Introduction to µSR
µSR is a condensed matter experimental technique that measures internal magnetic fields inside matter by means of the spin precession of implanted polarized muons. The muon is an unstable elementary particle. It is produced above a certain threshold energy by protons impinging on solid targets. Muons can be selected with 100% spin polarization along the beam axis and stopped inside the material that one wishes to investigate.
The muon decays emitting a positron preferentially in the direction of its spin. Thus the muon spin precession is seen as an oscillations in the positron count rates in a fixed detector. The frequency of this oscillation measures the internal field, for instance that due to surrounding ordered magnetic moments in an antiferromagnet.
The wiki index is just below, but if you want a really simple account of µSR watch this http://www.youtube.com/watch?v=S4iSqS1rEX8 and if you want to catch a glimpse of the ISIS Muon Training Schools watch this http://www.youtube.com/watch?v=q5ydGCg4ZsM