Chemistry Reference
In-Depth Information
because of symmetry breaking or at the interfaces of multilayers or at the grain
boundaries of nanostructured systems because of the topological atomic disorder.
In addition, in the case of Fe containing magnetic nanomaterials,
57
Fe transmission
Mössbauer spectrometry including in-field measurements does contribute to better
understand both the intrinsic magnetic structure and the dynamics of magnetic
structures, i.e. the superparamagnetic relaxation phenomena which occur in non-
interacting single domain nanoparticles. The exploitation of the low temperature
magnetic hyperfine structure and the estimation of the blocking temperature pro-
vide relevant information to understand the dynamics of magnetic nanostructures.
But it is also important to emphasize that the analysis of the Mössbauer hyperfine
structure originates large debate because of the difficulties in the modelling leading
to various ambiguities.
In the next sections, attention will be paid to the effect of confinement on
magnetic properties with different environments but some general features of
57
Fe
Mössbauer spectrometry will be reported hereafter. Then, some general features
are given to introduce nanomagnetism and magnetism of nanoparticles. The last
sections are concerned by the review of different situations illustrating the role of
57
Fe Mössbauer spectrometry in investigating the structural and physical proper-
ties in several types of magnetic nanostructures: nanoparticles, nanocrystalline
alloys, nanostructured powders, and mesoporous systems. Most of the examples
have been the subject of our own research works developed in collaboration with
different groups of chemists. The strategy consists in the elaboration or the syn-
thesis of well controlled nanostructures which have to be well reproduced: thus it
requires to well understand the chemical mechanism and the role of all parameters
(temperature, pressure, pH, …). It is finally important to mention that the samples
reported in the next sections have been widely characterized by means of dif-
fraction techniques, electron microscopies and magnetic measurements.
57
Fe Mössbauer Spectrometry
4.2 General Features on
Mössbauer spectrometry is a powerful technique to study solid state materials
including magnetic nanostructures and frozen colloids, particularly their magnetic
dynamics. It is based on the recoil-free emission of a c-photon by a nucleus located
in a radioactive emitter (source) and the subsequent recoil-free absorption by a
similar nucleus located in the absorbing system (sample). The extremely narrow
width of the resonance which results from the finite lifetime of the excited state
(Heisenberg uncertainty principle) allows the hyperfine interactions between the
nuclear and electronic charges to be observed and to be determined. The recoilless
nuclear resonance or Mössbauer effect combined to the energy scanning obtained
from the periodic movement of the source originate the registration of Mössbauer
spectra. Their description gives rise to the hyperfine characteristics of the different
Fe species located in the studied sample, which are namely the isomer shift, the
quadrupolar splitting, the quadrupolar shift and the hyperfine field (a description
