17 Nanocomposites for Permanent Magnets
411
the Pt salt and the iron pentacarbonyl. Monodisperse nanoparticles of diameter up to
10 nm were isolated and purified by centrifugation and then redispersed at variable
concentrations in nonpolar solvents. If a drop of such a dispersion is spread on a
substrate and the carrier solvent is allowed to slowly evaporate, 3D superlattices of
FePt nanoparticles are produced. The as-produced FePt nanoparticles are chemically
disordered and thus magnetically soft. Annealing was used to transform the crystal
structure from face centred cubic to face centred tetragonal, and the evolution in the
XRD patterns of 1 μm-thick assemblies of 4 nm Fe 52 Pt 48 nanoparticles, as a function of annealing temperature (for a fixed annealing time of 30 min.), is compared
in Fig. 17.5. The observed shift in the position of the (111) peaks together with the
evolution of the (001) and (110) peaks attests to the chemical ordering induced by
annealing, though only partial ordering is achieved below 500 °C. Increasing the
annealing temperature or the annealing time (data not shown) leads to increased
chemical ordering. Representative in-plane coercivity values of a series of 140-nmthick assemblies of 4 nm Fe 52 Pt 48 nanoparticles, annealed in the same temperature
range and for the same time, are compared in Fig. 17.6. The gradual increase in
coercivity with annealing temperature is consistent with the concomitant increase
in chemical ordering evidenced by x-ray diffraction, which leads to an increase in
the magnetocrystalline anisotropy of the FePt phase. In-plane coercivity values of
another series of 140-nm-thick assemblies of 4 nm Fe x Pt 1–x nanoparticles of variable
composition are also shown in Fig. 17.6. The highest values of coercivity were found
Fig. 17.5 XRD patterns of
as-synthesised 4-nm
Fe 52 Pt 48 particle assemblies
(a) and a series of similar
assemblies annealed under
N 2 for 30 min at
temperatures of 450 °C (b),
500 °C (c), 550 °C (d), and
600 °C (e). The indexing is
based on tabulated fct FePt
reflections [41]
411
the Pt salt and the iron pentacarbonyl. Monodisperse nanoparticles of diameter up to
10 nm were isolated and purified by centrifugation and then redispersed at variable
concentrations in nonpolar solvents. If a drop of such a dispersion is spread on a
substrate and the carrier solvent is allowed to slowly evaporate, 3D superlattices of
FePt nanoparticles are produced. The as-produced FePt nanoparticles are chemically
disordered and thus magnetically soft. Annealing was used to transform the crystal
structure from face centred cubic to face centred tetragonal, and the evolution in the
XRD patterns of 1 μm-thick assemblies of 4 nm Fe 52 Pt 48 nanoparticles, as a function of annealing temperature (for a fixed annealing time of 30 min.), is compared
in Fig. 17.5. The observed shift in the position of the (111) peaks together with the
evolution of the (001) and (110) peaks attests to the chemical ordering induced by
annealing, though only partial ordering is achieved below 500 °C. Increasing the
annealing temperature or the annealing time (data not shown) leads to increased
chemical ordering. Representative in-plane coercivity values of a series of 140-nmthick assemblies of 4 nm Fe 52 Pt 48 nanoparticles, annealed in the same temperature
range and for the same time, are compared in Fig. 17.6. The gradual increase in
coercivity with annealing temperature is consistent with the concomitant increase
in chemical ordering evidenced by x-ray diffraction, which leads to an increase in
the magnetocrystalline anisotropy of the FePt phase. In-plane coercivity values of
another series of 140-nm-thick assemblies of 4 nm Fe x Pt 1–x nanoparticles of variable
composition are also shown in Fig. 17.6. The highest values of coercivity were found
Fig. 17.5 XRD patterns of
as-synthesised 4-nm
Fe 52 Pt 48 particle assemblies
(a) and a series of similar
assemblies annealed under
N 2 for 30 min at
temperatures of 450 °C (b),
500 °C (c), 550 °C (d), and
600 °C (e). The indexing is
based on tabulated fct FePt
reflections [41]