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