Muons, Magnetic Frustration and TRIUMF

The photograph above (taken by Dominic Ryan) shows the Meson Hall at TRIUMF. Those with keen eyesight may spot Seán Cadogan in the picture overseeing the experiment which is counting away down below !

Muons can be thought of as either heavy electrons or light protons, depending on whether one is a nuclear physicist or a condensed matter physicist. The muon mass is about 207 times that of the electron and around 1/9^th that of the proton. In an on-going collaboration between Seán Cadogan (UNSW) and Dominic Ryan (McGill University, Montreal, Canada), we have been using muons as subatomic magnetometers to study the dynamic and static magnetic behaviour of the Fe atoms in amorphous materials based on Fe-Zr. The muons are produced at the TRIUMF cyclotron (the world's largest) in Vancouver, Canada, where H- ions are accelerated to ~75% of the speed of light in the cyclotron, which is basically a set of 6 huge magnets arranged in a pinwheel pattern and weighing 4000 tonnes. The accelerated H- ions then pass through a stripper-foil which removes the two electrons to leave a proton beam which smashes into a target of graphite. This produces pions which decay after around 26 nanoseconds to produce our muon beam. Magnets are used to steer the muon beam and we implant the polarized muons into our Fe-Zr magnetic glasses, one muon at a time. The muon quickly localises in the sample and precesses like a top around any magnetic fields present at the muon's location. After around 2.2 microseconds, the muon decays and emits a positron, which we count. The positron is emitted preferentially along the direction of the muon's magnetic moment at the time of decay.

We have used muon spin relaxation to investigate the phenomenon of magnetic frustration in magnetic glasses where one has both positive (ferromagnetic) and negative (antiferromagnetic) exchange interactions, leading to magnetic structures called spin-glasses. The advantage of muon spin relaxation is that we are able to probe the magnetism of the Fe atoms in our sample without having to apply any magnetic fields.