Flux Dynamics in NdO1−x F x FeAs Bulk Sample

We present data of multiharmonic magneto-dynamic experiments. In particular, we performed ac magnetic susceptibility experiments on layered pnictide-oxide quaternary compound NdOFeAs doped with fluorine. The experiments allow one to measure the critical temperature and probe the flux dynamic behavior using the third harmonic component of the ac susceptibility of an NdF_0.16FeAsO_0.84 bulk sample as a function of temperature and frequency of the applied ac magnetic fields. Measured signals are connected with the nonlinear superconducting flux dynamic behavior and are characterized by a “flux critical states” sustaining a superconducting critical current. In this framework the irreversibility line that describes the stable superconducting state has been extracted from the onset of the third harmonic signal vs. frequency. Finally we present also the analysis of the flux dynamic dimensionality in the investigated sample.     

AC losses in multilayer superconducting tapes

ac losses have been investigated experimentally as well as theoretically in tapes having on the surface several normal conducting and/or superconducting layers. The superconducting layers under investigation have been Nb₃Sn on a niobium substrate and Nb₃Ge on a stainless steel substrate. It has been proved that the layered structure of the tapes is well reflected by the stepwise character of the ac losses dependence on the amplitude of the surface magnetic field. The magnetic flux passing through a surperconducting layer or surface barrier into the inside of the tape enhances the losses in the passed barrier or layer.     

High-Frequency Tunneling Magnetic Loss in Soft Ferrites

This paper considers high-frequency (200 kHz-1.0 MHz) losses in MnZn power ferrites and shows that none of the three well-known magnetic loss mechanisms (namely, hysteresis, classical eddy-current loss, and excess eddy-current loss) can account for the experimentally observed dependence of the loss on the frequency and flux density. In order to investigate the origin of this discrepancy, the electric field that is induced in the typical core when the material is driven at high frequencies and flux densities was estimated. The estimates show that these electric fields can be quite large. The paper presents experimental data on the electrical conductivity for such large electric fields, which shows a highly nonlinear behavior that can give rise to a modified eddy-current loss mechanism. By a simple curve fit to the nonlinear conductivity, the experimentally observed flux density dependence of the high-frequency loss, which previously could not be explained, can be reproduced by using this modified eddy-current loss mechanism. A modified ferrite structure can eliminate most of these extra losses by reducing the electric field generated at the grain boundaries due to high frequencies and flux densities     

Static and dynamic forces on a permanent magnet levitating between superconducting surfaces

A charged magnetic microsphere (radius 100 µm) is levitating inside a superconducting niobium capacitor. Because of its charge of about 1 pC, oscillations about the equilibrium position can be excited and detected electrically. The properties of this oscillator are investigated in order to study the static and dynamic forces of magnetic levitation. We find resonance frequencies between 200 Hz and 600 Hz. The resonance frequency and the damping are amplitude dependent due to nonlinear return forces and nonlinear friction, respectively. At small amplitudes the Q value is about 10⁶ in vacuum. A dc field can be applied to change the equilibrium position and consequently the resonance frequency. From the data the spatial dependence of the static force and of the dynamic stiffness can be determined and an empirical relation between both is established. Quite often, we find a hysteretic behavior of the static force whereas the stiffness is a reversible function of the position. Amplitude dependent damping is observed both in the decay of the free oscillations and in the dependence of the amplitude of the forced oscillations on the driving force.     

Losses in superconducting cables in pulsed magnetic fields

Measurements have been carried out of superconducting cables of different types in pulsed magnetic fields. Three types of samples have been made from multifilamentary Nb_0.5  Ti_0.5 superconductors: one, a cluster of isolated wires; two, a double-layer twisted flat cable; and three, one-layer twisted tube samples. Dependences have been studied of ac losses on the amplitude, direction and change velocity of the magnetic field as well as on the diameter of filaments and twist pitch. It is shown that the ac losses in unsoldered samples are close to those in the cluster of isolated wires.     

Pulsed magnetic field losses and critical current densities of V3 Ga and Nb3Sn multifilament superconductors

Pulsed magnetic field loss measurements have been performed on special V₃Ga filament superconductors, which are known to have very high critical current densities, and on a NB₃Sn filament superconductor.The dependence of the hysteresis losses on the diffusion heat treatment parameters for the V₃Ga layer formation is reported. To analyse the results, the hysteresis losses of hollow superconducting filaments in parallel and perpendicular magnetic fields were calculated.From E.J. Kramer's theory of flux pinning we postulate that, for a given material, upper and lower intrinsic limits of the hysteresis losses are due to the flux line shearing effect. A part of the results was presented at the ICEC 7.     

