Amplitude Of AC Magnetic Field Research Articles

Trapped field characteristics of Y-Ba-Cu-O bulk in time-varying external magnetic field

The trapped field characteristics of disk-shaped YBaCuO bulk exposed to an external AC magnetic field are investigated experimentally. The magnetic flux density on the top surface of the disk, defined as "trapped flux" in this paper, was measured in both short and long terms as functions of amplitude and frequency of the AC external magnetic field. The observed trapped flux attenuation was obviously different from that of flux creep, i.e., with no external magnetic field; this implies that the trapped flux density within the YBCO disk is reduced by a temperature rise due to AC loss. The abrupt attenuation of trapped flux density in the first several cycles was observed at frequencies of 0.1 and 1 Hz while not observed at 10 Hz. The attenuation rate after seven minutes of applying AC magnetic field, however, became almost the same regardless of frequency. The trapped flux attenuation in one cycle of AC magnetic field decreases with the frequency and increases with the amplitude of AC magnetic field. These results imply that the characteristic of trapped flux is closely related to AC loss, especially the hysteresis loss, and the AC loss depends on the frequency and the amplitude of the AC external magnetic field. It ran be considered that transient electromagnetic behavior within HTS bulk, especially supercurrent distribution, is a key factor of the relationship between the trapped flux attenuation and the AC loss.

Low frequency AC absorption in YBa 2[(CuO) 1− x(MS) x] 3O 4− δ (M=Fe ( x=0.01, 0.02, 0.05), Ni ( x=0.01) or Zn ( x=0.02, 0.05)) samples

We report and analyze AC susceptibility data of polycrystalline high- T c superconductors. The purpose of these investigations is to study the effect of doping YBa 2Cu 3O 7− δ with M=Fe, Ni or Zn sulfides on the low frequency AC losses of these materials. We measured the AC susceptibility of sintered YBa 2[(CuO) 1− x (MS) x ] 3O 4− δ pellets as a function of temperature and AC magnetic-field amplitude and frequency. The imaginary part of the AC susceptibility ( χ″) in terms of temperature and magnetic field intensity show remarkable characteristics. As the applied magnetic field intensity is increased, the three loss peaks (intergranular, intragranular, intrinsic) that can be observed, show shifts to lower temperatures. The intergranular and intragranular peak shift (because of doping and magnetic field variation) is explained by the Müller critical state model and shows hysteretic effect. The intrinsic peak shift follows the T c changes and shows no hysteretic effect.

Intergrain magnetic properties and critical currents of T1-1223 bulk and tape superconductors

The magnetic field and the temperature dependencies of the dispersive and the absorption components of the low frequency ac magnetic susceptibility for Pb and Bi single doped bulk superconductors as well as for Pb and Bi co-doped Ag-sheathed superconducting T1-1223 tapes have been measured and analyzed. The studies were accompanied by optical and electron microscopic observations of the microstructure. The ac susceptibility measured at various ac magnetic field amplitudes showed both intergrain and intragrain features. The temperature dependencies of the intergrain critical current densities were extracted from the absorption susceptibility via a modified Bean model. They were at relevant temperatures one order of magnitude larger for the tape than for the bulk specimens.

AC losses of arrays of superconducting strips on metallic substrates

The loss component of the AC susceptibility, χ″, for x-arrays of thin-film superconducting YBCO strips on metallic substrates was investigated both theoretically and experimentally. The contributions to χ″ due to the hysteretic loss in the superconducting strips and the eddy current loss in the substrate were calculated as a function of temperature for varying width and lateral separation of the strips, AC magnetic field amplitude and frequency, as well as substrate and film thickness. At low frequencies ( f∼100 Hz ), χ″ is dominated by the frequency-independent hysteretic component which decreases as the strips become narrower. With decreasing inter-strip separation, the hysteretic χ″ increases, while the associated peak in the temperature dependence of χ″ broadens and shifts to lower temperatures. The frequency-dependent eddy current contribution to χ″ becomes more prominent as the frequency increases.

