Ronald B. Goldfarb

Kim model for magnetization of type‐II superconductors

We have calculated the initial magnetization curves and complete hysteresis loops for hard type‐II superconductors. The critical‐current density Jc is assumed to be a function of the internal magnetic field Hi according to Kim’s model, Jc(Hi)=k/(H0+‖Hi‖), where k and H0 are constants. As is the case for other critical‐state models, additional assumptions are that bulk supercurrent densities are equal to Jc, and that the lower critical field is zero. Our analytic solution is for an infinite orthorhombic specimen with finite rectangular cross section, 2a×2b (a≤b), in which a uniform field H is applied parallel to the infinite axis. Assuming equal flux penetration from the sides, we reduced the two‐dimensional problem to a one‐dimensional calculation. The calculated curves are functions of b/a, a dimensionless parameter p=(2ka)1/2/H0, and the maximum applied field Hm. The field for full penetration is Hp=H0[(1+p2)1/2−1]. A related parameter is H*m=H0[(1+2p2)1/2−1]. Hysteresis loops were calculated for the di...

Demagnetizing factors for cylinders

Fluxmetric (ballistic) and magnetometric demagnetizing factors N/sub f/ and N/sub m/ for cylinders as functions of susceptibility chi and the ratio gamma of length to diameter have been evaluated. Using a one-dimensional model when gamma >or=10, N/sub f/ was calculated for -1 >

Evidence for two superconducting components in oxygen-annealed single-phase YBaCuO☆

Abstract The complex susceptibility of a sintered YBaCuO superconductor is strongly dependent on a.c. field amplitude, h . Very small values of h must be used for the real part of susceptibility, χ′, to reach a value corresponding to bulk diamagnetism just below the critical temperature, T c . The imaginary part, χ″, represents hysteresis loss in the sample. Thus, χ″ versus temperature becomes positive when h exceeds the lower critical field, H cl of the superconductor. Annealing the material in oxygen gives rise to two distinct components, a relatively high- T c , high- H cl superconductor (denoted as ‘G’ or ‘good’) and a relatively low-T c , low- H cl superconductor (denoted as ‘B’ or ‘bad’). Curves of susceptibility versus increasing temperature reflect the dual nature of the annealed sample: χ′ has an inflection point at T c of the B component and approaches zero at T c of the G component, while χ″ has a peak at each T c . Both critical temperatures decrease linearly with increasing h , though at very different rates. H cl of the G component is considerably greater than H cl of the B component. The lower critical fields are linearly decreasing functions of temperature. Two models might explain the susceptibility data. In the grain model, the G component consists of superconducting grains and the B component is either intergranular material, unfavourably orientated anisotropic grains, or oxygen-depleted grain boundaries. In the surface model, the G component is in the interior of the sample and the B component is at the samples surface. This condition could arise if there was oxygen depletion at the surface subsequent to total enrichment during annealing.

Magnetic susceptibility of sintered and powdered Y-Ba-Cu-O

The real and imaginary parts of ac susceptibility of a sintered Y1Ba2Cu3O7−δ superconductor were measured before and after powdering. The temperature‐dependent susceptibility may be separated into two contributions, one sensitive and the other relatively insensitive to the magnitude of the measuring field. The former is partially suppressed by coarsely crushing the sample. It is completely suppressed after finely powdering, whereupon the susceptibility curves become insensitive to the magnitude of the measuring field. Several models apparently consistent with the results are discussed.

Calibration of ac susceptometer for cylindrical specimens

The absolute magnetic susceptibility of cylindrical specimens is obtained with an ac susceptometer whose calibration is based on a calculation of mutual inductance. An axially magnetized cylinder is modeled as a solenoid of the same size. The mutual inductance between such a solenoid and a pickup coil of arbitrary dimensions is computed. The susceptibility is then a function of the mutual inductance, the cylinder length, the magnitude and frequency of the ac magnetizing field, and the voltage induced on the pickup coil. Demagnetization factor and eddy‐current effects are considered, an example is given, and pickup coil compensation is discussed. Other calibration methods are also presented.

