MAGNETIC FIELDS INSIDE SUPERCONDUCTORS have been measured with high spatial resolution using low energy muons. Like scouts sent ahead to survey a landscape, muons (essentially heavy electrons) can be sent into a material filled with magnetic fields. When the muons (which act like little magnets all carefully oriented in the same direction beforehand) enter the sample, the magnetic fields there cause the muon's magnetic axis to rotate (precess) in a characteristic fashion. When the muons finally decay (see figure at Physics News Graphics), one of the daughter particles, positrons, carry information about the local magnetic environment out of the sample into detectors.

This whole process, called muon spin rotation (muSR), has previously used relatively high energy (several MeV) muons, rendering magnetic maps with resolutions of tenths of mm. Now, researchers at the University of Birmingham (U.K.) (Tim Jackson, jackson@eee-fs8.bham.ac.uk, 011-44-121-414-7506) and the Paul Scherrer Institute (Switzerland) have been able to use much lower-energy (10 eV-30 keV) muons and are able to achieve a corresponding improvement in the magnetic map down to a resolution of tens of nm. Thus magnetic fields can now be surveyed in thin film samples of high-temperature superconductors. Such films are important in SQUID microcircuits (e.g., magnetometers) and filters at mobile phone base stations. (Jackson et al., Physical Review Letters, 22 May /pnu/2000/; Select Article.)