Application of the group pulse processes in the theory of Barkhausen noise

In the paper the 2nd-order periodic group pulse process is considered a statistical model of the Barkhausen voltage. Using this model an analytical formula for the power spectrum is derived and further specified to fit the measured spectrograms of Barkhausen noise. The results are interpreted with respect to the generating physical mechanism and examples of several numerical values are given.     

Scaling exponents for barkhausen avalanches in polycrystalline and amorphous ferromagnets

We investigate the scaling properties of the Barkhausen effect by recording the noise in several soft ferromagnetic materials: polycrystals with different grain sizes and amorphous alloys. We measure the Barkhausen avalanche distributions and determine the scaling exponents. In the limit of vanishing external field rate, we can group the samples in two distinct classes, characterized by exponents tau = 1.50+/-0.05 or tau = 1.27+/-0.03, for the avalanche size distributions. We interpret these results in terms of the depinning transition of domain walls and obtain an expression relating the cutoff of the distributions to the demagnetizing factor which is in quantitative agreement with experiments.     

Visualization of the Barkhausen effect by magnetic force microscopy

By visualization of the Barkhausen effect using magnetic force microscopy we are able to provide detailed information about the physical principles that govern the magnetization reversal of a granular ferromagnetic thin film with perpendicular anisotropy. Individual Barkhausen volumes are localized and distinguished as either newly nucleated or grown by domain wall propagation. The Gaussian size distribution of nucleated Barkhausen volumes indicates an uncorrelated random process, while grown Barkhausen volumes exhibit an inverse power law distribution, which points towards a critical behavior during domain wall motion.     

Propagation of the electrodynamic disturbance following a Barkhausen jump in metallic ferromagnetic samples

An expression is derived for the space-time distribution of the magnetic field of a magnetic dipole with time dependent moment, which is used as a model of a Barkhausen jump. The medium surrounding the dipole is a conductor of infinite dimensions.     

Use of time-delay correlators and wave-shaping techniques in the statistical analysis of Barkhausen pulses

A new class of digital correlators and other related instruments have been developed recently for photoelectron statistics applications. In this work we explore the possibility of using these low cost devices for analyzing trains of Barkhausen pulses. We also outline the applications of the theory of random point processes (specifically, self-exciting point processes) to Barkhausen noise. Finally, we present a new technique for exposing ferromagnetic samples to a linearly variable induction field. Our technique is fully digital and offers several advantages in terms of accuracy and reproducibility of control over the conventional tape-recorder method.     

Der Elektromagnetische Aspekt der Magnetomechanischen Erscheinung in Gleich und im Monoalternant Pulsierenden Magnetfeld

The electromotive tension induced by Barkhausen discontinuities is studied. These Barkhausen discontinuities are excited by periodic mechanical stress in a d-c and a mono-alternating pulse magnetic field.     

Evaluation of surface decarburization depth by magnetic Barkhausen noise technique

Industrially unfavorable process of steel surface decarburization was induced by annealing in air. Two methods of after-anneal surface treatment were used: an acid pickling and a sand blasting. The obtained decarburized layers were examined by optical microscope, wave dispersive spectrometer, and surface X-ray diffraction method. Magnetic Barkhausen noise technique was tested for applicability of non-destructive characterization of the decarburized layer depth. A newly introduced parameter, Barkhausen noise coercivity, was proposed for practical use due to its sensitivity to decarburization and stability to measurement conditions. Other magnetic parameters, e.g. number of Barkhausen noise counts, were found to be sensitive to the compressive residual stress caused by the sand blasting.     

Scaling behavior of the first arrival time of a random-walking magnetic domain

We report a universal scaling behavior of the first arrival time of a traveling magnetic domain wall into a finite space-time observation window of a magneto-optical microscope enabling direct visualization of a Barkhausen avalanche in real time. The first arrival time of the traveling magnetic domain wall exhibits a nontrivial fluctuation and its statistical distribution is described by universal power-law scaling with scaling exponents of 1.34+/-0.07 for CoCr and CoCrPt films, despite their quite different domain evolution patterns. Numerical simulation of the first arrival time with an assumption that the magnetic domain wall traveled as a random walker well matches our experimentally observed scaling behavior, providing an experimental support for the random-walking model of traveling magnetic domain walls.     

