Planar Superconductor-Ferromagnet-Superconductor Josephson Junctions as Scanning-Probe Sensors

Abstract

We propose a novel type of magnetic scanning probe sensor, based on a single planar Josephson junction with a magnetic barrier. The planar geometry together with high magnetic permeability of the barrier helps to focus flux in the junction and thus enhance the sensitivity of the sensor. As a result, it may outperform equally sized SQUID both in terms of the magnetic field sensitivity and the spatial resolution in one scanning direction. We fabricate and analyze experimentally sensor prototypes with a superparamagnetic CuNi and a ferromagnetic Ni barrier. We demonstrate that the planar geometry allows easy miniaturization to nm-scale, facilitates an effective utilization of the self-field phenomenon for amplification of sensitivity and a simple implementation of a control line for feed-back operation in a broad dynamic range.

Two mechanisms of Josephson phase shift generation by an Abrikosov vortex

“…This creates an actual magnetic field in the junction with the sign opposite to that of the vortex. This field is large because of flux-focusing effect (large demagnetization factor) [27]. It induces correspondingly large edge currents I 1x,2x in the junction banks, which have equal amplitudes but opposite directions, I 1x = −I 2x , as sketched in Figs.…”

“…If vortex-junction distance is (much) larger than the effective penetration depth z v > λ * , we may neglect circulating vortex currents in the bulk of electrodes at the junction. In this case vortex stray fields become dominant due to a large demagnetization factor of planar junctions [27]. Fig.…”

Josephson junctions in a local inhomogeneous magnetic field

“…(15) [25]. [28] that spatial resolution of a scanning probe sensor based on a single planar junction is potentially not limited by its size. Such a device could obviate a trade-off problem between field sensitivity and spatial resolution inherent for scanning SQUID sensors [29,30], as mentioned in the Introduction.…”

“…Another motivations of this work is related to a recent proposal to use a single planar JJ as a scanning probe sensor [28]. The leading superconducting scanning probe technique today is the scanning SQUID (superconducting quantum interference device) microscopy [29,30].…”

“…In Ref. [28] it was argued that a sensor based on a single planar JJ would be able to obviate the trade-off problem at least in one spatial direction. Similar to a SQUID, the field sensitivity of a planar JJ is also inversely proportional to the area.…”