A Low-cost and Accurate Method for Measuring Weak Magnetic Susceptibilities.

CLAUDIO J. BONIN; FERNANDO J. BONETTO

Sao Paulo, Brasil

Encuentro; Autumn Meeting 2022 of the Brazilian Physical Society; 2022

Sociedade Brasileira de Fisica

Themagnetic properties of any substance in nature can be condensed in a uniqueparameter: the volume magnetic susceptibility c. Its value andsign indicate if a given material is diamagnetic, paramagnetic orferromagnetic. Accurate measurements of c for paramagneticor diamagnetic substances are generally performed with high-tech devices withrelatively complex methods. Simple methods proposed to determine c foreveryday materials result in inaccurate measurements (≥ 50 %). Here we proposea simple, practical and low-cost method to accurately obtain c, viameasurements performed with an ordinary balance and a set of neodymium magnets.The force exerted by the magnets on a given sample is sensed by the balanceand, via comparison with exact numerical calculations of the magnetic force onthe whole sample, the magnetic susceptibility can be determined. The shape,size and position of the sample relative to the magnets as well as the numberof neodymium magnets are optimized to maximize the magnetic force on the sampleand, therefore, minimize the experimental uncertainties in the determination ofc. The analysis of the dependence of the total forcewith the number of stacked neodymium magnets indicates that a large proportionof the total force caused by an infinite number of stacked magnets is obtainedwith a quite limited number of them. In the present contribution we used fourstacked magnets that can generate more than 80% of the total force exerted byan infinite number of magnets. The analysis of the optimum shape and dimensionsof the sample for maximizing the force per volume unit (force density)indicates that a closed rectangular ring wire, with the magnet?s externaldimensions, positioned right below the magnets would be ideal in case lowsample volumes are available (with no constraints on the sample shape). Forhigher sample volumes centered positioned, flat extended samples, not beyondthe magnet dimensions, should be preferred over thicker samples.We have determined the volume magnetic susceptibility of four pureelements (one diamagnetic and three paramagnetic), three diamagnetic solidmaterials and pure liquid water. In all the cases studied, the values obtainedagree with that previously reported in the literature, evidencing the accuracyand reliability of the proposed method.