Sonochemical preparation of nanosized amorphous Fe-Ni alloys

Nanosized amorphous alloy powders of Fe20Ni80, Fe40Ni60, and Fe60Ni40 were prepared by sonochemical decomposition of solutions of volatile organic precursors, Fe(CO)5 and Ni(CO)4 in decalin, under an argon pressure of 100 to 150 kPa at 273 K. Magnetic susceptibility of Fe40Ni60 and Fe60Ni40 indicates blocking temperatures of 35 K and a magnetic particle size of about 6 nm. Thermogravimetric measurements of Fe20Ni80 give Curie temperatures of 322 °C for amorphous and 550 °C for crystallized forms. Differential scanning calorimetry exhibits an endothermic transition at 335 °C from a combination of the magnetic phase transition and alloy crystallization. The Mossbauer spectrum of crystallized Fe20Ni80 shows a sextet pattern with a hyperfine field of 25.04 T.

Alternating-Field Susceptometry and Magnetic Susceptibility of Superconductors

This review critically analyzes current practice in the design, calibration, sensitivity determination, and operation of alternating-field susceptometers, and examines applications in magnetic susceptibility measurements of superconductors. Critical parameters of the intrinsic and coupling components of granular superconductors may be deduced from magnetic susceptibility measurements. The onset of intrinsic diamagnetism corresponds to the initial decrease in electrical resistivity upon cooling, but the onset of intergranular coupling coincides with the temperature for zero resistivity. The lower critical field may be determined by the field at which the imaginary part of susceptibility increases from zero. Unusual features in the susceptibility of superconductor films, such as a magnetic moment that is independent of film thickness and the variation of susceptibility with angle, are related to demagnetization. Demagnetizing factors of superconductor cylinders are significantly different from those commonly tabulated for materials with small susceptibilities. Rules for the susceptibility of mixtures with specific demagnetizing factors are used to estimate the volume fraction of superconducting grains in sintered materials. Common misunderstandings of the Meissner effect, magnetic units, and formula conversions are discussed. There is a comprehensive summary of critical-state formulas for slabs and cylinders, including new equations for complex susceptibility in large alternating fields. Limitations on the use of the critical-state model for deducing critical current density are listed and the meaning of the imaginary part of susceptibility is considered.

Reduction of interfilament contact loss in Nb3Sn superconductor wires

Interfilament contact in Nb3Sn wires made by the internal‐tin‐diffusion process causes excess hysteresis loss beyond the intrinsic magnetic hysteresis loss of the filaments. In analogy with eddy‐current and proximity‐effect coupling losses, the excess contact loss can be reduced by decreasing the twist‐pitch length of the filaments in the wire. One consequence of interfilament contact is that volume magnetization measurements are strongly dependent on sample length below about one twist pitch. We define a characteristic length whose reciprocal is equal to the sum of the reciprocals of the sample length and the twist pitch. Hysteresis loss is a universal function of characteristic length for different sample lengths and twist pitches. We discuss several experimental parameters for the magnetic determination of hysteresis loss.

Method for measuring complex permeability at radio frequencies

An established method for measuring complex rf magnetic permeability is based on the change in inductance and resistance of a coaxial transmission line upon insertion of a sample toroid. It is not necessary to wind coils on the toroid or correct for geometric demagnetization factors. The use of modern commercial impedance analyzers, as described in this paper, makes measurements from 1 kHz to 1 GHz particularly easy, fast, and accurate.

Exchange‐anisotropy field in disordered nickel‐manganese alloys

Magnetization data for disordered and weakly ordered Ni‐Mn alloys from 22.1 to 32.1 at.% Mn have been obtained upon warming from 4.2 to 300 K, for measuring fields up to 100 kOe, after first cooling from 300 to 4.2 K in zero field or in 100 kOe. The measurements show that there is a minimum measuring field for which the data for zero‐field cooling match the data for field cooling. This field, for the disordered alloy, increases from 10 kOe at 22.1 at.% Mn to 90 kOe at 32.1 at.% Mn. Partial atomic ordering reduces this coupling field. The coupling field is smaller than expected from previous studies. The data are qualitatively explained by a recently proposed local‐environment model which considers explicitly the nearest‐neighbor environment of Mn atoms in the alloy.