Defect related switching field reduction in small magnetic particle arrays

An array of 42 mum square, 3 mum thick garnet particles has been studied. The strong crystalline uniaxial anisotropy of these particles results in the stable remanent state being single domain with magnetization parallel to the film normal. Magneto-optic measurements of individual particles provide distribution statistics for the easy-axis switching field H-sw, and the in-plane hard-axis effective anisotropy field, H-eff, which induces the formation of a metastable stripe domain structure. Both H-sw and H-eff are much smaller than the crystalline anisotropy field. Micromagnetic simulations show that the small H-sw cannot be attributed to shape anisotropy, but is consistent with smooth, localized reductions in the crystalline anisotropy caused by defects in either the particles or the substrate

Domain-wall coercivity in ferromagnetic systems with nonuniform local magnetic field

Domain-wall (DW) coercive field, H-CW, which characterizes pinning of DW's in soft magnetic materials, decreases strongly with increasing value of gradient, G, of the effective local DW-position-restoring magnetic field. Particular shapes of the dependence, H-CW(G), can be calculated from the mean energy dissipation of the DW moving over the particular profile of the DW pinning field, H-p. In this paper, H-CW(G) is calculated from a wall-pinning field, H-p, which is expressed as a stochastic function of the DW coordinate, x(DW). The wall-pinning field, H-p, is described as a Wiener-Levy stochastic process modified by two correlation lengths in such a way that H-p is stationary for large DW displacements and dH(p) /dx(DW) is well defined for small DW displacements. The computed H-CW(G) is close to a hyperbolic decrease, but it approaches finite values if G-->O and it decreases in a much steeper way than alpha 1/G for high values of G, which agrees with the experimental observations. Experimentally, the dependence H-CW(G) was measured on close-packed arrays of cylindrical bubble domains in two thin films of magnetic garnets, where the local field gradient, G, was controlled within the range 10(9)-10(10) A/m(2) by changing distances between neighboring DW's. The DW coercive field, H-CW, extrapolated from the measured values for G-->O was close to 80 A/m for both samples, while H-CW(G approximate to 10(10) A/m(2)) was several times smaller. Fitting the calculated H-CW(G) dependence to the experimental data, we obtained values of the Wiener-Levy correlation lengths well comparable to the DW width parameters

Modifying the temperature dependence of magnetic garnet film coercivity by etching

The temperature dependence of the domain-wall coercive field of epitaxial magnetic garnet films was modified in a defined temperature range by removing the surface layer of the films. Outside the given temperature range the coercivity versus temperature curve did not change. The result supports a model of coercivity according to which different sets of material imperfections are responsible for pinning the domain walls in different temperature regions. Appropriate processing of the samples enables some of the pinning sets to be modified independently of each other

Random constructions for translates of non-negative functions

Suppose A is a discrete infinite set of nonnegative real numbers. We say that A is type 2 if the series s(x) = Sigma lambda Lambda f (x + lambda) does not satisfy a zero-one law. This means that we can find a non-negative measurable "witness function" f : R -> [0,+ infinity) such that both the convergence set C(f, Lambda) ={x : s(x) < + infinity} and its complement the divergence set D (f, Lambda) = {x : s(x) = +infinity} are of positive Lebesgue measure. If Lambda is not type 2 we say that A is type 1. The main result of our paper answers a question raised by Z. Buczolich, J-P. Kahane, and D. Mauldin. By a random construction we show that one can always choose a witness function which is the characteristic function of a measurable set. We also consider the effect on the type of a set A if we randomly delete its elements. Motivated by results concerning weighted sums Sigma c(n)f(nx)and the Khinchin conjecture, we also discuss some results about weighted sum

SURVEY OF THE DEPENDENCE ON TEMPERATURE OF THE COERCIVITY OF GARNET-FILMS

The temperature dependence of the domain-wall coercive field of epitaxial magnetic garnets films has been investigated in the entire temperature range of the ferrimagnetic phase, and has been found to be described by a set of parametric exponents. In subsequent temperature regions different slopes were observed, with breaking points whose position was found to be sample dependent. A survey ba.ed on literature Data as well as on a large number of our own samples shows the general existence of this piecewise exponential dependence and the presence of the breaking points. This type of domain-wall coercive field temperature dependence was found in all samples in the large family of the epitaxial garnets (about 30 specimens of more than ten chemical compositionsj and also in another strongly anisotropic material (TbFeCo)

Review paper: Application of the Pulsed Eddy Current Technique to Inspect Pipelines of Nuclear Plants

Local wall thinning in pipelines affects the structural integrity of industries, such as nuclear power plants (NPPs). In the present study, a development of pulsed eddy current (PEC) technology that detects the wall thinning of pipelines covered with insulation is reviewed. The methods and experimental results, which have two kinds of probe with a single and double core, were compared. For this purpose, the single and double core probes having one and two excitation coils have been devised, and the differential probe with two Hall sensors has been fabricated to measure the wall thinning in insulated pipelines. The test sample is a stainless steel having different thickness, laminated by plastic insulation to simulate the pipelines in NPPs. The excitation coils in the probe is driven by a rectangular current pulse, the difference of two Hall sensors has been measured as a resultant PEC signal. The peak value of the detected signal is used to describe the wall thinning. The double core probe has better performance to detect the wall thinning covered with insulation; the single core probe can detect the wall thinning up to an insulation thickness of 18 mm, whereas the double probe can detect up to 25 mm. The results show that the double core PEC probe has the potential to detect the wall thinning in an insulated pipeline of the NPPs

TEMPERATURE-DEPENDENCE OF DOMAIN-WALL COERCIVE FIELD IN MAGNETIC GARNET-FILMS

The coercive properties of magnetically uniaxial liquid-phase epitaxy garnet films were investigated between 10 K and the Neel temperature (T(N) less-than-or-equal-to 500 K). Two independent methods, the results of which are nearly identical (magnetical response of oscillating domain walls and the method of coercive loops measured in a vibrating sample magnetometer), were used. Besides the usual domain-wall coercive field, H(dw), the critical coercive pressure, p(dw), was also introduced as it describes in a direct way the interactions of the domain walls with the wall-pinning traps. Both H(dw) and p(dw) were found to increase exponentially with decreasing temperature. Three different types of wall-pinning traps were identified in the sample and their strength, their rate of change with temperature, and their temperature range of activity were determined