Muons act as sensitive magnetometers to explore static / dynamic internal magnetic fields. They are a very sensitive probe, able to detect very small internal fields. Muons can provide a unique probe of small / dilute / randomly oriented moments, and are sensitive to fluctuation rates intermediate between those probed by neutrons and NMR. In systems which are multi-phase or partially ordered, the muon signal is proportional to the volume fraction of each component. A wide variety of magnetic systems can be studied: ordered, spin glass, frustrated, low-dimensional, CMR, heavy fermion, organic, etc., . . .

Picture: The structure of bilayer manganites [A I Coldea et al., 'Spin Freezing and Magnetic Inhomogeneities in Bilayer Manganites', Phys. Rev. Lett. 89 (2002)277601]