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Remnant Fields in Superconducting Magnets

Superconducting magnets can present challenges when trying to accurately set fields of less than 10s of Oersted due to e.g. flux trapping and pinning. In the case of the Quantum Design instruments, the PPMS-DynaCool and MPMS3, the magnetic field reported in MultiVu is based only on the current from the magnet power supply, and the remnant fields can produce field errors of up to ~20-40 Oe in a 7-9 Tesla magnet, and up to ~200 Oe in a 14 Tesla magnet.

Such remnant fields can cause significant measurement artefacts to e.g. hysteresis loop measurements of ferromagnetic samples with low coercivities, often resulting in an "inverted" loop where the switching field along the descending (ascending) branch of the hysteresis loop occurs for positive (negative) fields.

Fortunately, the remanence properties of the magnets are fairly reproducible as long as the same field charging procedure is followed for each measurement. As a result, it is possible to characterise these remnant fields using a  standard paramagnetic test sample, and the procedure can be carried out prior to each actual measurement to obtain a good correction for the absolute field error.

Quantum Design has produced two documents detailing the issues and precautions in using the superconducting magnets at low fields, and the procedure for the field correction. These can be downloaded below for members of the University with Raven access.