Magnetic Field Sweep Rate Research Articles

Effects of the sweep rate of the magnetic field on the changes of ultrasonic wave velocity in magnetic fluid

An external magnetic field applied to a homogeneous magnetic fluid induces a change in its structure manifested as the appearance of chains of magnetic particles arranged along the direction of the field. This change implies changes in the acoustic and magnetic properties of the magnetic fluid such as the acoustic wave propagation velocity and coefficient of ultrasonic wave absorption, and in magnetisation. The paper reports results of a study on the influence of the magnetic field sweep rate and the magnetic fluid temperature on the value of ultrasonic wave velocity, proving that the degree of the aggregation of the magnetic particles (chain formation process) depends on the above-mentioned parameters.

Thermally activated sweep-rate dependence of magnetic switching field in nanostructured current-perpendicular spin-valves

We report on the thermal-activation nature of magnetic switching in magnetic nanostructures, using the junction magnetoresistance of a current-perpendicular magnetic spin-valve device as a probe. A spin-valve junction structure was fabricated using electron-beam lithography. A sweep-rate-dependent magnetic switching field was obtained in the quasi-static limit. Results confirm the predictions of a single-domain thermal activation model. The scaling relation between the magnetic field sweep rate, the magnetic switching field, and the sample size is verified for sample dimensions of 0.1×0.2 μm 2 .

Dynamics of metastable vortex states in weakly pinned superconductors: A phenomenological model

We present a phenomenological model for the vortex dynamics in the peak effect region of weakly pinned superconductors. We explain the history dependent dynamic response of the metastable vortex states subjected to a transport current and the hysteretic voltage-current characteristics observed in the vicinity of peak effect in weakly pinned superconductors. A strong variation in voltage current characteristics with the current sweep rate and the anomalous dependence of critical current density $J_c$ on the magnetic field sweep rate have also been accounted for by this model.

MnSb：GaAsグラニュラー薄膜の室温光誘起巨大磁気抵抗効果

Nano-size MnSb clusters with a nominal thickness of 3 ML were grown on GaAs(111)B substrates by molecular beam epitaxy. The films were capped with GaAs, and the magnetoresistance effect was investigated at room temperature. A large positive magnetoresistance effect of over 50% was observed. The dependency on the sweep rate of magnetic field and an effect of Cr-doping in GaAs capping layers were investigated. The photoinduced MR effect was also observed under laser-irradiation with a photon energy above the band gap of GaAs. It is shown that these phenomena can be attributed to the enhancement of the conductivity by photogenerated carriers.

Field sweep rate effect on the flux creep in HTSC

We report the effect of the magnetic field sweep rate on the relaxation of the magnetization as a function of temperature and magnetic field H parallel to the c-axis in an aligned powder, thin film and melt textured YBaCuO superconductors. The real M vs. In t relationship can be deduced from the experimental one by scaling the time t to t+ τ+ t 0. The resulting curves collapse all into a single one. The correlation time τ is calculated and it obeys to the relation (dH/dt) −1 for small T and H. The behavior of the M(In t) curves cannot be described by the Kim-Anderson model for all the explored temperatures and fields.

Flux jumps driven by a pulsed magnetic field

The understanding of flux jumps in the high temperature superconductors is of importance since the occurrence of these jumps may limit the perspectives of the practical use of these materials. In this work, we present the experimental study of the role of heavy ion irradiation in stabilizing the HTSC against flux jumps by comparing un-irradiated and 7.5×10 10 Kr ion/cm 2 irradiated (Y x Tm 1− x )Ba 2Cu 3O 7 single crystals. Using pulsed field magnetization measurements, we have applied a broad range of field sweep rates from 0.1 to 1800 T/s to investigate the behavior of the flux jumps. The observed flux jumps, which may be attributed to thermal instabilities, are incomplete and have different amplitudes. The flux jumps strongly depend on the magnetic field, on the magneto-thermal history of the sample, on the magnetic field sweep rate, on the critical current density j c, on the temperature and on the thermal contact with the bath in which the sample is immersed.

