Abstract
We present the first μSR study of the dynamic magnetic behaviour of iron clusters supported in a non-magnetic thin film matrix. Thin film samples afford the possibility of very tight control over the cluster size, but are too thin for study with conventional μSR owing to the relatively long (of order 0.5 mm) stopping range of 4 MeV muons. In this study, the thin film 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 μSR 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 μs −1 was observed and interpreted as due to a static Lorentzian distribution of dipolar fields with half-width 0.5 mT. At high temperatures, a motional narrowing of the field distribution, due to fluctuations in the orientation of the cluster moments, was observed. The fluctuations 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.
Published Version
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