Muons and gammas: nuclear probes and dynamic magnets

Full text: Dynamic behaviour is central to the properties of both frustrated systems, such as spin glasses, and also fine-particle magnets such as recording media. However, there are few direct, microscopic probes of magnetic dynamics available, and they are under-used, and when employed they are often mis-used. Using two very different systems (a ferrofluid and a frustrated magnetic metallic glass), I will discuss how conventional Moessbauer spectroscopy can be used to develop a consistent definition of the blocking temperature for a superparamagnetic system. I will then move on to show how muon spin relaxation (μSR) and Selective excitation double Moessbauer (SEDM) can yield detailed microscopic information on both static and dynamic behaviour. In particular, one can distinguish blocked and fluctuating particles in a superparamagnetic assembly. Also, it is possible to both observe, and identify the fluctuations associated with the ordering of transverse spin components in a frustrated magnet