increased value of the threshold field is based on the effect of retardation of bulk screening. The derived formulas were applied successfully for analysis of the field dependence of the total switching time. The durations of wall jumps and wall stays (rest times) extracted from the switching current data are analyzed separately. The deceleration of the wall motion velocity during jump is controlled by the trail of residual depolarization field produced by bound charges and screening charges in the area behind the wall. The duration of the rest time is governed by the bulk screening of residual depolarization field. The value of Hurst exponent 0.75 obtained by fractal analysis of the switching current data has confirmed the essential influence of prehistory on the domain wall motion. The measurements of the coercive field by switching in bipolar triangular pulses in wide range of the field ramp rate have allowed us to extract the record low value of coercive field 60 V/mm for quasi-static polarization reversal.

Measurement and modeling of dielectric properties of Pb(Zr,Ti)O3 ferroelectric thin films

In this study, the real and imaginary parts of the complex permittivity of lead zirconate titanate ferroelectric thin films are studied in the frequency range of 100 Hz to 100 MHz. The permittivity is well fitted by the Cole-Cole model. The variation of the relaxation time with the temperature is described by the Arrhenius law and an activation energy of 0.38 eV is found. Because of its nonlinear character, the dielectric response of the ferroelectric sample depends on the amplitude of the applied ac electric field. The permittivity is composed of three different contributions: the first is due to intrinsic lattice, the second is due to domain wall vibrations, and the third is due to domain wall jumps between pinning centers. This last contribution depends on the electric field, so it is important to control the field amplitude to obtain the desired values of permittivity and tunability.

Domain-wall dynamics driven by short pulses along thin ferromagnetic strips: Micromagnetic simulations and analytical description

Micromagnetic simulations are used to describe the domain-wall dynamics along thin ferromagnetic strips driven by short pulses of magnetic field or electrical current with sinusoidal shape. For perfect strips without pinning centers, the net displacement of the domain wall is proportional both to the amplitude and the duration of the field pulse. A similar behavior is observed under current pulses if some nonadiabatic corrections are taken into account. On the contrary, the net displacement is null in the perfect adiabatic case. The domain-wall dynamics driven by these pulses is also characterized for strips which contain a single constriction, which acts as pinning site for the wall. The results reveal that an initially pinned domain wall can be eventually expelled far away from the constriction but if the maximum displacement does not surpass a given threshold the domain-wall experiences an attractive force which pushes it again toward the initial pinning site. Finally, the analysis of the domain wall jumps between two pinning sites is carried out both at zero and at room temperature for several separations between them. The simulations point out that the jumps can be achieved by means of short field or current pulses in the subnanosecond regime, an observation which could find application for a fast and easily controlled writing mechanism for future magnetic random access memory devices based on a pinned domain wall.

Direct observation of saw‐tooth typed domain wall jumps in epitaxial ferromagnetic MnAs film on GaAs(001)

AbstractWe report the direct observation of saw‐tooth domain wall jumps in epitaxial ferromagnetic MnAs film on GaAs(001) via a magneto‐optical Kerr effect microscope. Interestingly, it is found that the saw‐tooth domain wall jumps are generated along an easy axis orientation and the saw‐tooth angle is nearly constant irrespective of the applied field direction. These observations can be understood on the basis of strong uniaxial anisotropy on the sample plane. The analytical study reveals that the saw‐tooth domain wall structure results from competition between the domain wall energy and the dipolar energy. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Modeling microstructural effects on Barkhausen emission in surface-modified magnetic materials

Relationships between magnetic properties and microstructure in surface hardened steel were studied through measurements and model simulations, with the objective of developing techniques for quantitative measurements of case depth of surface hardened steels. The depth profiles of the magnetic properties were found to correlate with the hardness profile. Theoretical analysis using the hysteresis model and the stochastic model of domain wall dynamics indicates that the Barkhausen model parameters are inversely proportional to the domain wall pinning coefficient k. These relationships can be interpreted in terms of the effects of pinning site density on the local pinning field and distance of domain wall jumps which determine the Barkhausen activity.

Dynamics of a single-planar domain wall in ferroelectric–ferroelastic Gd2(MoO4)3

The field-induced jump-like dynamics of a single planar domain wall are investigated in single crystal uniaxial ferroelectric–ferroelastic Gd2(MoO4)3 containing extended bulk defects. The kind and location of the defects interfering with the free motion of the domain wall determines which type of elastic vibration is excited by the discontinuous jump of the domain wall. Time coincidence between optically observed jumps, Barkhausen pulses, and acoustic emissions each occur for particular jumps only. The time coincidence conditions correlate with the vibration spectra and differ for the initial release of the wall, large domain wall jumps, and clamped motions.

