Localized superconductivity and Little-Parks effect in superconductor/ferromagnet hybrids

Abstract

On the basis of Ginzburg-Landau theory we investigate order-parameter ͑OP͒ nucleation in superconducting films of a finite thickness in the presence of inhomogeneous magnetic fields induced by ferromagnetic nanoparticles. In particular, we consider a generic system, consisting of a small perpendicularly magnetized nanoparticle, placed at a height h above the disk center. We study the oscillatory dependence of the critical temperature T c of the disk on an external magnetic field H, similar to the classical Little-Parks effect ͓Phys. Rev. Lett. 9, 9 ͑1962͔͒. Two OP nucleation regimes have been found: an appearance of superconductivity either near the disk center ͑under the magnetic particle͒ or near the disk edge. Switching between these regimes at varying the external field has been shown to result in an abrupt modification of the T c ͑H͒ dependence-both the amplitude and the period of the Little-Parks oscillations become much larger, provided that the OP pattern is localized under the particle and R ӷ h, R is the disk radius. The OP nucleation near the magnetic particle is strongly suppressed by increasing the film thickness. Using both analytical and numerical approaches we have found the range of system parameters that are optimal for the experimental observation of the different regimes of localized superconductivity in this hybrid system.

Anisotropy of the critical temperature of a superconducting niobium thin film with an array of nanoscale holes in an external magnetic field

“…When the dimensions of a material are reduced to a characteristic length, new behavior emerges governed by a new set of mechanisms due to confinement-induced changes in the wave function of the charge carriers and other fundamental quantities [1][2][3][4][5][6][7][8][9][10] . For example, the circulation of Cooper pairs confined in a path with lateral dimensions comparable to the superconducting coherence length ξ produces the Little-Parks effect 1,[8][9][10][11][12][13] . A superconducting strip narrower than its penetration depth λ cannot sustain a complete flux exclusion, resulting in lower diamagnetic energy which then leads to a higher critical field H c 14, 15 .…”

Localization of superconductivity in superconductor–electromagnet hybrids

“…c2 ) (the similar argumentation was presented, e.g., in [44]). Substituting B 0 ≃ 0.36 B (0) c2 (estimated for I w = 10 mA), we get the threshold temperature T * ≃ 0.6 K. It means that for intermediate temperatures (T = 0.90 K, panel (b) in Fig.…”

Vortex patterns in a mesoscopic superconducting rod with a magnetic dot

“…Recently Aladyshkin et al 35 have treated the vortex pattern of the rod in presence of a magnetic moment on its top added to a homogeneous magnetic field, both oriented along the central axis. His approach is limited to a one-component order parameter, fitted to obtain the upper critical field using the linearized theory.…”