Visualizing domain wall and reverse domain superconductivity

Abstract

In magnetically coupled, planar ferromagnet-superconductor (F/S) hybrid structures, magnetic domain walls can be used to spatially confine the superconductivity. In contrast to a superconductor in a uniform applied magnetic field, the nucleation of the superconducting order parameter in F/S structures is governed by the inhomogeneous magnetic field distribution. The interplay between the superconductivity localized at the domain walls and far from the walls leads to effects such as re-entrant superconductivity and reverse domain superconductivity with the critical temperature depending upon the location. Here we use scanning tunnelling spectroscopy to directly image the nucleation of superconductivity at the domain wall in F/S structures realized with Co-Pd multilayers and Pb thin films. Our results demonstrate that such F/S structures are attractive model systems that offer the possibility to control the strength and the location of the superconducting nucleus by applying an external magnetic field, potentially useful to guide vortices for computing application.

Observation of superconducting vortex clusters in S/F hybrids

“…However, due to renormalization of the coherence length, Pb thin films behave as type-II superconductors and magnetic field penetrates in form of flux quanta. Moreover, in a twin 30-nm Pb film a ξ Pb (0) ≈ 48  nm 11 has been measured using STM vortex imaging, in agreement with values of 40 ÷ 50 nm reported in literature for ultrathin Pb films62.…”

“…In the last decade several studies have focused on the influence of different types of pinning centers on the dynamics of superconducting vortices in type-II thin film superconductors1234567, which resulted in better fundamental understanding of vortex physics. Compared to nano-structured inclusions made of normal metals or insulators, magnetic pinning leads to intriguing effects, such as magnetostatic interaction8, proximity effect9 and domain wall superconductivity10111213, to name a few. Moreover, magnetic pinning in magnetically coupled superconductor/ferromagnet (S/F) heterostructures, appears to be stronger than other pinning mechanisms414.…”

Doppler-scanning tunneling microscopy current imaging in superconductor-ferromagnet hybrids

“…The conductance maps in applied magnetic field show the vortex cores as regions of higher zero bias conductance due to spatial variation of the superconducting pair potential D in the vortex core (on the order of $ n) and the supercurrents flowing around the vortex core at a distance on the order of $ k. Since the increase in states is dominated by the divergent drop in the pair wavefunction within the vortex core, the resulting lateral size of the vortex in ZBC maps gives an accurate measurement of the coherence length, which for this system is around n GL (1.5 K) % 50 nm; in agreement with other ultrathin Pb films in clean limit. 20,36 The maps in Figures 3(c)-3(f) clearly show that vortices arrange preferentially along stripes, with an increasing density for higher applied magnetic fields. For opposite polarities, vortices are located along previously unoccupied stripes (Figures 3(c)-3(f)).…”

“…We would also like to emphasize that the values for the perpendicular stray field found from this simulation agree well with the estimated values from STM measurements on similar systems. 20 In Figure 2(a), we show a large scale topographic STM image of the Pb surface of the film. The film is 30 nm thick with surface corrugation on the order of few Angstroms.…”

“…This could lead to a variety of interesting opportunities such as the ability to guide the supercurrent by altering the magnetic domain structure in the ferromagnet, 17 control vortex mobility, 18 and induce the nucleation of filamentary superconductivity. [19][20][21] The majority of experiments on planar magnetically coupled superconductor/ferromagnet heterostructures have so far been primarily focused on global measurements 13,[22][23][24][25] with theoretical insights based on London formalism. 26,27 However, the increased use of local probes, such as the scanning tunneling microscopy 28,29 and magnetic force microscopy (MFM), 30,31 has begun to reveal a rich body of new physics on the mesoscopic scale.…”