mu SR study of the superparamagnetic mesoscopic cluster Fe8

Muon longitudinal relaxation rate (lambda) measurements as a function of te mperature and magnetic field in Fe8 molecular clusters are presented. The t emperature dependence of lambda is characterized by broad maxima centered a t 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 la mbda is anomalously strong below T = 2 K. In both types of Fe8 under partic ular magnetothermal cycling conditions, a muon precession signal was observ ed to set in in zero field for T < 2.2 K, suggesting the presence of a loca l field at the muon site associated with the freezing of the cluster magnet ic moment. (C) 2000 Elsevier Science B.V. All rights reserved.