Nanoscale superconducting quantum interference devices (SQUIDs) fabricated on the apex of a sharp tip can provide spin sensitivities that are nearly two orders of magnitude better than previous SQUID sensors.

A scanning superconducting quantum interference device with single electron spin sensitivity

Nano Superconducting Quantum Interference device: A powerful tool for nanoscale investigations

Due to a growing interest in quantum information processing based on spin systems, the micro-SQUID technique has been finding new interest and fabrication improvements have led to nano-SQUIDs. There are two types of nano-SQUIDs: either the cross section of the Josephson junctions is reduced to about 1 nm by using carbon nanotube junctions, or the loop size is reduced to a few 100 nm. This paper reviews the basic ideas of the micro-SQUID technique applied to magnetic nanostructures and shows that both types of nano-SQUIDs lead to a significant improvement concerning the detection of magnetization switching of individual magnetic particles or molecules.

https://iopscience.iop.org/article/10.1088/0953-2048/22/6/064013/pdf

From micro- to nano-SQUIDs: applications to nanomagnetism

We present scanning superconducting quantum interference device microscopy data on the superconductors Sr2RuO4 Tc=1.5 K and PrOs4Sb12 Tc=1.8 K. In both of these materials, superconductivity-related timereversal symmetry-breaking fields have been observed by muon spin rotation; our aim was to visualize the structure of these fields. However, in neither Sr2RuO4 nor PrOs4Sb12 do we observe spontaneous superconductivity-related magnetization. In Sr2RuO4, many experimental results have been interpreted on the basis of a pxipy superconducting order parameter. This order parameter is expected to give spontaneous magnetic induction at sample edges and order parameter domain walls. Supposing large domains, our data restrict domain wall and edge fields to no more than 0.1% and 0.2% of the expected magnitude, respectively. Alternatively, if the magnetization is of the expected order, the typical domain size is limited to 30 nm for random domains or 500 nm for periodic domains.

/pdf/limits-on-superconductivity-related-magnetization-in-sr-2-4c5v61qup7.pdf

Limits on superconductivity-related magnetization in Sr 2 RuO 4 and PrOs 4 Sb 12 from scanning SQUID microscopy

In this review I discuss the application of scanning magnetic imaging to fundamental studies of superconductors, concentrating on three scanning magnetic microscopies—scanning SQUID microscopy (SSM), scanning Hall bar microscopy (SHM) and magnetic force microscopy (MFM). I briefly discuss the history, sensitivity, spatial resolution, invasiveness and potential future developments of each technique. I then discuss a selection of applications of these microscopies. I start with static imaging of magnetic flux: an SSM study provides deeper understanding of vortex trapping in narrow strips, which are used to reduce noise in superconducting circuitry. Studies of vortex trapping in wire lattices, clusters and arrays of rings and nanoholes show fascinating ordering effects. The cuprate high-Tc superconductors are shown to have predominantly d-wave pairing symmetry by magnetic imaging of the half-integer flux quantum effect. Arrays of superconducting rings act as a physical analog for the Ising spin model, with the half-integer flux quantum effect helping to eliminate one source of disorder in antiferromagnetic arrangements of the ring moments. Tests of the interlayer tunneling model show that the condensation energy available from this mechanism cannot account for the high critical temperatures observed in the cuprates. The strong divergence in the magnetic fields of Pearl vortices allows them to be imaged using SSM, even for penetration depths of a millimeter. Unusual vortex arrangements occur in samples comparable in size to the coherence length. Spontaneous magnetization is not observed in Sr2RuO4, which is believed to have px ± ipy pairing symmetry, although effects hundreds of times bigger than the sensitivity limits had been predicted. However, unusual flux trapping is observed in this superconductor. Finally, unusual flux arrangements are also observed in magnetic superconductors. I then turn to vortex dynamics: imaging of vortices in rings of highly underdoped cuprates places limits on spin-charge separation in these materials. Studies of spontaneous generation of fluxoids upon cooling rings through the superconducting transition provide clues to dynamical processes relevant to the early development of the universe, while studies of vortex motion in cuprate grain boundaries allow the measurement of current–voltage characteristics at the femtovolt scale for these technologically important defects. Scanning SQUID susceptometry allows the measurement of superconducting fluctuations on samples comparable in size to the coherence length, revealing stripes in susceptibility believed to be associated with enhanced superfluid density on the twin boundaries in the pnictide superconductor Co doped Ba-122, and indicating the presence of spin-like excitations, which may be a source of noise in superconducting devices, in a wide variety of materials. Scanning magnetic microscopies allow the absolute value of penetration depths to be measured locally over a wide temperature range, providing clues to the symmetry of the order parameter in unconventional superconductors. Finally, MFM tips can be used to manipulate vortices, providing information on flux trapping in superconductors.

https://iopscience.iop.org/article/10.1088/0034-4885/73/12/126501/pdf

Fundamental studies of superconductors using scanning magnetic imaging

Superconducting Quantum Interference Devices (SQUIDs) can have excellent spin sensitivity depending on their magnetic flux noise, pick-up loop diameter, and distance from the sample. We report a family of scanning SQUID susceptometers with terraced tips that position the pick-up loops 300 nm from the sample. The 600 nm - 2 um pickup loops, defined by focused ion beam, are integrated into a 12-layer optical lithography process allowing flux-locked feedback, in situ background subtraction and optimized flux noise. These features enable a sensitivity of ~70 electron spins per root Hertz at 4K.

A Terraced Scanning Superconducting Quantum Interference Device Susceptometer with Sub-Micron Pickup Loops

Superconducting quantum interference devices (SQUIDs) can have excellent spin sensitivity depending on their magnetic flux noise, pickup loop diameter, and distance from the sample. We report a family of scanning SQUID susceptometers with terraced tips that position the pickup loops 300nm from the sample. The 600nm–2μm pickup loops, defined by focused ion beam, are integrated into a 12-layer optical lithography process allowing flux-locked feedback, in situ background subtraction and optimized flux noise. These features enable a sensitivity of ∼70 electron spins per root hertz at 4K.

/pdf/a-terraced-scanning-super-conducting-quantum-interference-w2s7lscu2z.pdf

A terraced scanning super conducting quantum interference device susceptometer with submicron pickup loops

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A Terraced Scanning Superconducting Quantum Interference Device Susceptometer with Sub-Micron Pickup Loops

A terraced scanning super conducting quantum interference device susceptometer with submicron pickup loops