AC magnetic susceptibility by modified Bean model with Hc1

The analytical expressions of AC magnetic susceptibility χ AC were given by the modified Bean model with lower critical field H c1 of type II superconductor. The two steps of fH c1 and (1− f) H c1 were considered for the internal magnetic flux density at the surface and inner part of slab sample, respectively, where f=| M eq|/ H c1 was the dimensionless factor and the reversible magnetization M eq was approximated to be constant at | H|> H c1. Then the equilibrium flux density B eq was given by μ 0 H(1− fH c1/| H|) at the inner surface for all of the magnetizing and demagnetizing processes. The expressions of χ AC, including nonlinear parts, were given by three parameters of α= H*/ H AC, α 1= H c1 / H AC and f, where H AC was the amplitude of external AC magnetic field and the penetration field H p was given by H*+ H c1. At H= H p, the internal flux density B int became nonzero at the center of sample. The observed reductions of third harmonics of χ AC, χ 2′ and χ 2″, can be explained by this model when H c1≠0.

Irreversibility Lines in Halogen- and Hydrogen-Doped YBa2Cu3O7−δ Compounds

We measure the real and imaginary parts of the ac magnetic permeability of YBa2Cu3O7−δA x (A = Cl, F, H) ceramics and powders as functions of temperature and ac magnetic field amplitude H0, applying uncommonly low and widely ranged excitation fields (1 mOe ≤ Ho ≤ 200 Oe). We determine the temperature dependence of two loss peaks, Hp(T) ∼ (Tc − T)2.7 at low fields in ceramics and Hm(T) ∼ Ts0 − T at higher fields in both powders and ceramics. The extrapolated field Hp(0) and characteristic temperature Tc depend on x and δ, whereas Hm(0) and Ts0 depend mainly on δ. The real part of the permeability indicates a two-step flux-penetration process with two threshold fields for flux penetration. The latter govern the temperature dependence of maxima in the imaginary part as well as the peaks' widths. We propose a field-dependent percolation description of this process, derived from a previous temperature-dependent percolation model. It involves two penetration depths, the classic Ginzburg–Landau one and a low-field abnormally large penetration depth due to chemical disorder. Correspondingly, two types of vortices describe high- and low-field dissipation peaks.

Reversible fluxoid motion in a superconducting Bi-2223 tape with fine filaments

The energy loss density was measured for a Bi-2223 tape with superconducting filaments of average thickness 3 µm which were separated by about 40 µm so as to eliminate bridging of filaments. The result was compared with the prediction of the modified Kim model. Although it was lower than the prediction owing to the reversible fluxoid motion at temperatures above 77.3 K, it slightly increased at around 1 mT. It is considered that an increase in the pinning strength and a resultant shortening of the ac penetration depth suppressed the effect of the reversible fluxoid motion at such low ac magnetic field amplitudes.

Change of the HTSC harmonic susceptibility properties with electrolytic hydrogen loading using different aqueous solution as LiOH and NaOH

The higher harmonics magnetic susceptibilities (/spl chi/=/spl chi//sub i/'+i/spl chi//sub i/'') of HTSC hydrogen loaded as function of the temperature at 6 G of ac magnetic field amplitude at 107 Hz of frequency have been measured. The samples have been loaded by electrolysis in an aqueous solution using /spl mu/s current pulses at room temperature. The HTSC electrodes have been polarized by short pulse width (1 /spl mu/s) and high peak current (/spl ges/4 A) with a low duty-cycle (10/sup -3/) and a variable repetition rate (1-5 kHz). The hydrogen influence on HTSC (T/sub c/, ac losses) has been measured and the increasing of the superconducting properties, particularly T/sub c/ and J/sub c/, have been found. These effects appear strongly dependent to the loading process and to the stoichiometric ratio H/YBCO. It has been demonstrated that the electrolyte (Li, Na) does not affect the YBCO superconducting properties.

AC losses in Bi-2223 tapes and in the 1-kA transmission line model

The authors present here results of their study of the 5 m long AC transmission line model with 1 kA current capability at 77 K. Primary attention was paid to the current characteristics and AC losses in individual tapes and in the core of the cable. The losses were measured as a function of AC magnetic field amplitude in various orientations of magnetic field with respect to the plane of the tape and filaments. Hysteresis losses were close to the losses in the AC regime, meaning that eddy current losses in tapes may be neglected when compared with hysteresis losses. They designed and constructed a 5 m long model of the AC transmission line using multifilamentary Bi-2223 tapes. The current core of the model contains 120 tapes for the forward line and 120 tapes for the backward line. AC losses in the current core were substantially greater than those seen in the individual tapes. The reason for this is related to a complex magnetic field distribution inside the current core.

