Muon spin relaxation, often called µ+SR for short, is an experimental technique which can be used to study a wide variety of materials.
I use muons to study magnetic crystals. Muons are like tiny bar magnets and, if you put them in a magnetic substance, they will rotate around the magnetic field inside it. You create them all with the bar magnets pointing in the same direction. When they decay, they emit a positive electron along the direction they were pointing.
If you know where the muon was pointing to start with, and you know where it was pointing when it died, you can work out how many times it’s rotated its two microseconds of life. This means you can work out how fast it was spinning, which is directly related to the magnetic field it was sitting in while it was alive.
This allows us to work out the magnetic field inside a crystal.
Muon spin rotation can only be conducted at a few research facilities around the World. Most of my research is conducted at the Rutherford Appleton Laboratory near Oxford and the Paul Scherrer Institut in Switzerland.
If you’re a nerd and want more detail about µ+SR, and how it can be applied in a variety of different systems, then try Spin-polarized muons in condensed matter physics, by Stephen Blundell.