Monitoring neutron embrittlement in nuclear pressure vessel steels using micromagnetic Barkhausen emissions

In nuclear power plants, neutron embrittlement of pressure vessel steels has been one of the main concerns. The use of micromagnetic Barkhausen emissions is a promising method to monitor the variations in microstructural and subsurface stress states due to their influence on these emissions. Measurements of these emissions can reveal neutron irradiationdegradation in nuclear power plant components. Samples which were irradiated at differentneutron fluences and annealed at different temperatures were obtained from three reactor surveillance programs. The results of different neutron fluences and annealing procedures showed noticeable fractional changes in the magnetic Barkhausen effect signal parameter, ΔMBE/MBE, and in the mechanical properties of these specimens. For example, increased intensity of neutron fluence decreased the ΔMBE/MBE as well as impact energy and upper‐shelf energy, but increased Rockwell hardness and yield strength. Typical changes in this parameter were in the range from −20% to −45% for fluences of up to 25×1018 n cm−2

Comprehensive analysis of Barkhausen emission spectra using pulse height analysis, frequency spectrum, and pulse wave form analysis

The dependence of magnetic Barkhausen emissions (MBE) upon both field excitation and detection frequencies and excitation wave form was studied in order to investigate two of several crucial factors which affect the emissions. Sinusoidal, triangular, and square wave forms were used to generate the MBE and the pulse height spectra, frequency spectra, and pulse wave forms of these signals were analyzed. The frequency spectra of sinusoidal and triangular alternating field excitations showed similar behavior but the spectrum under square wave excitation was different due to the existence of high frequency components during square wave switching. As yet, no common standard has been agreed upon for parameterization and representation of Barkhausen signals. It appears from this work that field excitation wave form and frequency should define the inputs, while detection frequency range, pulse height spectrum, frequency spectrum, and emitted pulse wave form analysis should be used to quantify the output

Pooled Analysis of Resistance Patterns of Escherichia coli Strains Isolated From Urine Cultures in Turkey: Comparison of 1997–2001 and 2002–2007 Periods

WOS: 00026328780028

Nonuniversal scaling behavior of Barkhausen noise

We simulate Barkhausen avalanches on fractal clusters in a two-dimensional diluted Ising ferromagnet with an effective Gaussian random field. We vary the concentration of defect sites $c$ and find a scaling region for moderate disorder, where the distribution of avalanche sizes has the form $D(s,c,L) = s^{-(1+\tau (c))}{\cal{D}}(sL^{-D_s(c)})$. The exponents $\tau (c)$ for size and $\alpha (c)$ for length distribution, and the fractal dimension of avalanches $D_s(c)$ satisfy the scaling relation $D_s(c)\tau (c) =\alpha (c)$. For fixed disorder the exponents vary with driving rate in agreement with experiments on amorphous Si-Fe alloys.Comment: 5 pages, Latex, 4 PostScript figures include

Hysteresis in the Random Field Ising Model and Bootstrap Percolation

We study hysteresis in the random-field Ising model with an asymmetric distribution of quenched fields, in the limit of low disorder in two and three dimensions. We relate the spin flip process to bootstrap percolation, and show that the characteristic length for self-averaging $L^*$ increases as $exp(exp (J/\Delta))$ in 2d, and as $exp(exp(exp(J/\Delta)))$ in 3d, for disorder strength $\Delta$ much less than the exchange coupling J. For system size $1 << L < L^*$, the coercive field $h_{coer}$ varies as $2J - \Delta \ln \ln L$ for the square lattice, and as $2J - \Delta \ln \ln \ln L$ on the cubic lattice. Its limiting value is 0 for L tending to infinity, both for square and cubic lattices. For lattices with coordination number 3, the limiting magnetization shows no jump, and $h_{coer}$ tends to J.Comment: 4 pages, 4 figure

Charge and spin distributions in GaMnAs/GaAs Ferromagnetic Multilayers

A self-consistent electronic structure calculation based on the Luttinger-Kohn model is performed on GaMnAs/GaAs multilayers. The Diluted Magnetic Semiconductor layers are assumed to be metallic and ferromagnetic. The high Mn concentration (considered as 5% in our calculation) makes it possible to assume the density of magnetic moments as a continuous distribution, when treating the magnetic interaction between holes and the localized moment on the Mn(++) sites. Our calculation shows the distribution of heavy holes and light holes in the structure. A strong spin-polarization is observed, and the charge is concentrated mostly on the GaMnAs layers, due to heavy and light holes with their total angular momentum aligned anti-parallel to the average magnetization. The charge and spin distributions are analyzed in terms of their dependence on the number of multilayers, the widths of the GaMnAs and GaAs layers, and the width of lateral GaAs layers at the borders of the structure.Comment: 12 pages,7 figure

Modeling of micromagnetic Barkhausen activity using a stochastic process extension to the theory of hysteresis

Recent work by Bertotti [IEEE Trans. Magn. MAG‐24, 621 (1988)] and others has shown that it is possible to model the micromagnetic Barkhausen discontinuities at the coercive point using a two‐parameter stochastic model. However, the present formulation of the model is restricted to limited regions of the hysteresis curve over which dM/dH is approximately constant and whendH/dt is held at a constant rate. A natural extension of this model is to take the basic result, in which the level of Barkhausen activity in one time period is related to the activity in the previous time period, and increment it by a small amount which is dependent on the differential permeability. The extension of the model proposed here uses the theory of ferromagnetichysteresis to determine the differential permeability at any point of the hysteresis loop. The Barkhausen activity is then assumed to vary in proportion to the differential permeability. The resulting model allows the Barkhausen sum of discontinuous changes in magnetization to be modelled around the entire hysteresis loop, leading to an important generalization of the basic model