For the magnetic field sweep, an MS-2 magnetic shield (Twinleaf LLC, Princeton, U.S.A.) was used to provide a 10 5 shielding factor against external magnetic fields. No static shim fields were required, since the residual field within the shield was on the order of 1 nT. The time-dependent applied magnetic field was generated from the built-in By shim coil, with current supplied by a Kea2 spectrometer with 1 µs time precision.