Tj. Jackson et al., Superparamagnetic relaxation in iron nanoclusters measured by low energy muon spin rotation, J PHYS-COND, 12(7), 2000, pp. 1399-1411
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 fil
m was 500 nm thick and contained a volume fraction of 0.1% iron. Measuremen
ts 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 dis
tribution sensed by the implanted muons. An intrinsic cluster relaxation ti
me of tau(0) = 12 +/- 4 ns and an activation energy of 51 +/- 9 K were dedu
ced from fits to the data. SQUID magnetometry of thicker (1.5 mu m) but oth
erwise identical films on graphite substrates showed the clusters to have a
volume of the order of 10(-26) m(3), from which a cubic anisotropy constan
t of K = 2.3 +/- 0.4 x 10(5) J m(-3) was calculated. Remanence measurements
showed no evidence of a preferred orientation for the magnetization of the
cluster assembly.