Cryostat Design:
Schematic cross sectional view of the Vanderbilt University's Superconducting Quantum Interference Device (SQUID) Microscope. The magnetic field of the sample is sensed with a superconducting pickup coil coupled to a highly sensitive magnetic field sensor, the SQUID. The operating temperature of our superconducting sensors are -265 Deg C. Therefore the pickup coil and the SQUID are housed in the vacuum space of a cryostat behind a thin sapphire window and are cooled through a thermal link to a liquid Helium reservoir.


Dewar Tail:
Enlarged view of the tail of the SQUID microscope in Cryostat Design figure (above). A bellows mechanism is used to reduce the spacing between the 0.001-inch thick sapphire window and the superconducting pickup coil. We typically achieve a spacing between the -265 deg C coil and the room-temperature sample of 0.001 inch (25 micrometers). This means that the temperature change across the sapphire window is about 7 million degrees Fahrenheit per foot (10 million degrees C per meter). The thermal insulation is provided by the vacuum. The sample is situated just below the sapphire window at bottom.


Pickup Coil:
The pickup coil is wound from a 0.001 inch thick Niobium wire on a 0.01 or 0.02 inch diameter sapphire bobbin and has between ten and twenty turns in two layers. Sapphire is chosen because it is an insulator, has a high strength and a high thermal conductivity. Careful thermal shielding is required to reduce the heat load on the superconducting pickup coil. The sample is situated just below the sapphire window at bottom.


Experimental Setup:
Photograph of the Vanderbilt SQUID microscope in the magnetically shielded room. The Vanderbilt SQUID microscope has to be operated in a magnetically shielded room repelling magnetic fields like the earth's field, magnetic fields generated by power lines and electrically powered devices.


SQUID Microscope, Scanning Stage:
The room temperature sample is scanned in front of the sapphire window of the cryostat on a high precession non magnetic platform. The platform can be leveled relative to sapphire window with a precision of less than 0.001 inches. The platform is powered by optically encoded, piezo driven, non magnetic motors.