Design and performance aspects of pickup loop structures for miniature SQUID magnetometry

As miniature SQUID susceptometers and scanning SQUID microscopes come into wider use, it is important to understand in detail the magnetic coupling of objects of interest to small superconducting pickup loops as well as our ability to scale such pickup loop structures to sub-/spl mu/m dimensions. In this paper we address important aspects of both of these issues. First, assuming a perfect circular pickup loop, we examine the coupling to three-dimensional objects and arrive at a number of simple practical results for measurements on small samples with various geometries and positions. Second, we examine the nature of small superconducting pickup loop structures and the impact on scaling of factors such as connecting leads, local groundplanes and the superconducting penetration depth. We conclude that highly localized pickup areas of well under 1 /spl mu/m/sup 2/ are possible with existing technology.<>