NanoSQUID sensitivity for isolated dipoles and small spin populations

Abstract

The sensitivity of nanoscale SQUIDs (nanoSQUIDs; SQUID: superconducting quantum interference device) to single and small populations of magnetic dipoles is considered. The simple estimate given previously for the atomic spin sensitivity of a nanoSQUID coupled to an isolated magnetic dipole at its centre is confirmed. It is demonstrated that the sensitivity is constrained in most practical situations by the finite size of the SQUID loop and nanobridges. An exact analytic result is obtained for a nanoSQUID composed of an idealized filamentary circular loop. The issue of optimum placement and orientation of the dipole with respect to the nanoSQUID hole is also considered, and it is shown that the optimum position for the dipole depends upon the height of the dipole above the plane of the nanoSQUID. It is pointed out that the conclusion quoted by previous authors, that the optimum position is above one of the Josephson junctions or Dayem bridges, although true in the limit of very narrow bridges with tight magnetic coupling, is not true in general, and estimates of the potential sensitivity when the dipole is placed in this region based on simple filamentary models are likely to overestimate the sensitivity achievable in practice.

Hybrid superconducting quantum magnetometer

“…SQUIDs are nowadays exploited in a variety of physical measurements 4,5 with applications spanning, for instance, from pure science to medicine and biology 1,6 . Recently, the interest in the development of nanoscale SQUIDs [7][8][9] has been motivated by the opportunity to exploit these sensors for the investigation of the magnetic properties of isolated dipoles [10][11][12][13][14] , with the ultimate goal to detect one single atomic spin, i.e., one Bohr magneton.…”

A scanning superconducting quantum interference device with single electron spin sensitivity

“…Since the SOT can approach and scan a sample within a few nm of the surface 12,26 , an even higher spin sensitivity and enhanced spatial resolution can be obtained near the loop edges rather than in the center 8,27 . Figures 4a and 4b show the calculated flux coupling to the ø160 nm SOT from a single electron spin at a safe distance of 10 nm.…”

DC NanoSQUID from Nb Thin Films

“…Compared with the value of magnetic flux quantum (Φ 0 = 2.07 × 10 −15 Wb) the flux noise reaches Φ ns = 1.19 × 10 −6 Φ 0 which is a typical value of flux noise of SQIUDs [7]. Considering w = 120 nm and ideal coupling limit α = 0.5 ( Fig.…”