Superparamagnetic relaxation in iron nanoclusters measured by low energy muon spin rotation

Abstract

Low energy (16 keV) muons were used to probe the dynamic magnetic behaviour of iron nanoclusters embedded in a silver thin film matrix. The silver film was 500 nm thick and contained a volume fraction of 0.1% iron. Measurements were made in a field of 25 mT, applied normal to the plane of the film, in the temperature range 4.7 K to 300 K. At temperatures above 20 K thermal activation of the cluster moments was seen as a narrowing of the field distribution sensed by the implanted muons. An intrinsic cluster relaxation time of τ0 = 12 ± 4 ns and an activation energy of 51 ± 9 K were deduced from fits to the data. SQUID magnetometry of thicker (1.5 µm) but otherwise identical films on graphite substrates showed the clusters to have a volume of the order of 10-26 m3, from which a cubic anisotropy constant of K = 2.3 ± 0.4 × 105 J m-3 was calculated. Remanence measurements showed no evidence of a preferred orientation for the magnetization of the cluster assembly.