Chemistry Reference
In-Depth Information
main surface species would be a protonated binuclear species and the top layer
would be in the (111) plane [
41
].
At this stage, more complex nanoparticles can be designed, involving several shells
with different chemical, structural and magnetic compositions with antiferromagnetic,
ferromagnetic, and ferrimagnetic phases, in order to induce some bias exchange
phenomena as example. Such a layered spherical structure leads to the occurrence of
well defined interfaces and surfaces: this assumes a non atomic diffusive phenomena
during the synthesis procedure and a chemical stability versus time. An example of
core-shell-shell structure is found in [
90
] from exotic ''onion like'' core-shell mor-
phology of c-Fe nucleus surrounded by a concentric double shell of a-Fe and ma-
ghemite-like oxide iron-carbon nanocomposites, synthesized using a commercial
activated amorphous and porous carbon.
To go thoroughly into the complete modelling of the magnetic structure, it is
necessary to study the effect of size and dispersion of nanoparticles in order to
better understand the intrinsic and extrinsic magnetic properties of nanoparticles,
providing they are chemically and structurally homogeneous and rather mono-
disperse. The Zeeman spectrum gives rise to the magnetic effective field at the
nucleus probe while the variations of the hyperfine field result from the magnetic
and electronic properties. But the thermal fluctuations of the magnetization might
originate a reduction of the hyperfine field: consequently, to prevent from the
presence of dynamic superparamagnetic relaxation effects, Mössbauer spectra
have to be recorded at the low temperature and/or under an intense external field.
In such a case, the effective magnetic field characteristic of a Mössbauer atom can
be described as
H
eff
¼
H
hf
þ
H
app
þ
H
D
þ
H
dip
þ
H
L
where H
hf
,H
app
,H
D
,H
dip
and H
L
correspond to the hyperfine field, the applied
field,
the
demagnetizing
field,
the
dipolar
field
originating
essentially
from
neighbouring particles and Lorentz field, respectively.
Contrary to bulk microcrystalline materials where the demagnetizing field is
negligible because of the multidomain structure, the effect of the demagnetizing
field in the case of mono domain nanoparticles can be significant, particularly
when the hyperfine field is not so large, but dependent on their morphology. Some
experiments have been successfully performed to give evidence that the magnetic
field at the nuclear probes located in spherical single domain nanoparticles is
larger than that characteristic of multi-domain particles (about 0.7 T).
An other important point is related to the surface hyperfine field which may be
a priori smaller or larger than the bulk hyperfine field at low temperature in the case of
ultrathin films, according to the substrate and the material coating the surface.
Mössbauer studies of a-Fe nanoparticles have revealed that the hyperfine field values
of inner Fe nuclei are similar to those of bulk crystalline a-Fe at low temperature (i.e.
static blocked magnetic regime), but those characteristic of the superficial atomic
layer are found either lower or higher [
91
-
96
]. Such conclusions are well supported
by some examples of the literature with 2 nm a-Fe particles in organic liquids [
93
],
