As shown in Fig.42c, we have analyzed the LF
dependence of the relaxation rate using the T1 formula
for dilute spin systems (eq.33). The resulting Lorentzian
field width (a) and field fluctuation rate () are shown in
Table 5. It was suggested that
vacancy-doping results in faster field fluctuations () and a
larger field width (a) than the nominally pure system shows.
Qualitatively, this result may be understood, if muons detect the
dipolar fields from unpaired spins in the sample: as susceptibility
has indicated, the number of unpaired spins increases upon doping.
Therefore, the doped system should exhibit a larger field-width (a)
at muon location, if the muons detect the dipolar fields from the
doping induced moments. If the interactions between these unpaired spins are
also dipolar interactions, the field fluctuation rate () should
increase upon doping, as has been observed in the Mg doped systems. A
more quantitative discussion, which leads to the estimates shown in
Table 5, is given later.