8 Mössbauer Spectroscopy in External Magnetic Fields
435
Fig. 8.56 Temperature
dependence of cluster field
B cl in Y(Fe 0.75 Al 0.25 ) 2 at
13.5 T for different Fe
surroundings. Reprinted
from [19]
Fig. 8.57 Field dependence
of magnetization σ (open
symbols) and B n (full
symbols) for
Y(Fe 0.75 Al 0.25 ) 2 at 50 K
(squares), 160 K (circles) and
240 K (triangles). Adapted
from [155]
the correlated state but does not support the formation of the clusters. Since the reciprocal values of τ on and τ of f are correlated with the probability of formation and/or
decay of a correlated region, this implies, following general scaling arguments (e.g.
[164, 165]), that larger regions are integrated into the dynamics. Looking on the
autocorrelation function, which is proportional to ¯
τ on , a power law dependence on
reduced temperature t = T /T f is found with an exponent of −2 for the high Fe
concentration and −1 for the low Fe concentration (Fig. 8.61). The question why the
magnetic correlated regions are not stable after they have formed, but are decaying