416
I. de Moraes and N. M. Dempsey
theoretical value of non-exchange coupled isotropic L1 0 FePt. However, it is far
from the theoretical upper limit predicted for textured single-phase FePt (1/4μ 0 M
2
s
= 390 kJ/m
3
= 49 MGOe). Nevertheless, this case study demonstrated the possibility
to use chemical synthesis to fabricate exchange coupled hard-soft nanocomposites.
Further improvements in the energy product can be expected with advances made
in powder compaction and the inducement of crystallographic texture in the hard
magnetic phase.
17.2.2 Case Study #2—FePd/α-Fe Nanocomposites
The second case study we selected to highlight concerns FePd/α-Fe nanocomposites
[49]. The chemical approach used to fabricate these nanocomposites is somewhat
different to the previous case and the authors used first-order reversal curve (FORC)
analysis to study the magnetic switching behaviour. While FePd has a slightly lower
magnetisation and significantly lower magnetocrystalline anisotropy than FePt, it has
the advantage that according to its phase diagram it can co-exist with α-Fe, while the
annealing of α-Fe in proximity to FePt tends to transform high magnetisation α-Fe
into lower magnetisation Fe 3 Pt. The authors argued that the coexistence of FePd and
α-Fe in a thermodynamically stable state should favour the formation of coherent
interfaces. This is expected to improve exchange coupling compared to NdFeB or
FePt-based nanocomposites with amorphous or diffuse interfaces [50].
The sample preparation process began with fabrication of monodisperse
trioctylphosphine (TOP) coated Pd nanoparticles by thermal decomposition of a
Pd-surfactant complex [51]. Heterostructured Pd/γ-Fe 2 O 3 nanoparticles, in which
one, two or three γ-Fe 2 O 3 nanoparticles are attached to a Pd nanoparticle, were then
formed by dissolving the Pd nanoparticles in a solution of 1-octanol, Fe(acac) 3 , oleylamine and oleic acid and heating the solution (180 °C/1 h) while bubbling N 2 through
it [52]. TEM images of Pd nanoparticles and heterostructured Pd/γ-Fe 2 O 3 nanoparticles are shown in Fig. 17.11. The Pd/γ-Fe 2 O 3 nanoparticles were then mixed with
Pd nanoparticles in n-hexane, annealed under O 3 (200 °C/2 h) to remove surface
organic ligands and then annealed (10 h) at various temperatures under Ar + H 2 .
The volume fraction of the soft and hard phases was varied by changing the relative
amount of Pd and Pd/γ-Fe 2 O 3 nanoparticles in the mixture (Fig. 17.12). The second
annealing step caused reduction of the γ-Fe 2 O 3 and interfacial atom diffusion to
form FePd/α-Fe and led to chemical ordering in the FePd phase, though too high a
temperature (823 K) led to disordering of the phase, as evidenced by x-ray diffraction
(Fig. 17.13a). Increasing the annealing temperature also led to an increase in grain
size of both phases to above 30 nm, as estimated from Scherrer formula analysis of the
XRD patterns (Fig. 17.13b), and confirmed by TEM analysis (data not shown). High
resolution TEM analysis of some select samples annealed at 723 K (data not show)
revealed an interfacial orientation relationship: α-Fe(110)//L1 0 -FePd (111) and α-Fe
(111)//L1 0 -FePd(110), which indicates that the close packed planes of both phases
are in coherent contact, though slightly distorted at the interface (lattice mismatch
I. de Moraes and N. M. Dempsey
theoretical value of non-exchange coupled isotropic L1 0 FePt. However, it is far
from the theoretical upper limit predicted for textured single-phase FePt (1/4μ 0 M
2
s
= 390 kJ/m
3
= 49 MGOe). Nevertheless, this case study demonstrated the possibility
to use chemical synthesis to fabricate exchange coupled hard-soft nanocomposites.
Further improvements in the energy product can be expected with advances made
in powder compaction and the inducement of crystallographic texture in the hard
magnetic phase.
17.2.2 Case Study #2—FePd/α-Fe Nanocomposites
The second case study we selected to highlight concerns FePd/α-Fe nanocomposites
[49]. The chemical approach used to fabricate these nanocomposites is somewhat
different to the previous case and the authors used first-order reversal curve (FORC)
analysis to study the magnetic switching behaviour. While FePd has a slightly lower
magnetisation and significantly lower magnetocrystalline anisotropy than FePt, it has
the advantage that according to its phase diagram it can co-exist with α-Fe, while the
annealing of α-Fe in proximity to FePt tends to transform high magnetisation α-Fe
into lower magnetisation Fe 3 Pt. The authors argued that the coexistence of FePd and
α-Fe in a thermodynamically stable state should favour the formation of coherent
interfaces. This is expected to improve exchange coupling compared to NdFeB or
FePt-based nanocomposites with amorphous or diffuse interfaces [50].
The sample preparation process began with fabrication of monodisperse
trioctylphosphine (TOP) coated Pd nanoparticles by thermal decomposition of a
Pd-surfactant complex [51]. Heterostructured Pd/γ-Fe 2 O 3 nanoparticles, in which
one, two or three γ-Fe 2 O 3 nanoparticles are attached to a Pd nanoparticle, were then
formed by dissolving the Pd nanoparticles in a solution of 1-octanol, Fe(acac) 3 , oleylamine and oleic acid and heating the solution (180 °C/1 h) while bubbling N 2 through
it [52]. TEM images of Pd nanoparticles and heterostructured Pd/γ-Fe 2 O 3 nanoparticles are shown in Fig. 17.11. The Pd/γ-Fe 2 O 3 nanoparticles were then mixed with
Pd nanoparticles in n-hexane, annealed under O 3 (200 °C/2 h) to remove surface
organic ligands and then annealed (10 h) at various temperatures under Ar + H 2 .
The volume fraction of the soft and hard phases was varied by changing the relative
amount of Pd and Pd/γ-Fe 2 O 3 nanoparticles in the mixture (Fig. 17.12). The second
annealing step caused reduction of the γ-Fe 2 O 3 and interfacial atom diffusion to
form FePd/α-Fe and led to chemical ordering in the FePd phase, though too high a
temperature (823 K) led to disordering of the phase, as evidenced by x-ray diffraction
(Fig. 17.13a). Increasing the annealing temperature also led to an increase in grain
size of both phases to above 30 nm, as estimated from Scherrer formula analysis of the
XRD patterns (Fig. 17.13b), and confirmed by TEM analysis (data not shown). High
resolution TEM analysis of some select samples annealed at 723 K (data not show)
revealed an interfacial orientation relationship: α-Fe(110)//L1 0 -FePd (111) and α-Fe
(111)//L1 0 -FePd(110), which indicates that the close packed planes of both phases
are in coherent contact, though slightly distorted at the interface (lattice mismatch