Superconducting–magnetic heterostructures: a method of decreasing AC losses and improving critical current density in multifilamentary conductors

Abstract

Magnetic materials can help to improve the performance of practical superconductors on the macroscale/microscale as magnetic diverters and also on the nanoscale as effective pinning centres. It has been established by numerical modelling that magnetic shielding of the filaments reduces AC losses in self-field conditions due to decoupling of the filaments and, at the same time, it increases the critical current of the composite. This effect is especially beneficial for coated conductors, in which the anisotropic properties of the superconductor are amplified by the conductor architecture. However, ferromagnetic coatings are often chemically incompatible with YBa(2)Cu(3)O(7) and (Pb,Bi)(2)Sr(2)Ca(2)Cu(3)O(9) conductors, and buffer layers have to be used. In contrast, in MgB(2) conductors an iron matrix may remain in direct contact with the superconducting core. The application of superconducting-magnetic heterostructures requires consideration of the thermal and electromagnetic stability of the superconducting materials used. On one hand, magnetic materials reduce the critical current gradient across the individual filaments but, on the other hand, they often reduce the thermal conductivity between the superconducting core and the cryogen, which may cause destruction of the conductor in the event of thermal instability. A possible nanoscale method of improving the critical current density of superconducting conductors is the introduction of sub-micron magnetic pinning centres. However, the volumetric density and chemical compatibility of magnetic inclusions has to be controlled to avoid suppression of the superconducting properties.

AC losses in coated conductors

“…Here we look at an example of how this knowledge can be used to search for solutions improving the tape performance. In particular, we analysed the effect of a horse-shoe cover of tape edges [27][28][29]. The cross section of the composite tape used in the simulation is shown in figure 15.…”

Nature of the low magnetization decay on stacks of second generation superconducting tapes under crossed and rotating magnetic field experiments

“…1 . The reason of this choice, μ r = 100, is justified by the observations of other groups 24 , 25 , where it has been established than the use of a soft FM layer with a relative magnetic permeability of μ r = 100–200, is sufficient for an effective screening of a transverse magnetic field with no apparent difference in the limit μ r = ∞. Hence, for a fully premagnetized SC/FM metastructure, the FM layers act to prevent any change in the direction of the magnetic field over the surface of the SC layers.…”

Why energy lossless ferromagnetic materials can add energy losses onto type-II superconductors?

“…Due to the novel phenomena and applications that can be envisaged by the use or devising of metamaterials, the developing of Superconducting-Ferromagnetic heterostructures have been the focus of considerable attention in recent years. [1][2][3][4][5][6][7][8][9][10][11] Particular focus has been played to the role of magnetization and demagnetization properties of these kind of systems 11,12 , as well to the study of their magnetic cloaking features [4][5][6][7][8] , and the shielding properties of type II superconductors (SC) surrounded or in the near proximity of a soft ferromagnetic material (SFM) [13][14][15][16][17][18][19][20][21][22] . However, the influence of the physical coupling between the macroscopic electromagnetic properties of these materials on the overall hysteresis losses of SC/SFM heterostructures for AC applications is yet to be understood.…”