Chemistry Reference
In-Depth Information
from the collapsing of the magnetic hyperfine structure in the spectrum. The value
of hyperfine field corresponding to the full magnetic moment is estimated by
extrapolating to zero temperature. In ionic Fe
3+
compounds, the hyperfine fields
range from 50 to 60 T. In the case of Fe
2+
compounds, hyperfine fields are much
smaller, whereas a contribution of orbital angular moment is significant.
Mössbauer spectroscopy is very useful for the study of antiferromagnetic materials
since similar hyperfine fields are observed also in antiferromagnetic cases, which
means the major contribution of hyperfine field is the local spin polarization in the
Fe atom. If a sample including two or more phases with different magnetic
properties, the observed spectrum becomes a superposition of respective hyperfine
structures and the relative amounts of Fe atoms in each phase are estimated from
the observed absorption intensities. In the case of
57
Fe nucleus, since the gamma
ray energy is rather small and the Debye temperature is sufficiently high, the
relative amount of observed absorption is almost proportional to the amount of Fe
atom. Relative amount of each phase is easily estimated regardless of the spectrum
structure, single line, doublet, or magnetically split pattern.
The observed magnetic hyperfine field is an average value in the term of the
nuclear Larmor precession time (the order of 10
-8
s). Therefore in most of
magnetically ordered materials, the observed hyperfine field is rigorously pro-
portional to the temperature dependence of local magnetic moment. If a sample
includes two valence states, two hyperfine fields are observed as far as each
valence state is respectively stable in a much longer time than nuclear Larmor
precession time. If two valences are mixed in a faster rate than this characteristic
time, an averaged hyperfine field is observed. A typical example is Fe
3
O
4
(mag-
netite). This compound includes Fe
2+
and Fe
3+
ions with the ratio of 1:2. The
crystal structure (spinel) has 2 crystallographic sites A and B. A site is occupied by
Fe
3+
ions while B site by Fe
2+
and Fe
3+
with the ratio of 1:1. This material is
known to exhibit a transition (Verwey order) from insulator at low temperature to
metal at high temperature and the transition temperature is about 120 K, which
depends on the stoichiometry. At the high temperature phase, due to electron
hopping, the valence state of Fe in B site becomes the average of Fe
2+
and Fe
3+
.
Since this compound has a high Curie temperature, the Mössbauer spectrum at
room temperature shows two hyperfine splitting: One corresponds to Fe
3+
at A-site
and the other the averaged Fe
2.5+
at B-site. Since the electron hopping rate is much
faster than the nuclear Larmor precession time, the observed spectrum exhibits the
averaged hyperfine field of Fe
2+
and Fe
3+
.
In nanoscale magnetic systems, the relaxation phenomena may influence the
observed Mössbauer spectra. If the particle (or grain) size of a magnetically
ordered material becomes very small, on the order of 1 nm, or the film thickness
becomes a monolayer region, ordered spin structures are spatially no more stable
at room temperature but start to fluctuate by thermal agitation. Such a phenomenon
is called superparamagnetism. The Mössbauer spectra with magnetic splitting are
only observed if the superparamagnetic relaxation time is longer than the nuclear
Larmor precession time. When the particle size is smaller or the temperature is
higher, the superparamagnetic relaxation time becomes shorter and accordingly
