Chemistry Reference
In-Depth Information
The interpretation of in-field Mössbauer spectra which exhibit rather broadened
and overlapped lines, remains rather ambiguous because the presence of structural
defects in the core of the particle cannot be clearly distinguished from the surface
effects.
In the next section, we illustrate from selected examples how
57
Fe Mössbauer
spectrometry contributes to characterize structural and magnetic of Fe containing
nanoparticles. It is clear that numerous studies have been reported in the literature
but currently two different kinds of magnetic nanoparticles, Fe oxides including
ferrites and metallic, are mainly being produced, characterized for their potential
use in catalysis, data storage, biomedicine, magnetic resonance imaging, magnetic
particle imaging and environmental remediation.
Many routes to synthesize magnetic nanoparticles are now well established:
they derive from 3 main ''bottom up'' strategies which are co-precipitation (a
facile and convenient method to prepare Fe oxides from aqueous Fe
2+
/Fe
3+
salt
solutions by the addition of a base), thermal decomposition (endothermic reaction
from organometallic compounds in high-boiling organic solvents containing sta-
bilizing surfactants), micro emulsion technique (based on a reaction of thermo-
dynamically stable and isotropic liquid mixtures of oil, water and surfactant with a
co-surfactant).
As the Mössbauer spectra of both disordered structures and nanostructures
exhibit broadened lines, the first question is concerned by the origin of such a
feature: chemical or topological disorder (or both) and dynamics through the
presence of relaxation phenomena. Because one cannot a priori establish the
origin, the application of an external magnetic field (typically at least 0.5 T up to
10 T) should allow to conclude to the occurrence of dynamic effects by the
increase of the magnetic contribution at the expense of the quadrupolar doublet
decrease and a better resolution of the magnetic hyperfine structure while the
change of the intermediate line intensities is assigned to a magnetic alignment of
Fe moments, suggesting rather a disordered structure. The temperature evolution
of the hyperfine structure of an assembly of nanoparticles show a progressive
collapse from a quadrupolar to a magnetic structure coexisting in varying pro-
portions over a temperature range which is dependent on the particle size, the
dispersity in size and the volumetric concentration: the estimation of Mössbauer
blocking temperature T
Möss
which consists in describing the spectra by means of
quadrupolar splitting and hyperfine field distributions is not obvious because of the
higher and lower limits, respectively. Such a value is usually found much higher
than that established from magnetic measurements because the characteristic
measurement time is much smaller.
Great attention has been devoted to nanoparticles of ferrites: these compounds
are spinels with the chemical formula A
2
þ
B
3
2
O
4
;
where A
2
þ
and B
3
þ
correspond
to metallic 3d cations, respectively. The ferrite spinel structure has a face-centered
cubic (fcc) structure in the close packed cubic arrangement of oxide ions. The
structure contains two interstitial sites, occupied by oxygen coordinated metallic
cations, with tetrahedral (A)-site, and octahedral [B]-site, resulting in a different
