B-GREATER-THAN-1-T LOW-MASS MAGNETIC-FIELD SWEEP ASSEMBLY

When making linear or nonlinear conductivity measurements it is often desirable to apply a high magnetic field locally to a sample. If the w hole magnet assembly is integral to the sample probe, then, particular ly when making low temperature measurements, a low total mass is desir ed. We have designed, modeled, and tested a variable magnetic field sw eep unit suitable for use over a wide temperature range from cryogenic to approximate to 200 degrees C which provides a sweepable uniform ma gnetic field over an air gap of 2 mm and spatial extent of 4 mm x 4 mm . The unit consists of a magnet yoke structure with a mass of 18 g and a spur gear driven rotatable magnet to vary the gap field in a nearly sinusoidal manner as a function of the magnet rotation angle. In the present design, a 20 g SmCo magnet has been used which allows for low temperature operation to approximate to 10 K, when attached to a cryog enic refrigerator cold finger. The shape of the magnetic yoke structur e has been modeled and optimized using three-dimensional magnetic fiel d software. The gap field uniformity can thus be modeled and tested ex perimentally. In a present working model the field for a 2 mm gap at r oom temperature B(theta) = 1.05 . cos(theta)(T) where theta is the mag net rotation angle. The field sweep amplitude at 19 K only increased b y 3% over the 295 K value. With 0.25-mm-thick permunder pole tips, the field amplitude in a similar unit was increased to 1.20 T over a gap of 1.5 mm. (C) 1997 American Institute of Physics.