3 Tesla high homogeneity magnet system for muon resonance (μSR) spectroscopy
This magnet system was supplied in 2009 for muon spin resonance spectroscopy on a customer’s new and upgraded beamlines, and consists of a high homogeneity, low drift rate 3 T split pair magnet mounted within the tail set of a low loss cryostat.
The magnet incorporates independent shim coils to provide a high level of homogeneity correction. As it is important in this application to remove remanent field effects, ‘zero field’ coils are located within the magnet.
The cryostat is based on a standard Oxford Instruments NMR-type low loss cryostat, with modifications to suit the specific requirements of the customer's system specification, and provides liquid helium hold time of > 14 days. Ambient temperature access is provided along the two orthogonal directions in the horizontal plane. Vacuum sealing faces and mounting holes for sample cryostats and are provided on the bore tube end flanges for integration with the beamline and detectors.
A key success of this magnet system was in achieving the extremely tight clearance and precise alignment requirements of the magnet coils (right) within the cryostat tails. By comparing magnetic field plots from either end of the bore, the angular deviation of the flux density vector relative to the bore axis was calculated as only < 0.03º, compared to a specification requirement of < 0.2º – an excellent achievement.
Muon spin resonance (μSR) spectroscopy
The broadest application of μSR spectroscopy is as a magnetic probe with which to examine structural and dynamic processes in bulk materials on an atomic scale, providing a powerful technique to gain a deeper understanding of magnetic materials, semiconductors and superconductors. Beams of positive muons are created with aligned spins, which precess around the local magnetic fields within the material that the beam is fired into it. The muons decay into positrons which are typically ejected in the same direction as the muon spin. By measuring the directions in which the positrons are emitted, the μSR spectroscopist can investigate how the internal magnetic fields of the sample materials have influenced the muon spins.