Probing dynamics and pinning of single vortices in superconductors at nanometer scales

Abstract

The dynamics of quantized magnetic vortices and their pinning by materials defects determine electromagnetic properties of superconductors, particularly their ability to carry non-dissipative currents. Despite recent advances in the understanding of the complex physics of vortex matter, the behavior of vortices driven by current through a multi-scale potential of the actual materials defects is still not well understood, mostly due to the scarcity of appropriate experimental tools capable of tracing vortex trajectories on nanometer scales. Using a novel scanning superconducting quantum interference microscope we report here an investigation of controlled dynamics of vortices in lead films with sub-Angstrom spatial resolution and unprecedented sensitivity. We measured, for the first time, the fundamental dependence of the elementary pinning force of multiple defects on the vortex displacement, revealing a far more complex behavior than has previously been recognized, including striking spring softening and broken-spring depinning, as well as spontaneous hysteretic switching between cellular vortex trajectories. Our results indicate the importance of thermal fluctuations even at 4.2 K and of the vital role of ripples in the pinning potential, giving new insights into the mechanisms of magnetic relaxation and electromagnetic response of superconductors.

Probing the low-frequency vortex dynamics in a nanostructured superconducting strip

“…9,33 Since both components of the local permeability nearby the moving vortices are typically of the same order of magnitude, their motion is clearly phase lagged with respect to the ac excitation. If we take into account the very small frequency used in the experiment, this is a clear evidence of thermally activated hopping of these vortices.…”

“…1, we present the profiles of the local magnetic permeability b ac (x)/h ac of a superconducting strip, which is the typical quantity accessed in ac susceptibility imaging experiments, using either scanning Hall probes or SQUIDs. 3,8,9,33 This quantity was calculated for different values of Λ and Λ v , covering 0.05a ≤ |Λ ac | ≤ 2.5a, at a height z 0 = 0.005a above the plane of the strip. This value for z 0 is close to the typical probe height used in our experiments.…”

Diamond magnetometry of Meissner currents in a superconducting film

“…1 A variety of techniques, including magnetic force microscopy, 2,3 Hall sensing, 4-11 magneto-optical imaging 9,[12][13][14][15] and scanning superconducting quantum interference device magnetometry [16][17][18][19][20] have been used to perform spatially resolved measurements of magnetic properties of superconductors.…”

Electrically Tunable Multiterminal SQUID-on-Tip

“…In contrast to conventional two-terminal/two-junction SQUIDs that display optimal sensitivity when flux biased to about a quarter of the flux quantum, the additional terminals and junctions allow optimal sensitivity at arbitrary applied flux, thus eliminating the magnetic field "blind spots". We demonstrate spin sensitivity of 5 to 8 µ B /Hz 1/2 over a continuous field range of 0 to 0.5 T, with promising applications for nanoscale scanning magnetic imaging.KEYWORDS: superconducting quantum interference device, SQUID on tip, nanoscale magnetic imaging, current-phase relations 2 In recent years, there has been a growing effort to develop and apply nanoscale magnetic imaging tools in order to address the rapidly evolving fields of nanomagnetism and spintronics.These include magnetic force microscopy (MFM) 1,2 , magnetic resonance force microscopy (MRFM) [3][4][5] , nitrogen vacancy (NV) centers sensors [6][7][8][9] , scanning Hall probe microscopy (SHPM) 10-12 , x-ray magnetic microscopy (XRM) 13 , and micro-or nano-superconducting quantum interference device (SQUID) [14][15][16][17][18][19][20] based scanning microscopy (SSM) [21][22][23][24][25][26][27][28][29][30][31][32] . Scanning micro-and nanoscale SQUIDs are of particular interest for magnetic imaging due to their high sensitivity and large bandwidth 15,19 .…”