Magnetic instability of thin-film recording heads

Two forms of magnetic head instability are investigated: Write and read instability. Write instability generates noise directly after writing which impairs sector-servo performance. This noise peaks at write currents saturating the apex or backgap. The noise amplitude is exponentially distributed. The noise is caused by cooling off of the yoke after write as well as by thermally activated domain wall jumps in the upper yoke. Read instability produces read pulse distortion, predominantly on the signal's trailing edges. The only form of instability in dedicated-servo drives, read instability is best characterized by pulsewidth measurements. The degree of instability changes with intermediate writes. Read instability is caused by hysteretic domain wall motion in the pole tips in response to signal excitation and/or stray magnetic fields. >

Inductance fluctuation, domain instability and popcorn noise in thin film heads

The mechanisms of popcorn noise in thin film heads were studied by correlating the inductance fluctuations under DC bias current, domain instabilities after write and popcorn noise probabilities. Inductance fluctuations of heads were obtained by calculating the standard deviations of the inductance values measured from 20 traces by a HP network analyzer. Instantaneous domain patterns during and after write as well as popcorn noise probabilities of heads were observed by a 0.5-nsec exposure time wide-field Kerr effect microscope and a Barkhausen noise tester, respectively. A strong correlation was found among inductance fluctuations, domain instabilities and popcorn noise probabilities in a noisy head. This relationship was further confirmed by the quantitative correlation between the peak popcorn noise probabilities and peak inductance fluctuations of 12 different heads. Therefore, by measuring the inductance fluctuations of thin film heads under DC bias current, one can infer their susceptibilities to Barkhausen wall jumps, and thus popcorn noise probabilities. >

Wall jumps in temperature, partial pressures and populations of multicomponent mixtures of nonequilibrium polyatomic gases

The Maxwell-Loyalka method is used to derive formulas for the jumps in the macroparameters of a multicomponent nonequilibrium gas mixture. It is assumed that within the kinetic (Knudsen) wall layer one group of internal states of the molecules is close to equilibrium at the translational temperature, while another group and homogeneous chemical reactions are excited fairly slowly, so that they may be considered to be frozen and the corresponding inelastic collision integrals in the kinetic equations can be neglected. However, at the wall different groups of internal states of the molecules may be excited and chemical reactions may take place. The final calculation formulas are obtained under a series of simplifying assumptions in accordance with the recommendations made in [1, 2].

Field-driven domain-wall jumps in thin-film heads

Modeling voltage transients in thin-film heads produced by irreversible wall motion allows calculation of wall displacement versus time. Velocity modulation of large wall jumps traversing shallow interaction sites allows calculation of the distribution of these sites. In multiple wall jumps, sympathetic secondary jumps producing flux changes of the same polarity as the primary jump involve walls situated along the flux path. Antisympathetic secondary jumps producing flux changes opposite that of the primary jump involve walls situated parallel to the flux path. Finally, thermal agitation is shown to be able to release a wall below its critical field.

Barkhausen noise in thin-film recording heads

Barkhausen noise in thin-film heads was studied by exciting the heads with an external magnetic field and with a head-coil-generated field. Observed pulses show an exponential growth and decay, resulting in a second-order wall energy versus displacement model. Analytical expressions are given for the observed pulse shapes. Most pulses are essentially associated with unstable regions around attractive interaction sites where a wall in equilibrium cannot exist. A method is presented which can detect in which pole piece (P/sub 1/ or P/sub 2/) a wall jump occurs. It also differentiates wall jumps located close to the pole faces from those occurring deeper in the yoke. >

Plane turbulent denser wall jets and jumps

This paper presents the results of' an experimental (mainly photographic) study on the behaviour of plane turbulent denser Boussinesq-type wall jets and wall jumps on a smooth boundary. The distinction between the wall jet and wall jump has been brought out. Diagrams for predicting the growth rates of denser wall jets and wall jumps have been developed. Some interesting observations have been made regarding the temperature defect field in the denser wall jets for three values of the source Richardson number varying from 0.02 to about 0.2. A diagram has also been constructed to predict the flow regime for given source conditions and far-field stratified flow depth.

Wave propagation in a waveguide with discontinuous wall impedance

Nonlocally reacting porous liners are used extensively in ducts to achieve sound absorption. We consider wave propagation in a duct in which the boundary condition on one wall jumps from the impermeability condition to one exhibiting bulk reaction. The bulk‐reacting boundary condition of Bliss [J. Acoust. Soc. Am. 71, 533–545 (1982)] is used to model the behavior of the liner. There are three regions in the duct in which the fluid exhibits different overall behavior; two “outer” regions and one “inner” region in which a fast transition occurs in wave character. The method of matched asymptotic expansions will be used to obtain a solution that is uniformly valid throughout the three regions. [Work supported by the National Science Foundation.]