Doping Dependence of Magnetic Field Penetration Depth in Kinetic Energy Driven Cuprate Superconductors

The doping and temperature dependence of the magnetic field penetration depth in cuprate superconductors is studied based on the kinetic energy driven superconducting mechanism. It is shown that the magnetic field penetration depth shows a crossover from the linear temperature dependence at low temperatures to a nonlinear one in the extremely low temperatures.     

Experimentally Determined Magnetization ac Losses of Mono and Multifilamentary MgB2 Wires

Magnetization AC loss measurements have been performed by the calibration free method on mono, 19 filaments un-twisted and 19 filaments twisted MgB₂ superconducting wires with magnetic fields perpendicular to the wire axis at various frequencies and temperatures between 24 and 40 K. AC losses were also calculated based on critical state model for mono-filamentary Nb sheathed MgB₂ wire with conditions corresponding to experiments. Good agreement is observed between model and experimental data. AC losses go through a peak when the wire was cooled below T _c. In terms of AC magnetization losses, the twisted wire has the minimum losses in order of 10^?5 J/m at 30 K in AC magnetic field with 20 mT amplitude.     

ac losses of superconducting composites with 8 μm NbTi filaments in a dc magnetic field with a superimposed ac component at 1 f 500 Hz

Measurements of the ac losses of two superconducting coils, made from low-loss filamentary superconductors, are reported for a coil current consisting of a dc and a superimposed ac component. The matrix losses and hysteresis losses in the superconductor are experimentally and theoretically analysed, especially their frequency and amplitude dependence.     

Ferromagnetism of 2D Solid 3He Investigated by SQUID NMR

Pulsed NMR measurements have been performed on ³He films adsorbed on an exfoliated graphite sample of area 2 m². The spectrometer is constructed using a DC SQUID with additional positive feedback, operating in flux locked loop mode with a bandwidth of 3.4 MHz. The input circuit, is a superconducting flux transformer, and so is intrinsically broadband. This spectrometer can therefore operate from typical frequencies used in conventional NMR down to otherwise inaccessible low kHz frequencies. With this system studies at the “ferromagnetic anomaly” have been undertaken at frequencies from 2 kHz to 100 kHz (0.06 to 3.09 mT), with the static magnetic field oriented normal to the nominal direction of the graphite platelets. At 50 and 100 kHz, measurements of the susceptibility are fit by high temperature series expansions between 100 mK and 5 mK, to determine an exchange constant J=1.86 mK. Below 1.3 mK the dipolar frequency shift increases linearly with temperature down to 0.3 mK, extrapolating to 10 kHz, close to our calculated value of 9.6 kHz, for fully polarised spins with an assumed lattice spacing of 0.392 nm for the second layer solid. Below 1.5 mK, a significant field dependence to the magnetisation and dipolar frequency shift appears at fields comparable to the characteristic dipolar field, below which the equilibrium alignment of the magnetization is field dependent. Assuming the frequency shift, in applied fields of 1.54 and 3.09 mT, to be proportional to the sample polarisation we find that it is not possible to fit the observed temperature dependence by 2D ferromagnetic spin-wave theory, with a consistent set of parameters for both fields. The theory is applied in the range 0.15>T/J>0.5, takes into account the Zeeman gap and finite system size, and includes the k=0 spin wave term. At present, the data provide no unambiguous evidence for a finite ordering temperature induced by the anisotropic dipolar interaction.     

Magnetic Flux Diffusion Inside a Superconducting Rod of Square Cross Section Submitted to a Sinusoidal Magnetic Field

The flux diffusion into a superconducting long rod of square cross-section in the flux flow regime is investigated numerically for sinusoidal variations of the external magnetic field. The real and imaginary parts of first harmonic as well as its penetration depth are determined in function of the field frequency. This penetration depth, which decreases exponentially in function of the frequency, is influenced by the change of the flux front shape from square to circular at low frequencies and seems to be a scaling length for both components of the first harmonic.     

Effect of Critical Current Density and Critical Temperature on ac Susceptibility

Based on the flux creep equation, the effect of critical current density and critical temperature on ac susceptibility is investigated numerically in a superconducting slab immersed in an ac magnetic field. The current density dependence of the flux creep activation barrier is employed as the logarithmic law. The fundamental ac susceptibilities of the slab as a function of temperature for the same ac field have been derived in a unified picture. The results show that ac susceptibility in flux creep regime is affected by critical current density and critical temperature.     

