The Materials Science and Engineering Division has designed a special facility for advancing measurements of absolute magnetic quantities of bulk materials. The measurements are made using the Thorpe-Sentfle method. In this technique the vertical magnetic force created on the sample by a vertical field gradient is measured with a sensitive electrobalance. Consequently, the measurement is reducible to the fundamental unit of the gram, providing the advantage of not requiring knowledge of the field gradient anywhere in the apparatus. As that is the largest source of error, higher accuracy is possible.
The susceptibility is measured by physically moving the sample to various positions from the field center to some distance outside of the magnet, measuring the magnetic force on the sample, and then integrating that force over the displacement distance. With the whole system, exclusive of the electromagnet, mounted on a movable table with stepping motor precision, sample position is also very accurately determined.
With the electromagnet possessing large flat pole pieces, the sample field is very uniform over the sample volume, while an additional uniform field gradient is applied via the injection of two parallel electric currents through the gap between the pole pieces on either side of the sample. Consequently, the assumption that the sample is a point dipole, upon which the method is based, is valid.
Customers of these SRMs include the permanent magnet, magnetic data recording, and electronic device industries that presently require traceability to an NMI.
Four standard reference materials (SRMs) have been certified for room temperature values:
- a pure Ni sphere for calibrating the coarsest ranges in superconducting quantum interference device (SQUID) and vibrating sample magnetometers (VSM); the certified value for magnetic moment is 3.47 mA•m2 ± 0.01 mA•m2
- a pure Ni disc for calibrating induction magnetometers for thin films; the certified value for specific magnetization is 54.78 A•m2/kg ± 0.15 A•m2/kg
- an yttrium sphere for calibrating the most sensitive ranges in SQUID and VSM magnetometers; the certified value for specific magnetization is 27.6 A•m2/kg ± 0.1 A•m2/kg; also useful for calibrating the very sensitive alternating gradient magnetometers
- a pure platinum cylinder for calibrating the zero point in magnetometer as well as the magnetic field scale once the magnetic moment scale has been calibrated using one of the other SRMs, certified value for the specific susceptibility is 1.268 x 10-8 m3/kg ± 0.004 x 10-8 m3/kg
Magnetic SRMs offered
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