A high-resolution magnetic imaging system based on a SQUID magnetometer

Abstract

We have recently developed a high-resolution magnetic imaging system based on an array of five superconducting pickup coils located within 1 mm of room temperature. The pickup coils are 1.70 mm in diameter and spaced 2.5 mm apart allowing spatial resolution of order 1 mm. They are each connected to an rf SQUID and have a magnetic field sensitivity of 3 pT/√Hz. The system includes a three axis nonmagnetic translation table for mounting the sample, a stage for temperature control, and complete computer control of all functions.

Cold SQUIDs and hot samples

“…The SQUID-sample separation of about 140 fim. is a factor of 6 smaller than the lowest value quoted for a low-Tc SQUID microscope [38], but substantially higher than the value of 40 /xm achieved by Black and co-workers [41] with their high-Tc microscope using a 25 ^.m-thick, 1 mm-diameter sapphire window. …”

“…To our knowledge, the smallest value of z achieved in a system of this kind is 0.85 mm [38]. On the other hand, the much higher operating temperature of high-Tc SQUIDs implies that a radiation shield is no longer required, so that the SQUID can be brought substantially closer to the window.…”

“…For a given separation between the SQUID and the sample, one wishes to optimize the dimensions of the SQUID to achieve a compromise between spatial resolution and sensitivity to magnetic field. Others have considered this problem for circular sensors, and concluded that the best choice is to make the diameter comparable to z [38,40]. Reducing the diameter much below z decreases the amount of captured flux, thereby reducing the signal amplitude from a given source, without appreciably improving the spatial resolution.…”

“…In our implementation of the microscope, we have chosen to use a bare SQUID as the sensor, rather than to use a superconducting flux transformer as have some other authors [37,38]. For a given separation between the SQUID and the sample, one wishes to optimize the dimensions of the SQUID to achieve a compromise between spatial resolution and sensitivity to magnetic field.…”

“…Black and co-workers have pioneered the use of high-Tc SQUIDs operated at liquid nitrogen temperature to examine cold samples with a variety of techniques, including static magnetization [31], eddy current [32], radio frequency [33], and microwave imaging [34]. Low-Tc SQUID microscopes have been used extensively for nondestructive evaluation (NDE) of warm samples in many different modes [26], and a number of different systems have been described [35,36,37,38]. More recently, high-Tc systems for NDE of room temperature samples have been introduced [39,33].…”