Chemistry Reference
In-Depth Information
Numerous studies were devoted to nanostructured spinel ferrites and garnets
elaborated by either mechanochemistry from a mixture of subsequent precursors, or
by mechanical activation resulting from the synthesis using a conventional route
followed by mechanical treatment, and with further annealing treatments. As dis-
cussed in previous section, the characterization of these ''nanoferrites'' requires the
use of various experimental techniques, including
57
Fe Mössbauer spectrometry
combining zero-field and in-field conditions [
179
-
196
]. It is important to well control
the homogeneity of the powders in order to model both the structure and the micro-
structure, to check whether no contamination occurs during milling. In addition to
overcome some superparamagnetic relaxation phenomena, the in-field Mössbauer
spectra do bring clear and accurate information to estimate the proportions of
Fe located in the octahedral and the tetrahedral sites, the (mean) canting angle of Fe
magnetic moments, which are both crucial to understand the magnetic properties.
The estimate from zero-field Mössbauer spectra is too approximate and the solution is
completely dependent on the fitting assumptions, preventing thus a physical approach.
It is important to emphasize that the presence of grain boundaries has a relevant role on
the magnetic coupling between grains, favoring thus either some spin glass cluster like
behavior or superparamagnetic non interacting grains. Consequently, the size of grains
and the thickness of grain boundaries have to be considered to explain both local
hyperfine magnetic data and macroscopic magnetic features from coercive field and
saturation magnetization.
4.10 Conclusions
The selected examples did allow first to demonstrate the ability of
57
Fe
Mössbauer spectrometry to contribute in the understanding of chemical, structural
and magnetic properties of nanostructures. But, an excellent characterization of
the studied systems is previously required including a control of an homogeneous
morphology. Both those crucial keys are necessary to make thus easier the
modelling of the physical properties, particularly local scale structural and
magnetic ones, and to understand the role of surfaces and grain boundaries in
nanostructures. The fitting of the Mössbauer spectra remains an important step
and the results have to be compared to those obtained under different experi-
mental in situ conditions (temperature, external magnetic field). The know-how
established from ''ideal nanostructures'' opens new ways to revisit Mössbauer
studies some topics: the phase transformation in some polymorphs [
197
,
198
], the
passivation processes and corrosion products and [
199
], the study of archaeo-
logical ceramics [
200
] and rocks and soils in geology and minerals [
201
] where
non homogeneous nanostructuration does occur.
