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

We present the "rst lSR study of the dynamic magnetic behaviour of iron clusters supported in a non-magnetic thin "lm matrix. Thin "lm samples a!ord the possibility of very tight control over the cluster size, but are too thin for study with conventional lSR owing to the relatively long (of order 0.5 mm) stopping range of 4 MeV muons. In this study, the thin "lm samples 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 cluster diameter of 2.8 nm. Transverse low-energy lSR measurements in a "eld of 25 mT, applied normal to the plane of the "lm, were made in the temperature range 4.7}300 K. At low temperatures, a relaxation rate of 0.4 ls~1 was observed and interpreted as due to a static Lorentzian distribution of dipolar "elds with half-width 0.5 mT. At high temperatures, a motional narrowing of the "eld distribution, due to #uctuations in the orientation of the cluster moments, was observed. The #uctuations were found 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 deduced. ( 2000 Elsevier Science B.V. All rights reserved.