Chemistry Reference
In-Depth Information
interactions, although the spectra in the two modes have the upside-down shapes
with each other. A definite merit of CEMS is that the necessary amount of
57
Fe for
the measurement is much less than absorption measurements. For example, a
sample including only one monolayer of
57
Fe can be measured.
57
Fe Hyperfine Interaction in Magnetic Materials
5.2
In a magnetic atom, unpaired electron spins produce a very large magnetic field at
the nuclear site, which corresponds to 33 T (Tesla) in the case of pure Fe metal at
room temperature, that is called hyperfine field. Unpaired electron spins exist in 3d
orbitals but 3d electrons have no density at the nuclear site. Such a large field can
be produced by the spin polarization of s-orbital electrons. Due to an exchange
interaction with 3d electrons, the radial distributions of spin-up and spin-down
electrons in core s-orbitals are slightly differentiated. Although the degree of
polarization in core electrons is rather small, the induced magnetic field is quite
large since the core s-electrons have finite densities at the nuclear site (Fermi
contact interaction). If a nucleus experiences a magnetic field, the interaction
between the field and nuclear magnetic dipole moment causes a Zeeman splitting
of nuclear energy levels. As already introduced in
Chap. 1
, six transitions are
allowed in the case
57
Fe, and therefore six peaks are observed in the Mössbauer
spectrum. The magnitude of splitting in the Mössbauer spectrum is proportional to
the hyperfine field. In the case of pure Fe metal, the hyperfine field at room
temperature is 33.0 T and the overall splitting of six lines in the Mössbauer
spectrum is 10.657 mm/s in the unit of Doppler velocity. The spin polarization
induced at the nuclear site due to the Fermi contact interaction has the opposite
direction to the spin polarization of 3d electrons. If a strong external magnetic field
is applied, 3d spins are oriented to the field direction and therefore direction of the
hyperfine field becomes opposite to that of external field and accordingly the
observed hyperfine field is reduced by applying a strong external field. This sit-
uation is expressed as the sign of the hyperfine field being negative. The sign of the
hyperfine field in ferromagnetic Fe systems is normally negative.
The relation between 3d local magnetic moment and hyperfine field is not
straightforwardly proportional but theoretical studies suggest that the hyperfine
field delicately depends on the electronic structure. Therefore in general a quan-
titative argument of hyperfine field is difficult. Nevertheless, magnetic hyperfine
field measurements give us valuable information for the understanding of magnetic
properties of materials. A rough proportionality between the hyperfine fields and
local magnetic moment has been found in many cases of Fe atoms in ferromag-
netic alloy systems. The presence of magnetic hyperfine structure in a Mössbauer
spectrum is undoubted evidence for the existence of a magnetic order. The tem-
perature dependence of the hyperfine field is proportional to that of the local
magnetization, independently of the type of magnetic order, ferromagnetic or
antiferromagnetic. The magnetic transition temperature is therefore determined
