Chemistry Reference
In-Depth Information
Application to the study on the magnetic materials is frequently performed and
recognized as the most important subject in the field of material science. Recently
nano-meter size magnet has been interested in a magnetic device technology. It is
well known that the nano-meter size fine magnetic particles (ferromagnet and
antiferromagnet) show superparamagnetism. From the early stage of the devel-
opment in the Mössbauer spectroscopy, the 57 Fe Mössbauer effect has been used to
study on the behavior of superparamagnetic particles that contain 57 Fe Mossbauer
active atoms [ 25 ]. The Zeeman splitting of the nuclear excited state is resolved if
the spacing between the levels is larger than the width of the levels, that is,
x L ¼ g j l N H hf = h [ C N or s L \ s N . This condition is generally satisfied for 57 Fe
in magnetically ordered materials because of the large value of the hyperfine field.
Hyperfine field H hf = 50 T gives s L = 4 9 10 -9 s, whereas s N = 1.4 9 10 -7 s. It
implies there is always sufficient time for several complete Larmor precessions to
take place before the nuclear decays and s N is not time scale to determine the
relaxation behavior. s L is considered as the measuring time for the observation of a
hyperfine interaction. If the relaxation time s is such that s s L , the orientation of
H hf hardly changes during one Larmor precession time and one can observe the six
lines 57 Fe Mössbauer spectrum. If s s L , the orientation of H hf changes many
times before the completion of one Larmor precession and one can observe the
singlet or doublet Mössbauer spectrum like paramagnetic substance. However,
when s * s L, the complicated spectra as a function of s have been observed as
shown in Fig. 1.7 .
τ = 5.0 * 10 —9 s
57 Fe Mossbauer spectra as a function of relaxation time s from 10 -12
Fig. 1.7
Calculated
sto
10 -6
s. Hyperfine parameters like H hf and DE q are used for the values of a-FeOOH
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