ARTICLE IN PRESS
Y. Ramaye et al. / Journal of Magnetism and Magnetic Materials 289 (2005) 28–31
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2.2. Synthesis of iron particles dispersed in cyclohexane
depending on the oxidation of the particles. Even if
most of the manipulations are performed in the glove
box, oxidation is not completely avoided because of the
very small size of particles.
Kerosene (100 mL) was introduced in a 0.5 L four-
neckround-bottom flaskunder nitrogen. It was stirred
and degassed by nitrogen during half an hour. The
stabilizing agent (OA and BEA) was then introduced
and iron pentacarbonyl was added in one time using a
Hamilton syringe. The mixture was then heated to 90 1C
under nitrogen stream and kept at this temperature
during one hour. It was then refluxed between 180 and
200 1C. During the decomposition of iron pentacarbonyl,
orange fumes were observed. The end of the reaction was
characterized by the end of these fumes and the solution
turned blackupon the formation of iron nanoparticles.
This solution was cooled down to room temperature. A
mixture of acetone and methanol (50/50 v/v) was added,
allowing to get a blackprecipitate. This latter was
isolated by magnetic separation and dispersed in
cyclohexane allowing to obtain a ferrofluid. All the
washing steps have to be carried out in a glove box,
under nitrogen atmosphere in order to avoid oxidation.
Even when oxidation is allowed the dispersion stays
stable with regards to particles agglomeration.
3.2. Electron microscopy
Pictures of Fig. 1 show mixtures of spherical particles
and triangle-like particles. Spherical particles are almost
monodisperse (Fig. 1a). The diameters distribution is
well described by a log-normal law of parameters d0 ¼
5:2 nm and s ¼ 0:1: In some cases, almost only triangles
are observed. The triangles lookequilateral, of the same
size as spherical particles, which means between 5 and
10 nm by side (Fig. 1b).
3.3. SAXS
SAXS provides an averaged description of the system
[13]. Fig. 2 is the form factor of the dispersion
corresponding to the TEM Fig. 1b. At low q, P(q)
reaches the Guinier plateau indicating that no aggrega-
tion occurs. The double-logarithmic plot shows a
straight line in the intermediate q domain, with a
constant slope of ꢀ3. For comparison, solid lines are
2.3. Methods for characterization
Particle shape and size distribution were deduced
from transmission electron microscopy (TEM): a drop
of the dispersion, diluted by the solvent was deposited
on the carbon grid and the solvent was evaporated.
Observation was performed with a microscope JEOL
100 CX2. Electron diffraction allowed the determination
of the nature of the particles. The experimental particles
size distribution was fitted by
a log-normal law
characterized by the parameters d0 and s:
Atomic force microscopy (AFM) was performed in
the taping mode (digital instruments). The sample was
deposited on a carbon support.
Magnetization of the samples (dispersions or powders)
was measured as a function of the magnetic field up to
1 T, using a home made vibrating magnetometer [12].
Small angle X-ray scattering (SAXS) on diluted
dispersions of particles gives the form factor P(q)
charateristic of particles shape. It has been performed
in the LURE laboratory using synchroton (beamline
D22, Orsay, France). The dispersion placed between
‘‘captons windows’’ has a low volume fraction ðfo1%Þ
in order that the scattered intensity can be assimilated to
the form factor P(q).
3. Results
Fig. 1. TEM pictures: (a) of a mixture of spherical and
triangular particles of diameter around 5 nm (hexagonal
packing on the grid is observed attesting the monodispersity
of the samples). (b) Of trigonal particles, an enlargement of a
triangle illustrates that this latter looks equilateral.
3.1. Macroscopic aspect of the samples
At the end of the synthesis stable dispersions are
obtained. Their color ranges from blackto red,
Ramaye