Relaxation of rotational magnetization in annealed Co-based amorphous ribbons

The permeability spectra for various amplitudes of AC magnetic field h 0 were measured in an annealed Co 66Fe 4NiB 14Si 15 amorphous ribbon. The rotational magnetization was involved only for low-driving amplitude of h 0 ⩽ 0.4 mOe. The DC permeability and relaxation time, obtained from the spectra of rotational relaxation, were discussed in terms of the change of restoring constant by the magnetic softening and nucleated crystalline sites during annealing.

AC susceptibility of a monofilament Bi-2223/Ag superconducting tape in a perpendicular field

Measurements and calculations are presented of the real ( χ′) and imaginary ( χ″) parts of the AC susceptibility of a monofilament silver-sheathed Bi-2223 tape as a function of temperature T=4–130 K, frequency ω/2 π=5 Hz–5 kHz and ac magnetic field amplitude μ 0 H m=0.02–7 mT. The susceptibilities consist of a hysteretic component due to induced currents in the superconductor core and an eddy current component due to induced eddy currents in the silver sheath. At high temperatures (>70 K) the low frequency limit is used to rigorously calculate the hysteretic and eddy current susceptibilities while at low temperatures the susceptibility is approximately described as being due to eddy currents flowing along the edges of the tape. The measured χ″ and hence AC loss at low frequencies (<50 Hz) and high temperatures is dominated by the hysteretic loss which varies with amplitude but is essentially independent of frequency. At higher frequencies and low temperatures, the eddy current loss of the silver sheath becomes dominant and increases with increasing frequency.

Pinning field distribution and relaxation time of domain wall in 2605CO Metglas

The pinning field H/sub p/ is obtained from the discontinuous variation of permeability according to ac magnetic field amplitude. The probability distribution of pinning field P(H/sub p/) in 2605CO Metglas ribbon of as-quenched state shows a Gaussian distribution, indicating that the pinning sites with different pinning field are randomly distributed. The permeability spectra for pinned state of domain wall are caused by the pure rotational magnetization with 1 MHz relaxation frequency. For the magnetization overcoming the pinning field, the permeability spectra are superposition of rotational magnetization and wall displacement. The average pinning field and relaxation time of wall displacement decrease with the magnetizing angle from ribbon axis, and the relaxation times are found to be exponentionally proportional to the average pinning field.

Superconducting properties of YBa 2Cu 3− xFe xO y ceramics with small x

The influence of iron doping on the transition temperature to the superconducting state has been studied in ceramic YBa 2Cu 3− x Fe x O y (0≤ x≤0.01) samples by means of AC susceptibility χ′− i χ″ measurements. A statistical character of the onset transition temperature to the superconducting state T c on depending on iron concentration ( x) was found. Monotonously decreasing behavior for the intergranular weak link coupling temperature T m J depending on AC magnetic field amplitude h 0 was observed. The higher the x the higher the dropping rate of T m J was observed for relatively strong field. It is found that the dropping rate of T m J against x is essentially decreasing in the range 0.003< x<0.007 and again is sharply reduced for higher x up to 0.01. Possible explanations of obtained results are discussed.

Dynamic self-organization of magnetic domains in amorphous films with perpendicular anisotropy

Dynamic self-organization of magnetic domains is observed in amorphous gadolinium-cobalt films in narrow temperature intervals on both sides of the magnetic compensation point. Spiral dynamic domains form in a limited range of ac magnetic field amplitudes and frequencies.

