Development of a niobium nanosuperconducting quantum interference device for the detection of small spin populations

Abstract

Electron-beam lithography and reactive ion etching have been used to fabricate thin-film Au/Nb bridges with widths ∼50 nm. The Au layer was used as both a mask for etching the Nb superconducting bridge and as a resistive shunt in the completed devices. Using these junctions, a dc superconducting quantum interference device (SQUID) design with a hole size of 200 nm×200 nm (nano-SQUID) has also been fabricated and characterized. A flux noise of approximately 7×10−6 Φ0/Hz1/2 at 4.2 K has been achieved, from which a calculated spin sensitivity of 250 spin/Hz1/2 is predicted.

Direct measurement of penetration length in ultrathin and/or mesoscopic superconducting structures

“…The critical currents of the nanobridges, however, are temperature dependent, as previously reported [8]. As the operating temperature ranged from 4.5-6.5K in the present experiments, the bias current was adjusted as required to optimise the voltage amplitude of the SQUID's V-B characteristic.…”

“…The structures are prepared from sputtered Nb films on oxidized silicon wafers. The SQUID rings and absorber patch are prepared by conventional lithography, whereas the nano-bridge junctions are made either by focussed-ion-beam [7] or electron-beam techniques [8]. This approach enables a valuable simplification of the fabrication process, in that the nano-bridges can be introduced at a late stage directly into the previously-deposited single-layer Nb SQUID ring.…”

Fabrication and Analogue Applications of NanoSQUIDs Using Dayem Bridge Junctions

“…Although the first SQUIDs that demonstrated an approach to the standard quantum limit were of relatively large size (typically one tenth of millimetre or so in linear dimension) there is no intrinsic reason why smaller SQUIDs should not also approach this level of sensitivity. In addition SQUIDs which approach the nanoscale in size will open up sensitive measurement of a new range of quantities [2][3][4]. In this paper we describe fabrication methods which we have developed at NPL to realize nanoSQUIDs, then go on to outline the performance achieved to date.…”

Alternating current magnetic susceptibility of a molecular magnet submonolayer directly patterned onto a micro superconducting quantum interference device

“…This method opens the possibility of applying ac susceptibility experiments to characterize two-dimensional arrays of single molecule magnets within a wide range of temperatures and frequencies. The ac magnetic susceptibility of magnetic nanoparticles and single molecule magnets (SMMs) provides useful information on their spin and magnetic anisotropy, 1 as well as on the magnetic relaxation mechanisms.2-4 Miniaturized superconducting quantum interference devices [5][6][7][8][9] (SQUIDs) should eventually become capable 8,10 of measuring the magnetization reversal of a SMM (µ i ∼ 20µ B for the archetypal Mn 12 molecule). However, detecting the linear response sets even more stringent conditions: at T = 1 K, a magnetic field H = 24 A/m (0.3 Oe) induces a magnetic polarization µ ≃ 0.007µ B on the same Mn 12 cluster.…”

“…2-4 Miniaturized superconducting quantum interference devices [5][6][7][8][9] (SQUIDs) should eventually become capable 8,10 of measuring the magnetization reversal of a SMM (µ i ∼ 20µ B for the archetypal Mn 12 molecule). However, detecting the linear response sets even more stringent conditions: at T = 1 K, a magnetic field H = 24 A/m (0.3 Oe) induces a magnetic polarization µ ≃ 0.007µ B on the same Mn 12 cluster.…”