1,720,979 research outputs found
Shape of a Barkhausen pulse
No full textThe 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
Thermodynamics of fractal networks
No full textOptimal channel networks are fractal structures that bear a striking resemblance to real rivers. They are obtained by minimizing an energy functional associated with spanning trees. We show that large network development effectively occurs al zero temperature since the entropy scales subdominantly with system size compared to the energy. Thus these networks develop under generic conditions and freeze into a static scale-free structure. We suggest a link of optimal channel networks with self-organized critical systems and critical phenomena which exhibit spatial and temporal fractality, the former under generic conditions and the latter on fine tuning
Analytical and numerical study of optimal channel networks
No full textWe analyze the optimal channel network model for river networks Using both analytical and numerical approaches. This is a lattice model in which a functional describing the dissipated energy is introduced and minimized in order to find the optimal configurations. The fractal character of river networks is reflected ill the power-law behavior of various quantities characterizing the morphology of the basin, In the context of a finite-size scaling ansatz, the exponents describing the power-law behavior are calculated exactly and show mean-field behavior, except for two limiting values of a parameter characterizing the dissipated energy, for which the system belongs to different universality classes. Two modified versions of the model, incorporating quenched disorder, are considered: the first simulates heterogeneities in the local properties of the soil and the second considers the effects of a nonuniform rainfall. In the region of mean-field behavior, the model is shown to be robust for both kinds of perturbations. In the two limiting cases the random rainfall is still irrelevant, whereas the heterogeneity in the soil properties leads to different universality classes. Results of a numerical analysis of the model are reported that confirm and complement the theoretical analysis of the global minimum. The statistics of the local minima are found to resemble more strongly observational data on real rivers
Loss separation for dynamic hysteresis in ferromagnetic thin films
No full textWe develop a theory for dynamic hysteresis in ferromagnetic thin films, on the basis of the phenomenological principle of loss separation. We observe that, remarkably, the theory of loss separation, originally derived for bulk metallic materials, is applicable to disordered magnetic systems under fairly general conditions regardless of the particular damping mechanism. We confirm our theory both by numerical simulations of a driven random-field Ising model, and by reexamining several experimental data reported in the literature on dynamic hysteresis in thin films. All the experiments examined and the simulations find a natural interpretation in terms of loss separation. The power losses' dependence on the driving field rate predicted by our theory fits satisfactorily all the data in the entire frequency range, thus reconciling the apparent lack of universality observed in different materials
Dynamic Hysteresis in FINEMET Thin Films
No full textWe performed a series of dynamic hysteresis measurements on Finemet films with composition Fe/sub 73.5/Cu/sub 1/Nb/sub 3/Si/sub 13.5/B/sub 9/, using both the longitudinal magneto-optical Kerr effect (MOKE) and the inductive fluxometric method. The MOKE dynamic hysteresis loops show a more marked variability with the frequency than the inductive ones, while both measurements show a similar dependence on the square root of frequency. We analyze these results in the frame of a simple domain wall depinning model, which accounts for the general behavior of the data
Signature of effective mass in crackling-noise asymmetry
No full textCrackling noise is a common feature in many dynamic systems1-9, 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 systems 8-12 , but the cause of this asymmetry has lacked explanation1. Here we show that the leftward asymmetry observed in the Barkhausen effect 2 -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 phenomenamore generall
Scaling, optimality and landscape evolution
No full textA nonlinear model is studied which describes the evolution of a landscape under the effects of erosion and regeneration by geologic uplift by mean of a simple differential equation. The equation, already in wide use among geomorphologists and in that context obtained phenomenologically, is here derived by reparametrization invariance arguments and exactly solved in dimension d = 1. Results of numerical simulations in d = 2 show that the model is able to reproduce the critical scaling characterizing landscapes associated with natural river basins. We show that configurations minimizing the rate of energy dissipation (optimal channel networks) are stationary solutions of the equation describing the landscape evolution. Numerical simulations show that a careful annealing of the equation in the presence of additive noise leads to configurations very close to the global minimum of the dissipated energy, characterized by mean field exponents. We further show that if one considers generalized river network configurations in which splitting of the flow (i.e., braiding) and loops are allowed, the minimization of the dissipated energy results in spanning loopless configurations, under the constraints imposed by the continuity equations. This is stated in the form of a general theorem applicable to generic networks, suggesting that other branching structures occurring in nature may possibly arise as optimal structures minimizing a cost function
Universality classes of optimal channel networks
No full textEnergy minimization of both homogeneous and heterogeneous river networks shows that, over a range of parameter values, there are only three distinct universality classes. The exponents for all three classes of behavior are calculated
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