Magnetic relaxation induced by transverse flux shaking in MgB₂superconductors

Abstract

We report on measurements and numerical simulations of the behavior of MgB2 superconductors when magnetic field components are applied along mutually perpendicular directions. By closely matching the geometry in simulations and measurements, full quantitative agreement is found. The critical state theory and a single phenomenological law, i.e. the field dependence of the critical current density Jc(B), are sufficient for a full quantitative description of the measurements. These were performed in thick strips of carbon nanotube doped MgB2 samples. Magnetization was measured in two orthogonal directions using a SQUID magnetometer. Magnetic relaxation effects induced by the application of an oscillatory perpendicular field were observed and simulated numerically. The measurements confirm the numerical predictions, that two relaxation regimes appear, depending on the amplitude of the applied magnetic field. The overall agreement constitutes a convincing validation of the critical state model and the numerical procedures used.

Crossed-magnetic-field experiments on stacked second generation superconducting tapes: Reduction of the demagnetization effects

“…Remarkably, a strong reduction of the demagnetization factor has been found even after 100 cycles of the ac perturbation ($10.2% in the worst of our cases), when in the case of a superconducting bulk the expected demagnetization factor after just one cycle of the ac transverse field can be even much higher than 50%. 7,10,11,13 The universal linear behavior of the magnetic susceptibility predicted by Brandt in Ref. 14 for the case of superconducting bulks is now confirmed for the case of stacks of 2G-HTS tapes by showing the linear dependence of the demagnetization factor as function of the amplitude of the ac transverse magnetic field.…”

“…The most severe demagnetization effects are found when the ac field perturbations are perpendicular to the direction of the initial magnetization which cause a significant decay of the trapped magnetic field ($50%), even after applying just one cycle of an ac transverse magnetic field with an amplitude similar to the intensity of the trapped magnetic field. [7][8][9][10][11] Therefore, the need for a continuous and stable operation of such devices may be seen dramatically affected as result of a long-term decay of the field produced by any bulk HTS, when it is subjected to more than one cycle of the ac transverse magnetic field. Moreover, despite the fact that the most compact sources for very high trapped field are HTS bulks, these materials are frequently impractical due to their thermal instability at low temperatures and their poor mechanical strength.…”

“…From these measurements, a striking decrease in the demagnetization factor of the sample is noticed, in particular, when it is compared with similar studies carried on superconducting bulks. 7,10,11 In fact, when a superconducting bulk is subjected to a cross field experiment, even just after the first cycle of the applied ac transverse magnetic field, the original magnetization of the sample can be erased in over a 50% for jH peak ab =H kc j % 1:5 in comparable geometries. 7,10 However, in a stack of 2G-HTS tapes, we have found that the demagnetization factor of the sample is significantly lower ($5%) even after 100 cycles of the ac perturbation, which makes this configuration much more attractive for the development of practical applications.…”

Trapped magnetic field distribution above a superconducting linear Halbach array

“…The assembly process is more critical than for a classic Halbach array given that the trapped current loops in each magnetized sample may be altered by the proximity of another sample. The magnetization of two neighbouring superconductors in the array being perpendicular, the situation is actually comparable to a "crossed-field" configuration, in which the trapped field of a superconductor is affected by an external magnetic field which is not parallel to the main axis of magnetization [28][29][30][31][32][33][34][35][36]. In a few reports, the placement of several trapped field superconductors in close proximity was investigated [37].…”

Trapped magnetic field distribution above two magnetized bulk superconductors close to each other

“…The crossed-field effect has been investigated experimentally, analytically and numerically by considering a premagnetized bulk superconductor subjected to N successive cycles of transverse field [23][24][25][26][27][28][29]. In this case, the decay of the magnetization can be fitted using a power law M ∼ N −a , where a is an increasing function of the transverse field amplitude.…”