A low-energy muon study of thermal activation in single-domain iron particles

We present the first mu SR study of the dynamic magnetic behaviour of iron clusters supported in a non-magnetic thin film matrix. Thin film samples af ford the possibility of very tight control over the cluster size, but are t oo thin for study with conventional mu SR owing to the relatively long (of order 0.5 mm) stopping range of 3 MeV muons. in this study, the thin film s amples were a 500 nm thick, silver matrix containing iron clusters with an equivalent iron concentration of 0.1% by volume. SQUID magnetometry showed a mean duster diameter of 2.8 nm. Transverse low-energy mu SR measurements in a field of 25 mT, applied normal to the plane of the film, were made in the temperature range 4.7-300 K. At low temperatures, a relaxation rate of 0.4 mu s(-1) was observed and interpreted as due to a static Lorentzian dis tribution of dipolar fields with half-width 0.5 mT. At high temperatures, a motional narrowing of the field distribution, due to fluctuations in the o rientation of the cluster moments, was observed. The fluctuations were foun d to follow an Arrhenius activation law, from which an activation energy of 51 +/- 9 K and an intrinsic cluster relaxation time of 12 +/- 4 ns were de duced. (C) 2000 Elsevier Science B.V. All rights reserved.