Soft-Chemical
Synthetic Route to Superparamagnetic
FeAs@C Core–Shell Nanoparticles Exhibiting High Blocking Temperature
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Abstract
Superparamagnetic FeAs nanoparticles
with a fairly high blocking
temperature (<i>T</i><sub>B</sub>) have been synthesized
through a hot injection precipitation technique. The synthesis involved
usage of triphenylarsine (TPA) as the As precursor, which reacts with
Fe(CO)<sub>5</sub> by ligand displacement at moderate temperatures
(300 °C). Addition of a surfactant, hexadecylamine (HDA), assists
in the formation of the nanoparticles, due to its coordinating ability
and low melting point which provides a molten flux like condition
making this synthesis a solventless method. Decomposition of the carbonaceous
precursors, HDA, TPA and Fe(CO)<sub>5</sub>, also produces the carbonaceous
shell coating the FeAs nanoparticles. Magnetic characterization of
these nanoparticles revealed the superparamagnetic nature of these
nanoparticles with a perfect anhysteretic nature of the isothermal
magnetization above <i>T</i><sub>B</sub>. The <i>T</i><sub>B</sub> observed in this system was indeed high (240 K) when
compared with other superparamagnetic systems conventionally utilized
for magnetic storage devices. It could be further increased by decreasing
the strength of the applied magnetic field. The narrow hysteresis
with low magnitude of coercivity at 5 K suggested soft ferromagnetic
ordering in these nanoparticle ensembles. Mössbauer and XPS
studies indicated that the Fe was present in +3 oxidation state and
there was no signature of Fe(0) that could have been responsible for
the increased magnetic moment and superparamagnetism. Typically for
superparamagnetic nanoparticle ensemble, the need for isolation of
the superparamagnetic domains (thereby inhibiting particle aggregation
and enhancing the <i>T</i><sub>B</sub>) has been in constant
limelight. Carbonaceous coating on these as-synthesized nanoparticles
formed <i>in situ</i> provided the physical nonmagnetic
barrier needed for such isolation. The high <i>T</i><sub>B</sub> and room temperature magnetic moment of these FeAs@C nanoparticles
also make them potentially useful for applications in ferrofluids
and magnetic refrigeration. In principle this method can be used as
a general route toward synthesis of other arsenide nanostructures
including the transition metal arsenide which show interesting magnetic
and electronic properties (e.g., CoAs, MnAs) with finer control over
morphology, composition and structure