Systematic studies of magnetic critical exponents in local moment molecular magnets can be made by combining three different types of μ SR measurement; zero-field precession, longitudinal relaxation and the frequency shift in transverse field. Systems have been studied with 1D, 2D and 3D structures, having both simple and non-collinear magnetic interactions and the measurements have included investigation of the effect of chirality on the critical spin fluctuations. In the case of the model S = frac(1, 2) Heisenberg antiferromagnetic 1D chain Heisenberg antiferromagnetic quantum critical fluctuations suppress magnetic ordering and TCNQ radical ion salts can provide some excellent examples of this model. In particular, DEOCC-TCNQF4 behaves as a nearly ideal case with no ordering seen down to 20 mK even though the intrachain exchange interaction is 110 K. In contrast the DMTzNC-TCNQ compound orders at 1.5 K, but still shows evidence for significant residual quantum fluctuations in its ordered state in the form of a large suppression of the magnitude of the ordered part of the magnetic moment. From analysis of the characteristic field dependent muon relaxation behaviour as a function of temperature above the transition, the nature of the spin fluctuations is found to change as the transition is approached, suggesting a crossover either to 2D diffusion or to 1D ballistic motion.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Electrical and Electronic Engineering