Rev. Sci. Instrum, 54(2) 137-145 (1983)

Superconducting magnet image effects observed with a vibrating sample 
magnetometer

A. Zieba and S. Foner

The distortion of the sample flux by a superconducting material produces a 
change of magnetometer output known as an image effect. Image effects for 
three Nb-Ti magnets were examined using a small current-carrying coil as a 
source of field-independent and adjustable, controlled moment of a 
vibrating sample magnetometer. This allowed determination of the initial 
image effect (in zero field) and the field-dependent image effect from 
zero to maximum magnetic field. The initial image effect (not detected by 
conventional calibration) can be large, and the field-dependent part was 
0.25-0.35 of initial image effect. The variation of the field-dependent 
image effect on field sweep reate and magnet temperature, and the change 
of the spatial distribution of the detection coil sensitivity were also 
observed. The image effect is independent of sample moment and decreases 
rapidly with increasing detection coil radius. The method of images was 
employed to calculate the image effect for perfect shielding as the 
function of detection coil dimensions relative to the magnet bore. For a 
small superconducting volume fraction lambda, the initial image effect for 
multifilamentary wire magnets is smaller than calculated by an approximate 
factor lambda/(1-labda) in agreement with observations for the Nb-Ti 
magnets. Some influences of the multifilamentary structure of the 
superconducting wire on the image effect are discussed.