Asymmetry of sweep rate and magnetic relaxation effects in YBCO thin films

We report magnetic field sweep rate and magnetic relaxation effects in YBCO thin films (thickness t<450 nm) at various temperatures. It is observed that for the H∥ c configuration and at relatively high temperatures ( T>73 K) there is a pronounced asymmetry to both the field sweep rate and relaxation effects. Both the sweep rate and relaxation effects are much more pronounced for increasing fields as compared to decreasing fields. The sweep rate asymmetry is more evident for slow sweep rates and higher fields, consistent with increased relaxation and lowered activation barriers for increasing fields. On the other hand, low temperature measurements down to 35 K do not reveal any asymmetry in either effect. The dependence on temperature and orientation leads us to conclude that the asymmetry originates in the unique characteristics of the critical state for very thin films ( t≪ λ, where λ is the penetration depth) in perpendicular applied fields and the effects of the field line curvature therein.

Simulation on local flux creep in type-II superconductors

Using C programming language, we have simulated the flux creep process in nonideal type-II superconductors. Global and local magnetization curves are calculated and the logarithmic time dependence of local magnetic induction B under a constant external field is examined. The effects of nonuniform pinning potential and self-organized criticality (SOC) model on the simulations are discussed. The results show that the main feature of flux creep is the relaxation effect. The form of hysteresis loops is dependent on the magnetic field sweep rate. SOC can account for the occurrence of fluctuation to a certain extent and nonuniform pinning potential can enhance the fluctuation.

Current-voltage characteristics and flux creep in melt-textured YBa2Cu3O7-

We investigated the current-voltage (E - J) characteristics in melt-textured YBa2Cu3O7- strips by measuring the magnetic-field sweep rate dependence of magnetization. We took account of the current density J distribution in the specimen using a previously developed method (Mawatari Y et al 1997 Appl. Phys. Lett. 70 2300). For a wide temperature and magnetic-field range (60-80 K, 0.2-5.0 T), the E - J curves in the electric-field window E = 10-10-10-5 V m-1 exhibited power-law behaviour E Jn, and the power index n generally became smaller at higher magnetic fields and temperatures. In low magnetic fields (µ0Ha0.5 T) the n values were large (20), and thus the Bean model becomes a good approximation. The E- J characteristics in the lower E window were also derived from the relaxation of magnetization, the flux creep, and we found that the wide-range E- J characteristics exhibit near-power-law behaviour but that there exist slight downward curvatures in the log E versus log J plots. This downward curvature reveals that the dissipation approaches zero when the current is substantially reduced. The drastic decrease of the flux creep, which was observed when the sample temperature was decreased in a fixed magnetic field, is consistent with the observed E- J characteristics.

Influence of the magnetic field sweep rate on flux creep in melt textured sample of YBa2Cu3O7−d

We investigated the influence of the field sweep rate dH/dt (3Oe/s≤dH/dt≤1kOe/s) on the flux creep in a melt textured YBa2Cu3O7−d at different temperatures and for a fixed fields parallel to the c-axis. The initial stage of the relaxation is imposed by the macroscopic sample dimension and the field sweep rate. As a result, all the relaxation curves, at a given field and temperature, collapse into a single one that is independent from the experimental conditions by a transformation t→t+τ in which τ is a transient time. The experimental values of τ coincides well with literature.

Influence of the magnetic field sweep rate on flux creep in highly textured thin film of YBa 2Cu 3O 7−δ

We investigated the influence of the field sweep rate on the magnetic relaxation and the critical current density of a highly textured thin films of YBa 2Cu 3O 7−δ at different temperatures and fields parallel to the c-axis. We find that the actual M versus ln t relationship can be deduced experimentally by a transformation of the form t→ t+ τ+ τ vs in which τ is a transient time proportional to R×(d H/d i) −1, τ vs the time constant of the magnetometer (∼0.25 s) and R the effective radius of the superconducting film averaged along the a- b planes. As a result, all the relaxation curves collapse into a single-master one that is independent of the experimental conditions. The parameter τ obeys approximately the relationship τ× dH dt∼2 Oe in the range T<30 K and depends on the film thickness. The resulting J versus ln t relationship deviates from linearity for all the explored temperatures and fields.

