IMPROVEMENT OF REPRODUCIBILITY OF MAGNETIC MOMENT DETECTED BY A SQUID MAGNETOMETER THROUGH RADIAL OFFSET MEASUREMENT ON A YIG SPHERE
Cindi L. Dennis, Robert D. Shull, Nobuhiro Matsumoto
We report here on the reproducibility of measurements on a commercial SQUID Magnetometer of two different yttrium iron garnet (YIG) spheres, both having a diameter of 1 mm: (1) the NIST magnetic moment standard reference material (SRM) and (2) a commercial sample. It has been suggested that rotating the sample rod around its axis can move the sample center towards the center of the second-gradiometer coil. The observed value of the magnetic moment will be theoretically a minimum when the radial offset is zero and this value will increase in a quadratic manner with the radial offset. When the magnetic moment of the SRM was repeatedly measured as a function of rotation angle ϕ from 0o to 360o, we observed a sinusoidal variation in the measured values. The radial offset dependence of the observed magnetic moment was experimentally confirmed by measurements of the commercial sphere placed in a hole in several cylindrical PTFE containers, wherein the distance between the center of the hole and the center of the container was r. The r dependence of the minimum from each ϕ-dependent measurement series is qualitatively consistent with the theoretical curve. When the ϕ-dependent measurements for the SRM in a capsule were repeated 12 times over 21 months, the relative standard deviation of the minimums improved up to 0.1%. Knowledge of these facts will be necessary for accurate measurement of the magnetic moment of other sample forms (e.g., powders).