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Muons as probes of dynamical spin fluctuations: some new aspects

This paper describes the impact of dynamic spin fluctuations on the muon spin relaxation signal in the longitudinal field set-up, namely, when a field is applied along the initial muon spin direction. Our main objective is to show that the μSR technique can do more than determine the correlation time of the spins in the system under investigation. It can, in fact, determine if the concept of correlation time is valid to begin with, and if not suggest alternatives. Consequently, the paper shows what to expect from the muon signal over a range of situations starting from a simple antiferromagnetic hopping model to more complicated models involving power laws and other types of correlation functions. The application of all models to experimental data is demonstrated. The possibility that the muon, by itself, generates dynamic fluctuations is critically examined by comparing muon and neutron scattering data.