Nonlinear microwave properties of epitaxial HTS films

The surface resistance Rs of epitaxial HTS films of YBa2Cu3O7−δ on sapphire is studied experimentally at a frequency of 8.95 GHz in the presence of an additional powerful microwave signal of frequency 9.4 GHz ensuring the ac magnetic field amplitude up to 20 Oe on the film. It is found that the surface resistance Rs of the HTS film increases with the amplitude of the additional signal. The value of Rs of the HTS film virtually coincides with the value of the nonlinear surface resistance of the film at the same amplitude of the ac magnetic field. In other words, the surface resistances of the film for a weak signal in the presence of an additional powerful signal and for the powerful signal alone are close. The observed phenomena are explained under the assumption that the HTS film is a Josephson medium containing various types of Josephson junctions. Under the influence of a powerful microwave signal, the properties of this medium can vary due to switching of a part of the junctions, causing a transition of the medium to a new state which is manifested identically both for a weak and for a powerful signal.

Frequency dependence of the higher harmonics susceptibilities of hydrogen loaded and unloaded melted YBCO samples

We measured the first and higher components of ac susceptibilities of melted YBCO samples before and after hydrogenation by μs pulsed electrolysis. First component was measured versus temperature at fixed frequency while higher harmonics were measured versus frequencies at fixed temperature near critical temperature. All measurements were performed at fixed amplitude of ac magnetic field and zero dc magnetic field. The frequency and temperature behavior of the harmonic components before and after hydrogenation gave us information on possible dynamic losses in comparison to the flux pinning mechanism.

Collapse of the magnetic moment in a hard superconductor under the action of a transverse ac magnetic field

The suppression of the static magnetic moment M of a superconducting plate by an alternating magnetic field applied perpendicularly to a dc magnetic field has been studied both experimentally and theoretically. Complete disappearance of the hysteresis of the magnetization curve in YBCO melt-textured samples was observed. Nonmonotonous behavior of M with the increase of the ac magnetic field amplitude was found. The dynamics of the collapse of the magnetic moment was studied in the pulse regime of the transverse field. The experimental results indicate homogeneity of the static magnetic induction in the sample regions where the ac field penetrates. The data obtained are interpreted within the framework of a two-liquid hydrodynamic model.

Ac susceptibility of high-temperature superconductors

By numerically solving the flux creep equation, we have investigated the temporal and spatial evolution of the field profiles in a high-temperature superconducting slab immersed in an ac magnetic field together with a dc bias magnetic field, for the situation where the flux creep activation barrier U depends explicitly on current density j as U(j)=(${\mathit{U}}_{0}$/\ensuremath{\mu})[(${\mathit{j}}_{\mathit{c}}$/j${)}^{\mathrm{\ensuremath{\mu}}}$-1]. The fundamental ac susceptibilities of the slab as a function of temperature for different dc bias magnetic fields ${\mathit{B}}_{\mathit{d}}$, ac magnetic field amplitudes ${\mathit{B}}_{\mathrm{ac}}$, and frequencies f have been derived in a unified picture, which reproduce many of the features exhibited by experiments. We have shown that the frequency-independent critical-state model breaks down in explaining these results, which however, can be well described by means of flux creep. We have also shown that part of the loss in the high- temperature superconductors is due to flux creep. \textcopyright{} 1996 The American Physical Society.

Anomalous flux pinning of UPd2Al3

The ac magnetic susceptibility of UPd2Al3 in the superconducting state has been measured to test of the existence of the Fulde, Ferrell, Larkin and Ovchinnikov (FFLO) state. A dip in the susceptibility has been observed in the vicinity of the upper critical field and the temperature range from zero to Tc. This dip is very sensitive to the amplitude of the ac magnetic field. The anomaly is explained by the increase of the flux pinning effect. Recent theories suggest that the pinning force becomes stronger in the FFLO state, and the existence of the nonuniform state due to the order parameter mixing in a d-wave superconductor is possible near Tc.

Magnetically modulated microwave absorption and harmonic generation in YBa 2Cu 3O 7 thin films

We have performed a comparative study of field-modulated microwave absorption of YBa 2Cu 3O 7 textured and epitaxial thin films using an EPR spectrometer. The Fourier analysis of the signal obtained from microwave absorption of the films has been investigated as a function of temperature and AC magnetic field amplitude. The response is found to be essentially non-linear. We show that the model of Portis et al. based on the existence of a critical state, with pinning and depinning of vortices during a modulation cycle, can account for the observed non-linear absorption signals. The response at the excitation frequency observed close to T c is discussed in terms of flux diffusion.