SQUID microsusceptometry in applied magnetic fields

We describe the design and construction of a planar dc SQUID susceptometer to characterize microcrystallites as a function of temperature from 0.3 K to 4 K in magnetic fields exceeding 280 G. Eliminating the need for magnetic shielding permits broader application of SQUIDs to characterization of small spin ensembles. We present temperature- and field-dependent magnetic susceptibility data on crystallites mounted directly on the surface of a SQUID chip. Large, in-plane static fields are applied through an external coil while perpendicular fields up to 2 G can be applied by a series-wound on-chip field coil. Paramagnetic, antiferromagnetic, and superconducting transitions have been resolved on samples of order 20 to 50 /spl mu/m (30 to 500 picomoles, /spl sim/5/spl times/10/sup 16/ emu/Hz/sup 1/2/.) We discuss the effects of sample shape and location on the spin calibration of the assembled susceptometer.