Exactly solvable model of avalanches dynamics for Barkhausen crackling noise

We review the present state of understanding of the Barkhausen effect in soft ferromagnetic materials. Barkhausen noise (BN) is generated by the discontinuous motion of magnetic domains as they interact with impurities and defects. BN is one of the many examples of crackling noise, arising in a variety of contexts with remarkably similar features, and occurring when a system responds in a jerky manner to a smooth external forcing. Among all crackling system, we focus on BN, where a complete and consistent picture emerges thanks to an exactly solvable model of avalanche dynamics, known as the ABBM model, which ultimately describes the system in terms of a Langevin equation for the velocity of the avalanche front. Despite its simplicity, the ABBM model is able to accurately reproduce the phenomenology observed in the experiments on a large class of magnetic materials, as long as universal properties are involved. To complete the picture and to understand the long-standing discrepancy between the ABBM theory and the experiments, which otherwise agree exceptionally well, consisting of the puzzling asymmetric shape of the noise pulses, microscopic details must be taken into account, namely the effects of eddy current retardation. These effects can be incorporated in the model, and result, to a first-order approximation, in a negative effective mass associated with the wall. The progress made in understanding BN is potentially relevant for other crackling systems: on the one hand, the ABBM model turns out to be a paradigmatic model for the universal behaviour of avalanche dynamics; on the other hand, the microscopic explanation of the asymmetry in the noise pulses suggests that inertial effects may also be at the origin of pulses asymmetry observed in other crackling systems.     

Lattice-scale domain wall dynamics in ferroelectrics

Ferroelectric domain walls are atomically thin, and consequently their dynamics are sensitive to the periodic potential of the underlying lattice. Despite their central role in domain dynamics, lattice-scale effects have never been directly observed. We investigate local domain dynamics in thin film ferroelectrics using atomic-force microscopy. Upon combined dc and ac electric driving, fluctuations in the local piezoresponse are observed. Fourier analysis of the fluctuations reveals the presence of narrow band and broad band noise, and Barkhausen jumps. The narrow band noise is attributed to dynamics associated with lattice-scale pinning and is reproduced by a simple physical model.     

Investigation of the dependence of Barkhausen noise on stress and the angle between the stress and magnetization directions

The dependence of Barkhausen noise amplitude on stress, and on the angle between the stress and magnetization directions, has been investigated in specimens of AISI/SAE 4340, HY-80 and ASTM A-36 steels. In all three materials it was found that the Barkhausen noise amplitude measured with the magnetic field applied at 60° to the stress axis is independent of stress. This result is compared with the theory of elasticity which indicates that under a uniaxial or a pure bending stress, the normal component of a strain in steel is zero along the direction approximately 61° from the direction of the applied stress. It was also found that the Barkhausen noise follows the same angle dependence as that of the normal strain. These findings strongly indicate that the Barkhausen noise amplitude is directly dependent on the normal strain along the direction of the magnetization and not on the shearing strain. An explanation for such a direct normal strain dependence is given based on magnetoelastic energy. Possible application of this finding to quantitative nondestructive stress measurements is also discussed.     

Magnetic Barkhausen noise measurement by resonant coil method

This paper describes a powerful new technique for nondestructive evaluation of ferromagnetic material. A method has been developed for measuring magnetic Barkhausen signals under different coil resonance frequencies. The measurements allow one to establish the behavior relating the power spectral density maximum and the resonant coil frequency. Time-frequency analysis of Barkhausen signals puts in evidence the tuning regions for each coil, and allows clear identification of each contribution to the Barkhausen signal spectrum. This concept was used in order to evaluate the relation between the degree of plastic deformation in carbon steel samples, and the power spectral density maximum at different resonance frequencies. This result also makes it possible to the selectively modify measurement sensibility to the magnetic Barkhausen signal by using different resonance frequencies.     

Stability of the single-domain state in single crystals of triglycine sulphate

The stability of the single-domain state in single crystals of triglycine sulfate is investigated using the Barkhausen effect. Barkhausen jumps are detected upon invariable macroscopic polarization. The parameters of the observed Barkhausen jumps are estimated and compared.     

Model for domain wall avalanches in ferromagnetic thin films

The Barkhausen jumps or avalanches in magnetic domain-walls motion between successive pinned configurations, due the competition among magnetic external driving force and substrum quenched disorder, appear in bulk materials and thin films. We introduce a model based in rules for the domain wall evolution of ferromagnetic media with exchange or short-range interactions, that include disorder and driving force effects. We simulate in 2-dimensions with Monte Carlo dynamics, calculate numerically distributions of sizes and durations of the jumps and find power-law critical behavior. The avalanche-size exponent is in excellent agreement with experimental results for thin films and is close to predictions of the other models, such as like random-field and random-bond disorder, or functional renormalization group. The model allows us to review current issues in the study of avalanches motion of the magnetic domain walls in thin films with ferromagnetic interactions and opens a new approach to describe these materials with dipolar or long-range interactions.     

