Fundamental studies of superconductors using scanning magnetic imaging

Abstract

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-T c 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 can not 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 Sr 2 RuO 4 , which is believed to have p x ± ip y 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 currentvoltage 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, reveal stripes in susceptibility believed to be associated with enhanced superfluid density on the twin boundaries in the pnictide superconductor Co-doped Ba-122, and indicate 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 ...

A scanning superconducting quantum interference device with single electron spin sensitivity

“…In order to achieve a sensitive magnetometer with a low magnetic field noise, ܵ ଵȀଶ = ܵ ) ଵȀଶ / A, SQUIDs are commonly designed to have pickup loops with large effective area A. In recent years there has been growing interest in the development of nano-SQUIDs for the study of quantum magnetism and for nanomagnetic imaging [3][4][5][6][7][8][9][10][11][12] . In this case, field sensitivity is compromised for the benefit of spatial resolution and sensitivity to magnetic dipoles.…”

“…This is one of the lowest flux noise levels ever achieved [16][17][18] in SQUIDs. The ultimate flux noise in SQUIDs is determined by the quantum noise 8,19 …”

“…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.…”

Stray-field imaging of magnetic vortices with a single diamond spin

“…Furthermore, the spatial resolution is then limited both by the probe size and its distance to the sample. Among many stray-field microscopy techniques 5 , magnetic force microscopy has become ubiquitous, as it provides a spatial resolution o50 nm and operates under ambient conditions without any specific sample preparation 6 . It was for instance the first method that allowed the observation of the core of a magnetic vortex in a thin ferromagnetic disk 7 .…”

MicroSQUID Force Microscopy in a Dilution Refrigerator

“…Scanning probe microscopy has been widely used for the study [1] of superconductors, we briefly discuss advantages of each of the most common methods. Magnetic force microscopy compared to other scanning techniques has the huge advantage of its high spatial resolution of ∼ 50 nm, and moreover recently significant progress has been made increasing its magnetic sensitivity [2].…”