The thermal spin transition that occurs in the polymeric chain compound [Fe(NH(2)trz)3](NO3)2 above room temperature has been investigated by zero-field muon spin relaxation (microSR) over the temperature range approximately 8-402 K. The depolarization curves are best described by a Lorentzian and a Gaussian line that represent fast and slow components, respectively. The spin transition is associated with a hysteresis loop of width DeltaT = 34 K (T1/2 upward arrow = 346 K and T1/2 downward arrow = 312 K) that has been delineated by the temperature variation of the initial asymmetry parameter, in good agreement with previously published magnetic measurements. Zero-field and applied field (20-2000 Oe) microSR measurements show the presence of diamagnetic muon species and paramagnetic muonium radical species (A = 753 +/- 77 MHz) over the entire temperature range. Fast dynamics have been revealed in the high-spin state of [Fe(NH(2)trz)3](NO3)2 with the presence of a Gaussian relaxation mode that is mostly due to the dipolar interaction with static nuclear moments. This situation, where the muonium radicals are totally decoupled and not able to sense paramagnetic fluctuations, implies that the high-spin dynamics fall outside the muon time scale. Insights to the origin of the cooperative effects associated with the spin transition of [Fe(NH(2)trz)3](NO3)2 through muon implantation are presented.

The thermal spin transition which occurs reversibly in [Fe(phen) 2 (NCS) 2 ] (polymorph I) around T 1/2 ∼ 177 K upon cooling and warming has been investigated by muon spin relaxation (µSR) measurements over the temperature range (∼10-280 K). The depolarization curves in zero and longitudinal fields (50-2000 Oe) are well described by two Lorentzian lines that represent fast and slow components in the decay curves and a nonrelaxing component. Evidence of the scope to track the spin transition occurring in [Fe(phen) 2 (NCS) 2 ] (polymorph I) is provided via the temperature dependence of the zero-field initial asymmetry parameter of the fast component. Muonium-substituted radicals delocalized on the phenanthroline ring (hyperfine coupling constant A ∼ 500 MHz), as well as diamagnetic muonic species, have been identified on the basis of the applied field µSR experiments.