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RESEARCH PRODUCT

Positive Muons as Local Probes in Paramagnetic Rare Earth Systems

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In recent years a new research method has been developed in solid state physics which is based on the asymmetry of muon decay1. A positive muon at rest decays after a mean lifetime of 2.2 μsec into a positron and two neutrinos, the positron being emitted preferentially in the direction of the muon spin. Hence if spin polarized muons are implanted in a solid, the time evolution of the muon spin polarization can be determined by a measurement of the angular distribution of the positrons. In an external magnetic field transverse to the initial spin direction of the muon one can study the muon spin precession frequency, which is determined by the actual field at the muon, and τ2-relaxation effects. (It is this experimental arrangement from which the most commonly used name “muon spin rotation” [μSR] is derived.) A longitudinal field arrangement allows study of τ1-relaxation effects. The time scale which can be covered by (μSR) is determined by the magnetic field at the muon, the field fluctuations and the muon lifetime. In nonmagnetic substances the time scale ranges from 10−8 sec to 10−5 sec, in magnetic substances it can be extended down to 10−11 sec.