Crossing fields in thin films of isotropic superconductors

Abstract

We study interactions of perpendicular and longitudinal magnetic fields in niobium films of different thickness in a wide range of temperatures below the superconducting transition temperature (T C ) . In 100 nm Nb film at all temperatures the longitudinal field H || practically does not influence the dynamics of the normal flux. However, in 200nm Nb film, a considerable anisotropy in the vortex motion is found with advanced propagation of the normal flux along H || at T>T C /2 and the preferential jump-wise growth of the thermomagnetic flux dendrites across H || at T<T C . Appearance of the in-plane vortices and their cutting-reconnection with tilted vortices induced by the normal field H ⊥ is the reason of the observed anisotropy in the thicker film. Absence of the in-plane vortices and much smaller tilt of vortices generated by H ⊥ explain the isotropic normal flux dynamics in the thinner film. Our results open a new way of manipulating both slow vortex motion and fast thermo-magnetic avalanches. IntroductionDespite a long history and substantial efforts to describe magnetic flux crossing in superconductors, the vortex cutting-reconnection phenomenon is still far from even qualitative clear comprehension (see [1] and refs there). Present work adds one more experimental evidence that the flux crossing brings a considerable effect on the vortex dynamics.Recently, we investigated the normal magnetic flux (B ⊥ ) entry in a YBa 2 Cu 3 O 7-d (YBCO) crystal plate magnetized with an in-plane field parallel to the superconducting CuO 2 layers (H || ) [2]. Direct flux imaging revealed pronounced anisotropic vortex dynamics with preferential vortex motion along H || and formation of a sharp flux front, delaying the entry of new vortices across H || . We associated the novel observed flux patterns, similar to those reported earlier in [3], with the layered structure of YBCO, forcing the motion of the incoming pancake vortices along the Josephson-like, in-plane flux lines, and with the cutting processes of the in-plane and new entering vortices.. The simulations of the crossing process in [2] revealed time variations of the vortex shape and allowed us to conclude that defects can increase the flux crossing barrier due to the pinning of strongly bent segments of intersecting vortices, and hence enhance currents along the crossing flux fronts.In principle, the anisotropic dynamics of different flux components in thin plates of any type II superconductor (both isotropic and anisotropic, homogeneous and layered), defined by the thin plate geometry, was predicted by Brandt [4], where he considered effects of the Meissner currents induced by the crossing external fields on the changing shape of vortices and showed that various flux diffusivities occur along different in-plane directions depending on the plate shape and the field orientation. Later, [5] he discussed possible physical reasons for the anisotropic normal flux motion in thin superconducting plates magnetized with an inplane field. One mechanism of the anisotropic vo...

Magnetic gates and guides for superconducting vortices

“…In the described experiments H || remained unchanged after cooling, securing the stripe polarization. We checked that in-plane fields up-to 1 kOe do not modify the normal flux dynamics in the reference 100 nm Nb film [81]. Background images revealing the stray fields at the edges of the Py stripes, which depend on the stripe polarization, were taken before application of the normal (perpendicular to the Nb film surface) field H Z .…”

Force-free state in a superconducting single crystal and angle-dependent vortex helical instability

“…Vortices are objects which can be entangled, crossed, cut, and reconnected. The study of vortices as topological defects encompasses many fields, such as plasmas [1], fluids [2][3][4][5], superfluids [6][7][8][9], and superconductors [10][11][12][13][14][15]. Sometimes fundamental properties of superconducting vortices are blurred by the crucial role played by vortex pinning.…”

Propriedades magnéticas e supercondutoras de redes de nanofios de Ni acopladas a filmes de Nb

“…As avalanche de fluxo, comumente observadas a baixas temperaturas em supercondutores tipo II com forte potencial de aprisionamento (pinning) [128], manifestamse nos peculiares perfis dendríticos de penetração de fluxo que tem a sua origem em efeitos térmicos relacionados ao movimento de milhões de vórtices quando é aplicado um campo externo. O movimento de vórtices num supercondutor é acompanhado por uma dissipação de energia na forma de calor que cria gradientes de temperatura ao longo do seu caminho [122,123]. A energia gerada neste processo é assimilada pelo sistema formado pelo supercondutor e o substrato, no entanto se a quantidade de calor for suficientemente grande, picos localizados de temperatura podem surgir dentro da amostra.…”

“…102 6.2 Imagens de magneto-ótica de um filme de Nb de 200 nm de espessura a T=3,2 K para dois campos diferentes. [123] 6.8 (a) Imagens de magneto-ótica para a amostra B5a (amostra sem nanofios) a T = 2, 5K em campo magnético perpendicular H=10 Oe. (b) Acercamento da mesma imagem onde observa-se que o perfil de penetração de fluxo é dendrítico.…”

“…Figura 6.1: Imagens de magneto-ótica de um filme de M gB 2 de 400 nm de espessura a T=5 K para diferentes valores de campo aplicado [122]. Figura 6.2: Imagens de magneto-ótica de um filme de Nb de 200 nm de espessura a T=3,2 K para dois campos diferentes [123]. …”