mu SR study of the superparamagnetic mesoscopic cluster Fe8

Muon longitudinal relaxation rate (lambda) measurements as a function of temperature and magnetic field in Fe8 molecular clusters are presented. The temperature dependence of lambda is characterized by broad maxima centered at a temperature which increases with increasing field and with an intensity inversely proportional to the field itself. While smooth field dependence of lambda is observed in Fe8 : Br, in Fe8 : ClO4 the field dependence of lambda is anomalously strong below T = 2 K. In both types of Fe8 under particular magnetothermal cycling conditions, a muon precession signal was observed to set in in zero field for T < 2.2 K, suggesting the presence of a local field at the muon site associated with the freezing of the cluster magnetic moment.