Chemistry Reference
In-Depth Information
Fig. 4.2 Schematic representation of surface effect in the case of a nanoparticle (left) and of
grain boundaries in nanostructured powder (right)
size of nanoparticles when they are single domain. Example of analysis is given by
MAUD software based on the Rietveld method combined with Fourier analysis
[
9
]. Conventional TEM is used to establish the crystalline domain size distribution
(nanoparticle size distribution) while High Resolution TEM allows to describe
carefully only some nano-objects at the atomic scale. Therefore it is important to
note that the sampling remains an important key because the statistics of imaging
usually relates to only a few tens or hundreds of nanoparticles, a very small
number compared to the total sample which does contain an extremely higher
number of nanoparticles. To some extent, local probe techniques do provide
a priori relevant and complementary information concerning the atomic structure
providing that the confinement effects favour a strong enhancement of surface,
interface or grain boundaries. Contrarily to microcrystalline magnetic materials,
the magnetic properties of nanostructures require the use of both dc and ac
magnetic measurements and the comparison of field-cooled and zero field-cooled
(FC-ZFC) magnetization curves, i.e. the thermomagnetic cooling.
Such a view is relevant because the main fundamental questions concern in the
case of nanoparticles, the surface and magnetic surface states. Indeed one expects
a structural relaxation originating thus some distortions compared to the crystalline
lattice: consequently the superficial magnetic structure does result from combined
symmetry breaking, surface anisotropy and frustration topological effects arising
from the exchange integral, in addition to the reduction or enhancement of the
magnetic moment, giving rise a priori to either a dead magnetic layer or a 2 atomic
magnetic canted layer shell, as illustrated in Fig.
4.2
. In the case of nanostructured
powders, the main questions are relative to the chemical composition, the struc-
ture, the thickness and the porosity of the grain boundaries, and their influence on
the magnetic coupling on neighbouring grains and the bulk magnetic properties.
In this frame,
57
Fe Mössbauer spectrometry which is a local atomic probe tool
highly sensitive to the atomic neighbouring, appears as an excellent technique to
investigate nanomaterials [
10
-
13
]: one expects to distinguish atoms belonging to
the crystalline zones from those located either at the surfaces of nanoparticles
