Chemistry Reference
In-Depth Information
the temperature dependence of the exchange field at the normal site (other than
surface site) are assumed to have no surface effect. It is known to be a good
approximation that the temperature dependence of hyperfine field is proportional
to that of local magnetization at each site. As shown in Fig.
5.9
, the temperature
dependence of surface hyperfine field is well reproduced, if the exchange field at a
surface site is assumed to be 70 % of the value at the normal site. The reduction of
the exchange field at a surface site, 30 %, appears to be a reasonable size. Similar
results were also obtained in the Mössbauer studies for the surfaces of a-Fe
2
O
3
and
b-FeOOH [
15
]. These results are examples indicating that Mössbauer studies on
surface-selectively enriched samples furnish us unique information on the surface
(or interface) magnetic behaviors of non-metallic materials.
119
Sn Hyperfine Interaction in Magnetic Materials
5.5
As mentioned in
Sect. 5.1
,
119
Sn is the second most convenient isotope to measure
Mössbauer spectra. The nuclear spin quantum numbers are the same as those of
57
Fe, i.e., 1/2 for the ground state and 3/2 for the 1st exited state. Eventually,
Mössbauer spectra show six-line patterns when an effective magnetic field is
induced at the
119
Sn nuclear sites (Fig.
5.10
). Sn is essentially a non-magnetic
element and the nuclei do not feel a hyperfine field in bulk metallic Sn or in other
nonmagnetic compounds. However, in magnetic compounds, the conduction
electrons of Sn atoms are spin-polarized due to the hybridization with spin-
polarized electrons of the neighboring magnetic elements. This imbalance between
the numbers of up-spin conduction electrons and down-spin conduction electrons
creates a hyperfine field at the Sn nuclear sites through the Fermi contact inter-
action. In this way,
119
Sn nuclei can also be used as probes to detect electron-spin
(a)
(b)
119m
Sn
m
I
μ
0
H
hf
0 T
≈
−
3
/
2
−
1
/
2
3
+
1
/
2
I
=
+
3
/
2
2
γ
-rays 23.8 keV
5 T
≈
≈
≈≈≈ ≈≈
m
10 T
I
v
E
=
E
1
+
+
1
/
2
0
c
1
15 T
I
=
2
119
Sn
−
1
/
2
Source
Sample
Velocity (mm/s)
119
119
Fig. 5.10
Energy levels of
Sn nuclei and simulated
Sn Mössbauer spectra for hyperfine
fields of 0, 5, 10, and 15 T
