G. Durin

Signature of effective mass in crackling-noise asymmetry

Crackling noise is a common feature in many dynamic systems1,2,3,4,5,6,7,8,9, the most familiar instance of which is the sound made by a sheet of paper when crumpled into a ball. Although seemingly random, this noise contains fundamental information about the properties of the system in which it occurs. One potential source of such information lies in the asymmetric shape of noise pulses emitted by a diverse range of noisy systems8,9,10,11,12, but the cause of this asymmetry has lacked explanation1. Here we show that the leftward asymmetry observed in the Barkhausen effect2 — the noise generated by the jerky motion of domain walls as they interact with impurities in a soft magnet—is a direct consequence of a magnetic domain wall’s negative effective mass. As well as providing a means of determining domain-wall effective mass from a magnet’s Barkhausen noise, our work suggests an inertial explanation for the origin of avalanche asymmetries in crackling-noise phenomena more generally.

Dynamic hysteresis in Finemet thin films

In this paper, we present a series of dynamic measurements on a set of Finemet thin films, having thickness ranging from 200 /spl Aring/ to 5 /spl mu/m, by using both magneto-optical Kerr effect (MOKE) and the fluxmetric inductive method. Unexpectedly, the data show two completely different hysteresis behavior, showing relevant differences in their frequency dependences. In particular, optical hysteresis is easily explained by a simple depinning model which can be solved analytically.

On the power spectrum of magnetization noise

Abstract Understanding the power spectrum of the magnetization noise is a long standing problem. While the earlier work considered superposition of ‘elementary’ jumps, without reference to the underlying physics, recent approaches relate the properties of the noise with the critical dynamics of domain walls. In particular, a new derivation of the power spectrum exponent has been proposed for the random-field Ising model. We apply this approach to the experimental data, showing its validity and limitations.

Shape of a Barkhausen pulse

Abstract The average shape of the pulse in Barkhausen noise has been recently proposed as a tool to compare models and experiments. We compute theoretically the pulse shape of Barkhausen noise in a model describing the motion of a domain wall in an effective Brownian potential. In this framework, the pulse shape is related to the properties of the excursion of a random process in a c log (x)−kx potential. We record the Barkhausen noise in polycrystalline FeSi materials, and compare the pulse shape with the one predicted by the domain wall model.

Statistical properties of Barkhausen noise in amorphous ferromagnetic films.

We investigate the statistical properties of the Barkhausen noise in amorphous ferromagnetic films with thicknesses in the range between 100 and 1000 nm. From Barkhausen noise time series measured with the traditional inductive technique, we perform a wide statistical analysis and establish the scaling exponents τ,α,1/σνz, and ϑ. We also focus on the average shape of the avalanches, which gives further indications on the domain-wall dynamics. Based on experimental results, we group the amorphous films in a single universality class, characterized by scaling exponents τ=1.28±0.02,α=1.52±0.3, and 1/σνz=ϑ=1.83±0.03, values compatible with that obtained for several bulk amorphous magnetic materials. Besides, we verify that the avalanche shape depends on the universality class. By considering the theoretical models for the dynamics of a ferromagnetic domain wall driven by an external magnetic field through a disordered medium found in literature, we interpret the results and identify an experimental evidence that these amorphous films, within this thickness range, present a typical three-dimensional magnetic behavior with predominant short-range elastic interactions governing the domain-wall dynamics. Moreover, we provide experimental support for the validity of a general scaling form for the average avalanche shape for non-mean-field systems.

Investigation of scaling properties of hysteresis in Finemet thin films

Abstract We study the behavior of hysteresis loops in Finemet Fe73.5Cu1Nb3Si18.5B4 thin films by using a fluxometric setup based on a couple of well-compensated pickup coils. The presence of scaling laws of the hysteresis area is investigated as a function of the amplitude and frequency of the applied field, considering sample thickness from about 20 nm to 5 μm . We do not observed any scaling predicted by theoretical models, while dynamic loops show a logarithmic dependence on the frequency.

Is demagnetization an efficient optimization method

Abstract Demagnetization, commonly employed to study ferromagnets, has been proposed as the basis for an optimization tool, a method to find the ground state of a disordered system. Here we present a detailed comparison between the ground state and the demagnetized state in the random field Ising model, combing exact results in d=1 and numerical solutions in d=3. We show that there are important differences between the two states that persist in the thermodynamic limit and thus conclude that AC demagnetization is not an efficient optimization method.

Universal properties of magnetization dynamics in polycrystalline ferromagnetic films.

We investigate the scaling behavior in the statistical properties of Barkhausen noise in ferromagnetic films. We apply the statistical treatment usually employed for bulk materials in experimental Barkhausen noise time series measured with the traditional inductive technique in polycrystalline ferromagnetic films having different thickness from 100 to 1000 nm and determine the scaling exponents. Based on this procedure, we group the samples in a single universality class, since the scaling behavior of Barkhausen avalanches is characterized by exponents τ∼1.5, α∼2.0, and 1/σνz∼ϑ∼2.0 for all the films. We interpret these results in terms of theoretical models and provide experimental evidence that a well-known mean-field model for the dynamics of a ferromagnetic domain wall in three-dimensional ferromagnets can be extended for films. We identify that the films present an universal three-dimensional magnetization dynamics, governed by long-range dipolar interactions, even at the smallest thicknesses, indicating that the two-dimensional magnetic behavior commonly verified for films cannot be generalized for all thickness ranges.

Complex dynamics of magnetic domain walls

We resume the recent theoretical and experimental results which point towards a deeper comprehension of the complex dynamics of magnetic domain walls, i.e., the Barkhausen noise. In particular, we show how the theoretical framework of depinning transition is able to correctly describe the various experimental scaling exponents, included the power spectral exponent which has been investigated without success since the earlier papers.