Chemistry Reference
In-Depth Information
Two regimes can be a priori well distinguished: either blocked magnetic states
(multi domain particles) or superparamagnetic relaxation phenomena when the
thermal energy is prevailing, i.e. fluctuations of the magnetization corresponding
to two minima of energy (fine single domain nanoparticles). Those dynamic
magnetic states can be well described by the Néel-Brown model [
30
] and by the
Dormann-Bessais-Fiorani model [
32
] in the case of an assembly of non-inter-
acting and weakly interacting particle. On the contrary, the individual fluctuations
are cancelled in the case of strongly interacting nanoparticles, favouring thus a
collective magnetic state, as observed in spin glasses [
33
]. The modelling of the
intermediate regime is not yet clearly established.
The main characteristics of a single domain isolated particle are thus its
blocking temperature T
B
, which corresponds to the progressive transition (not a
phase transition!) from the superparamagnetic state to the magnetic state. T
B
decreases when the size of the particle decreases and when the distance between
particles increases. In addition, T
B
increases with decreasing time measurement,
indeed its estimation is strongly dependent on the time scale characteristic of the
measuring technique. One of the first contribution of zero-field Mössbauer spec-
trometry to study assembly of non-interacting or interacting single domain oxide
nanoparticles is the evaluation of the mean blocking temperature (T
Möss
), defined
as the magnetically split and un split components representing 50 % each of the
spectral area, allowing thus an estimation of the mean anisotropy constant. But it is
important to emphasize that the estimate of T
Möss
is not so obvious because it
remains quite difficult to distinguish and to estimate accurately the absorption area
attributed to paramagnetic and magnetic contributions to the hyperfine structure.
The application of rather intense external field parallel to the easy axis reduces
the fluctuations of the magnetization, giving rise to a blocked like magnetic state.
The hyperfine structure of in-field Mössbauer spectra have to be compared then to
the zero-field Mössbauer spectrum particularly, the comparison between the
effective field(s) and the corresponding hyperfine field(s) and the intensity of the
intermediate lines: they should allow to better understand the dynamics and the
magnetic structure of the nanoparticles, describe the magnetic particle by means of
a core shell model. In the case of ferrimagnetic nanoparticles, the magnetization of
the core is oriented parallel to the external field while the random distribution of
moments occur at the outer shell [
34
,
35
]. In practice, the spectrum is decomposed
as the sum of two components corresponding to saturated and random configu-
ration: the thickness of the magnetic shell can be thus estimated, and it is usually
found at about 2 atomic layers [
34
-
46
]. One does note that such an effect was
confirmed from
57
Fe NMR experiments [
42
]. This model is well supported by the
surface spin disorder originating from the large surface anisotropy which over-
comes the exchange energy contribution [
37
-
39
]. Most of studies tend to conclude
that the outer layer is preferentially composed of octahedral units, in agreement
with chemical considerations. Let us emphasize that it is also supported by
computer simulation that predicts different structures according to the surface/
magnetocrystalline anisotropy ratio [
47
-
52
].
