New lows for magnetic shielding

We are awash in magnetic fields. Power lines, earbuds, and motors each generate their own fields, and some cars have upwards of 150 magnets; Earth’s field varies between about 25 µT and 65 µT at the surface, roughly 1/200 that of a refrigerator magnet. For precision measurements, whether of fundamental physical quantities or of brain activity, magnetic fields can be unwanted sources of noise, and experiments require environments shielded from fields both constant and varying in space and time. As part of a quest to put new limits on the electric dipole moment of the neutron (see the article by Norval Fortson, Patrick Sandars, and Steve Barr, Physics Today, June 2003, page 33 ), an international team led by Peter Fierlinger of the Technical University of Munich has demonstrated the ability to reduce field fluctuations at millihertz frequencies by a million-fold over a 4-m³ volume. That shielding represents an order-of-magnitude improvement in a challenging frequency range. To achieve that level of attenuation, the researchers fashioned five nested boxes of a highly magnetizable alloy; the three inner shells formed an insert that, as seen here, can be rolled out from within the surrounding shells. Among the design elements the team implemented are thin shells spaced close together and a refined procedure for using interlayer coils to equilibrate the layers. The approach reduced the fields from typical external sources to below 1 pT and limited gradient drifts to 1 pT/m over several hours. (I. Altarev et al., J. Appl. Phys. 117, 183903, 2015, doi:10.1063/1.4919366 ; photo by Astrid Eckert/TUM.)