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.
A bit of history.
The experimental technique of µSR applied to condensed matter originates from the discovery of the violation of parity in decays regulated by weak interactions. The Nobel Prize was awarded to Lee and Young (1957) "for their penetrating investigation of the so-called parity laws'' (from the motivation of the prize). These investigation had been confirmed by C.S. Wu at al., Phys. Rev. 105 (1957) 1413 in their experiment on the β-decay of .
The bottom line in the right column indicate that the authors had already the intuition of the potential use of this effect as a solid state technique.