Chemistry Reference
In-Depth Information
4.7 Fe in Situ Consolidated Clusters
A detailed study was early performed on Fe clusters generated by inert gas con-
densation and in situ consolidated [
158
]. A sample pellet of 30-80 lm thick was
obtained from compaction of the nanocrystalline powder in high vacuum (5.10
-6
Pa)
applying a unidirectional pressure of 5 GPa. As illustrated in Fig.
4.17
left [
158
], the
Mössbauer spectra clearly exhibit the presence of two magnetic sextets, one consists
of narrow lines while the second one is composed of broadened lines. It is clearly
observed a magnetic texture from the low intensity intermediate lines which results
from the sampling conditions. The evolution of their respective hyperfine field values
is strongly temperature dependent with a cross over at about 170 K (as is shown in
Fig.
4.17
right [
158
]).
The narrow line sextet is clearly attributed to the nanocrystalline Fe grain while
the second component is assigned to Fe atoms located at the interfaces. Indeed the
larger linewidth is consistent with a structural disorder, originating larger Fe-Fe
distances. The higher value hyperfine field at low temperature for the grain
boundaries can be interpreted in terms of reduced interface density, well supported
by the Bethe-Slater curve: indeed the increase of Fe-Fe distance favours that of the
Fe magnetic moment, and consequently that of the hyperfine field. In addition, the
enhancement of the isomer shift value of Fe atoms located in grain boundaries is
consistent with a reduction of the electron density and with a volume expansion.
From the temperature dependence of the relative absorption area of the two
components where that of the interface decreases faster, one can estimate that the
Debye temperature of grain boundaries is lower than that of nanocrystalline grains,
supporting that the grain boundaries possess a disordered atomic structure. Finally,
from the relative absorption at low temperature (4:1), the mean thickness of
interfaces is estimated at about 4 atoms, taking into account the size of nano-
crystalline grains.
Fig. 4.17 Mössbauer spectrum at 77 K showing the presence of 2 magnetic components (left);
temperature dependence of the hyperfine field of the 2 magnetic components (right)[
158
]
