SPM: Scanning SQUID Microscope

A scanning Superconducting Quantum Interference Device (SQUID) Microscope images magnetic fields above a sample surface. It has the advantages of high magnetic field sensitivity and an easily calibrated, linear response, and the disadvantages of modest spatial resolution and the requirement of a cooled SQUID sensor. The user facility instrument at Stanford is a modification of Attocubes cryogenic magnetic force microscope. It has a spatial resolution of between 1 and 10 microns, depending on the SQUID sensor mounted. The sample temperature can be varied between 4.3 K and 110 K while scanning. There are three imaging modes available: magnetometry, susceptometry, and current imaging. In magnetometry, magnetic fields of a few nanoTesla and dipole moments of a few hundred Bohr magnetons can be detected with a signal to noise of 1 in a 1Hz bandwidth. Similarly, in susceptibility mode a volume susceptibility of a few times 10^-7, and in current sensing mode a current of a few nano amps can be detected. Although the user facility SSM is in principle a fast turnaround system, in practice at least 2 days should be allowed to align, cool, and run a sample. An area of the sample 5 mm by 5 mm can be accessed in a particular cooldown. Individual areas of up to 150 microns by 150 microns can be scanned in a particular image. Multiple scans can be merged to image larger areas.