Sensors for scanning SQUID microscopy approaching and surpassing single electron spin sensitivity

Superconducting quantum interference devices (SQUIDs) are among the most sensitive

sensors for magnetic field, and nanoscale SQUIDs, in particular, also have high 

sensitivity to magnetic dipole fields. The sensitivity of SQUIDs with submicron sensor 

areas is better than ~100 µB/√Hz, where µB is the Bohr magneton, the magnetic dipole 

moment of the electron. When coupled with a scanning platform, these nanoscale 

SQUIDs become powerful tools for studying properties of magnetic systems, including 

persistent currents in normal metal rings, local measurements of penetration depths in 

high Tc

 superconductors, vortex dynamics in type II superconductors, and imaging edge 

currents in topological insulators. I will describe two such nanoscale SQUID sensors, 

planar SQUID susceptometers with sensitivities of ~70 µB/√Hz and needle-like SQUID 

magnetometers with sensitivities better than 1 µB/√Hz, concluding with recent results of, 

and future plans for, studies using these sensors.