Stochastic dynamics of a sheared granular medium

We experimentally investigate the response of a sheared granular medium in a Couette geometry. The apparatus exhibits the expected stick-slip motion and we probe it in the very intermittent regime resulting from low driving. Statistical analysis of the dynamic fluctuations reveals notable regularities. We observe a possible stability property for the torque distribution, reminiscent of the stability of Gaussian independent variables. In this case, however, the variables are correlated and the distribution is skewed. Moreover, the whole dynamical intermittent regime can be described with a simple stochastic model, finding good quantitative agreement with the experimental data. Interestingly, a similar model has been previously introduced in the study of magnetic domain wall motion, a source of Barkhausen noise. Our study suggests interesting connections between different complex phenomena and reveals some unexpected features that remain to be explained.     

Die Temperaturabhängigkeit des magnetischen Barkhausen-Effekts IV

The size distribution of Barkhausen discontinuities (magnetic moment > 6.5 · 10^?7 cgs) of very pure nickel was measured between ?195 and 360 °C on the hysteresis loop. The mean value of internal stresses was changed in a well defined manner by stretching the specimens. For different stress values the variation with temperature of the following properties is given in diagrams: The specific number and the “mean volume”² of discontinuities, the maximal value of Barkhausen susceptibility, the mean deviation of the volume of discontinuities along the hysteresis curve, and some characteristic field strengths defined by reversible and irreversible processes. Conclusions are drawn concerning the coupling of individual wall movements composing a Barkhausen jump.     

Der elektromagnetische Aspekt der Energiezufuhrerscheinung von einem longitudinalen Wechselmagnetfeld an ein Torsionspendel mit ferromagnetischer Aufhängung

It is shown that under conditions in which a torsion pendulum with ferromagnetic suspension thread oscillates with torsional oscillation being excited by the energy transferred from an a-c magnetic field, Barkhausen discontinuities are released by the torsional deformations. A study is made of the dependence of the voltage pulse, which induced these Barkhausen discontinuities in the coil in the field of which the ferromagnetic suspension thread is placed, on the intensity of the a-c magnetic field, the amplitude of torsional deformation and the angle of phase shift between these two sine quantities. The influence of an underlying longitudinal d-c magnetic field on these Barkhausen discontinuities is also investigated.

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Wir danken I. Schreck für seinen wertvollen Beitrag zur Herstellung der in dieser Arbeit benützten elektronischen Apparatur.     

Effect of the length of action of a steady electric field on the polarization buildup process in triglycene sulfate and barium titanate single crystals

During the investigation of the Barkhausen effect and the reversal current [1] in ferroelectric crystals, it was observed that, together with other factors, the length of time during which the specimens had been kept in a steady electric field has a substantial effect on the polarization reversal process. In this connection, it is of interest to investigate parallely the Barkhausen effect and the reversal current in ferroelectric crystals at various lengths of action of the electric field. In this paper we have investigated the number of Barkhausen jumps, the charge being reversed, the buildup time of the total and irreversible polarization, as well as the starting field for jumps and reversal current as a function of the length of action of the field on specimens of TGS and BaTiO₃ single crystals during the quasistatic polarization reversal process.     

Barkhausen effect in magnetic thin films: Experimental noise spectra

In this paper Barkhausen noise spectra are shown that were measured on thin uniaxial 83-17 Ni?Fe films and these spectra were compared with the known magnetic behaviour of these films. On these types of sample the Barkhausen effect had been investigated by Lambeck [1], but their Barkhausen noise spectra have not been investigated so far. The film thicknessd _m in our experiments ranged between 400 and 2400 Å. In this range a change occurs in the dynamics of the magnetization behaviour caused by transitions of the type of domain wall. The results presented here show the very strong dependence of the Barkhausen noise spectra on the type of domain wall. Films with Bloch walls always show a frequency dependence off ^?1.7 in the higher frequency range. Samples in the thickness range where the transition occurs from the Néel wall via the cross-tie wall to the Bloch wall, have exponents between ?1 and ?2. The frequencyf _c above which the measured noise intensity begins to decrease varies very much for the different films. The curve off _c versus film thicknessd _m has the same form as the curve of the domain wall mobilitym versusd _m.