Demagnetization and effective susceptibility of magnetic suspensions for turbulent dynamo experiments

Wednesday, 17 December 2014
Kunlun Bai, Aubrey Wahl, Ethan Kyzivat and Eric Brown, Yale University, New Haven, CT, United States
In order to achieve a turbulent dynamo in the laboratory, we propose to use eutectic Gallium-Indium alloy (eGaIn) with suspended magnetic particles. One of the key challenges is to quantify the magnetic properties of the suspensions, such as the magnetic susceptibility. We measure the susceptibility of materials in both suspension and powder forms. The effective susceptibility is much smaller than the true material susceptibility due to the demagnetization field inside the body resulting from the long-range nature of the dipolar force acting between individual magnetic moments. The demagnetization field tends to decreases the local magnetic field inside the body. It is found that the effective susceptibility is a function of global aspect ratio of the sample (defined as the ratio of length to diameter in a cylindrical sample), local aspect ratio of individual particle, and packing fraction. The effective susceptibility increases with increasing global and local aspect ratio, and packing fraction. The predicted effective susceptibility based on a model of the demagnetization effect in powders (Skomski et al. 2007) is tested against our measured data. The model, which incorporates three affecting factors, can describe powders with relatively high packing fraction (>40%). For powders with low packing fraction (<40%), the measured susceptibility deviates from the model. We propose a correction of the model for the magnetic samples. The new model turns out to quantify the measured effective susceptibility of both solids and powders, which cover a wide range of aspect ratio (from 3 to 56) and packing fraction (from 4% to 45%). The present studies may also be useful to understand the effects of magnetic material distributions in biasing magnetic measurements in rock samples that are used in studies of paleomagnetism for example.