IMAGE AND SAMPLE GEOMETRY-EFFECTS IN SQUID MAGNETOMETERS

This review concerns the interpretation of the signal provided by SQUI D magnetometers designed for measurements of magnetization and magneti c susceptibility. ''Image effect'' refers to the influence of supercon ducting elements in the vicinity of the sample and detection coil. Sev eral exact and approximate methods are presented for calculation of ho w the device sensitivity G(r) varies with position r of a unit dipole sample in the presence of a superconducting shielding tube. In particu lar, the relative decrease of G(r) is approximately given by the third power of the ratio of the detection coil and shield diameters, and th e effect of the shield's finite length is found to be negligible in pr actical situations. A survey of complex detection coils (Helmholtz, sa ddle-type, gradiometer, and second derivative coil) includes calculati on of the optimum spacing of the Helmholtz pair in the presence of a s uperconducting cylinder. The image effects due to the properties of th e SQUID circuit and the field-dependent effects in high-field SQUID ma gnetometers are also discussed. Change in the instrument calibration d ue to sample size, shape, and location is considered for arbitrary sam ples as well as for specific cases of small, medium, and very long sam ples. A spherical harmonic expansion of G(r) makes it possible to deri ve simple formulas describing sample geometry effects for medium-sized samples with regular geometries (thin rod, cylinder, and rectangular parallelepiped). The results are compared to published experimental da ta.