Observation of magneto-thermal instabilities in (YxTm1?x)Ba2Cu3O7 single crystals in pulsed magnetic field magnetization measurements

The understanding of flux jumps in the high temperature superconductors is of importance since the occurrence of these jumps may limit the perspectives of the practical use of these materials. In this work we present the experimental study of the role of heavy ion irradiation in stabilizing the HTSC against flux jumps by comparing un-irradiated and 7.5 · 1010 Kr-ion/cm2 irradiated (YxTm1−x)Ba2Cu3O7 single crystals. Using pulsed field magnetization measurements, we have applied broad range of field sweep rates from 0.1T/s up to 1800 T/s to investigate the behavior of the flux jumps. The observed flux jumps, which may be attributed to thermal instabilities, are incomplete and have different amplitudes. The flux jumps strongly depend on the magnetic field, on the magneto-thermal history of the sample, on the magnetic field sweep rate, on the critical current density jc, on the temperature and on the thermal contact with the bath in which the sample is immersed.

Magneto-thermal oscilations in a granular YBCO superconductor

It is shown that in a granular superconductor magneto-thermal oscillations precdde a series of subsequent flux jumps. We study these oscillations experimentally in three YBCO samples at temperatures 3K<T<7K and field sweep rates 10G/s<Be<45G/s. We find that the frequency of the oscillations is a linear function of the magnetic field sweep rate and is inversely proportional to the square root of the heat capacity.

Dynamic contribution to the fishtail effect in a twin-free DyBa 2Cu 3O 7−δ single crystal

Superconducting current densities j s and dynamic relaxation rates Q  d ln j s/d In(d B e/d t), where d B e/d t is the sweep rate of the external magnetic field B e, were measured as a function of temperature (5 K < T < 65 K) in magnetic fields up to 7 T on a twin-free DyBa 2Cu 3O 7−δ single crystal by means of a high-sensitivity capacitance torque magnetometer. Above 15 K, we observe a “fishtail” effect, i.e. a pronounced minimum in the j s( B e) curve at fields around B e = 1 T. The relaxation rate Q shows an anomalous increase at low fields which is correlated to the minimum in the j s( B e) curve. Both the j s versus B e and Q versus B e data are used as input parameters into the generalized inversion scheme developed by Schnack et al. [Phys. Rev. B 48 (1993) 13178] to calculate the true critical current density j c which is by definition independent of relaxation effects. Interestingly, the j c( B e, T) curves derived in this way do not show a minimum. This points clearly to a dynamic contribution to the fishtail effect. The true critical current density j c( B e, T) decreases weakly with increasing B e over the entire measured temperature and field range, as expected for single-vortex pinning. This indicates that the observed fishtail effect is not caused by a crossover from single-vortex pinning to pinning of flux bundles. The temperature dependence of j c is in good agreement with the predictions of a model based on single-vortex pinning caused by spatial fluctuations in the charge-carrier mean free path.

Critical current, magnetization relaxation and activation energies for YBa 2Cu 3O 7 and YBa 2Cu 4O 8 films

By means of high-sensitivity capacitance torque magnetometers we have measured the superconducting current j s and the dynamic magnetic-moment relaxation of YBa 2Cu 3O 7 and YBa 2Cu 4O 8 films of typically 100 nm thickness at temperatures between 2 K and T c in magnetic fields up to 6 T. For the measurements of the dynamic relaxation rate Q≡d ln j s/d ln (d B e/d t) magnetic-field sweep rates were varied between 0.5 and 40 mT/s. At low fields (typically 0.5 T) the dynamical relaxation rate exhibits a plateau at Q≈0.06 in YBa 2Cu 3O 7 and 0.04 in YBa 2Cu 4O 8. At high fields ( B e= μ 0 H e≈ 6 T) the plateaus have completely disappeared and Q increases almost linearly with increasing temperature. At all fields a sharp increase up to Q≊1 is observed when the irreversibility line is approached. By means of the generalized inversion scheme (GIS), the j s( T, B e) and Q ( T, B e) data are used to determined the current dependent activation energy U ( j, T, B e) for thermally activted flux creep. Although the GIS does not make any a priori assumptions about the explicit functional dependences on T and j, the U( j, T=0, B e) function derived from the experimental data by means of the GIS can remarkably well be described with the collective-creep interpolation formula U( j)=( U c/ μ)[( j c/ j) μ −1] with μ≈0.6 for currents j>0.15 j c ( T=0, B e) where J c( T=0, B e) is the critical current at T=0, and wi U c depending on B e. At lower current densities U( j, T=0, B e) does not diverge as j −0.6 but shifts gradually to a weaker ln( j c/ j) dependence. At low temperatures the current and relaxation data cannot be explained in terms of a thermally activated flux-motion model. Quantum creep has an influence up to ∼13 K.