The influence of thermally activated flux creep on the ac response of a hard superconductor

A theoretical and experimental study is presented of the influence of thermally activated flux creep on the ac response of a type II superconductor in the mixed state. The theory describes the ac response of a cylindrical superconductor in the mixed state in an axial dc magnetic field and a superposed parallel ac field of low amplitude and low frequency. Thermally activated flux creep gives rise to flux motion at the peak and the valley of the sinusoidal ac applied field and a frequency-independent phase shift of the voltage induced in a pickup coil wound about the sample. The instantaneous voltage at the peak or valley of the ac applied field is related to the critical-current density J _c and the pinning-energy barrier normalized by the temperature U/kT. The ac loss voltage and the voltage waveform were measured for a cold-worked NbZr sample. For temperatures T below 4.2 K, the measured values of kT/U are nearly proportional to the temperature. Both J _c and U were observed to depend on magnetic field history.Research sponsored by the Division of Materials Science, U.S. Department of Energy, under contract W-7405-eng-26 with the Union Carbide Corporation.     

The vibrating wire viscometer as a magnetic field-independent3He thermometer

An analysis of the semicircular loop, vibrating-wire³He viscometer has been made. Emphasis was placed upon the magnetic field dependence of the resonance frequency and frequency width (damping) in order to study the suitability of the device for a field-independent thermometer. The analysis was complemented by experiments with Ta, Nb-Ti, Cu, Ag, Au, and Cu-Ni wires. It was found that the various wires, whether superconducting or pure or impure normal metals, have advantages and disadvantages. The superconducting wires show a large field-dependent frequency shift due to induced magnetization and a field-dependent damping due to flux motion. Pure normal metals also show induced magnetization (eddy-current) frequency shift and damping, and impure metals and alloys give Joule heating. Nevertheless, it is shown that the device can be used in the range 1–10 mK and for fields from 20 mT to 1 T.     

Dendritic flux avalanches in superconducting Nb3Sn films

The penetration of magnetic flux into a thin superconducting film of Nb₃Sn with critical temperature 17.8 K and critical current density 6 MA/cm² was visualized using magneto-optical imaging. Below 8 K an avalanche-like flux penetration in form of big and branching dendritic structures was observed in response to increasing perpendicular applied field. When a growing dendritic branch meets a linear defect in the film, several scenarios were observed: the branch can turn and propagate along the defect, continue propagation right through it, or “tunnel” along a flux-filled defect and continue growth from its other end. The avalanches manifest themselves in numerous small and random jumps found in the magnetization curve.     

Paramagnetic Effect in Nano-opal-lead Structure

We report magnetic studies of the paramagnetic effect observed in the superconducting nano-structured opal-lead system. Positive magnetization is clearly observed when the sample is cooled in field. The paramagnetic effect is strongly dependent on the applied field and the cooling rate. The results suggest that the paramagnetic moment is due to flux trapping and the competition between the positive and negative moments due to the temperature dependence of the penetration depth.     

Pattern of flux tubes in superconducting lead films

At low magnetic fields the intermediate-state structure of type I superconducting films can consist of multiquantum flux tubes arranged in an ordered or disordered configuration. We have studied the number of flux tubes in small, rectangular, lead films of 6–8 µm thickness as a function of the magnetic field applied perpendicular to the films, using a high-resolution magnetooptical method. In order to facilitate the establishment of the equilibrium flux distribution and to minimize the influence of the Gibbs free energy barrier against magnetic flux penetration, the specimens were cooled at constant field from aboveT _c to 4.2 K. From the results the average flux tube diameter was calculated using flux conservation. The flux tube diameter was found to be larger by a factor of 2–3 than the value expected from the Goren-Tinkham model.Based on work performed under the auspices of the U.S. Atomic Energy Commission.     

The intermediate state in superconducting mercury

Using the magneto-optic technique, an extensive study has been made of the intermediate state in single crystals of superconducting mercury. A regular laminar structure, closely approaching that envisaged by Landau, was realized in the thinnest sample examined (47 Μm). The field dependence of the lamina periodicity displays excellent agreement with the Landau model and the deduced surface energy is in good agreement with the Ginzburg-Landau theory. At applied magnetic fields very close to the critical one, the stable magnetic structure consists of isolated superconducting filaments. A number of observations are reported on their growth and stability.     

Measurement of ac losses in multifilament super-conducting wires at frequencies between 1 and 100 Hz

Calorimetric measurements of the losses of a superconducting coil wound from a moderately twisted multifilamentary superconducting wire have been made with a coil current consisting of a direct current and a superimposed ac component at frequencies between 1 and 100 Hz. There exists a frequency range where the loss per cycle decreases with increasing frequency. Magnetization measurements on a sample of the multi-filament wire in an applied inductance B = B_s + B₀sinω t confirmed the results of the calorimetric measurements and indicated that with increasing frequency the flow of magnetic flux into and out of the composite during a cycle becomes more and more diminished.