Shielding currents and current-voltage characteristics of Bi 2Sr 2Ca 1Cu 2O 8+δ single crystal

The dependence of the irreversible magnetization on the external magnetic field sweep rate, d H e/d t, and its time relaxation are studied in a Bi 2+ x Sr 2+ y Ca 1+ z Cu 2O 8+δ ( x=0.11, y=0.12, z=-0.25) single crystal. Two characteristic regions are found with a transition temperature T∗ between 15 and 20 K depending on d H e/d t and relaxation time. Below T∗ the magnetic field dependence of the shielding currents follows a scaling law J s ( H e , T)/ J s (0, T)= f[ H e / J s (0,T)] with a universal function f independent of temperature and sweep rate of magnetic field. Voltage-current characteristics are described quite well by a power-like law E/ E c =( J s / J c ) n with n∼1/ T. Above T∗ a deviation from the J s( H e) scaling behavior and a transition to much steeper voltage-current characteristics are observed. Possible reasons of these different shielding current behaviours are discussed.

Flux jumps in high Tc single crystals

Abstract The magnetic responce of LaSrCuO and BiSrCaCuO single crystals has been investigated under quickly varying applied magnetic field. Sharp repetitive jumps in the magnetization vs. field curve were observed and studied as a function of the magnetic field sweep rate. The effect is interpreted as an avalanche — like flux flow, caused by a run-away break of the superconductor stability and the transition from isothermal to adiabatic limit of the flux propagation.

Strong evidence for vortex-glass-collective-pinning theory in YBa2Cu3O7 superconductors.

Magnetization studies on a melt textured-growth sample of YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]] have tested several theoretical models to see which of them best describes the relaxation of supercurrents in this high-[ital T][sub [ital c]] superconductor. By combining conventional flux-creep experiments with measurements of magnetization versus magnetic-field sweep rate, we are able to detect the decay of magnetization in both its intermediate and very early stages, thereby expanding the observational window up to 5--6 decades of time. Among four recent models tested, only vortex-glass--collective-pinning theory consistently describes both stages. The resulting values for the characteristic exponent [mu] strongly depend on temperature and magnetic field. Values for the effective hopping time for vortices, [ital t][sub [ital e][ital f][ital f]], are [similar to]0.1 s.

Thermal measurements of the magnetic field induced dissipation in YBa 2Cu 3O 7−δ at low temperature

We have studied the magnetic field induced dissipation in YBa 2Cu 3O 7−δ for magnetic fields in the ranges (0–2 T) during virgin runs by directly measuring the heat produced. The magnetic field sweep rates are in the range (0–100 G/s). The measurements are performed at low temperatures, between 4.2 and 6.5 K. The dissipation induced by the electric field produced by vortex movement has been modeled using the Bean critical state model. The dissipation was found to consist of two terms: the first one is due to the existence of the pinning force and the second is due to the viscous movement of the vortices. The fits of the experimental results, according to the theoretical form we have used for the dissipation j·E , led to the determination of the Bean field, in good agreement with the one obtained by magnetic measurements. We have also determined the effective resistivity corresponding to the viscous dissipation at low temperature. The very small value obtained for the latter is discussed.

Extension of the time window for investigation of relaxation effects in high- Tc superconductors

Measurements of the dependence of the magnetic hysteresis loop amplitude in a superconductor on the magnetic field sweep rate d H e/d t are shown to provide essentially the same information as magnetic relaxation experiments (flux-creep) at fixed external field H e. For practical reasons, however, magnetic hysteresis loop measurements are better suited to investigate fast flux-line movements than conventional flux-creep measurements. From the general flux movement differential equation it is shown that each magnetic hysteresis loop recorded at a given d H e/d t can be labelled by an effective time t M 0 so that this experimental information can be used to extend the time window of flux-creep measurements to short times. This method is applied to the experimental data obtained on a YBa 2Cu 3O 7 single crystal. In this way the time window is extended to less than 10 -2 s which is valuable for a more precise check of existing theoretical models for the activation energy of flux motion. It is found that the experimental data are well described by using an activation energy U( j)=( U c / μ)[( j c / j) μ -1] with μ=0.74 at 16 K and 1.8 T.