98
E. L. Winkler and R. D. Zysler
average of the parameters of both phases, H SW = 2(K H V H + K S V S )/(M H V H +
M S V S ), where K, M and V are the magnetic anisotropy, saturation magnetization,
and volume, respectively, and the subindex S and H correspond to the soft and hard
component.
When the thickness of the soft phase is larger than the critical thickness, t s >
δ c , the magnetization of both phases remains parallel until the Bloch-type domain
wall nucleates in the soft phase at the magnetic field H N = π
2 AS/(2M S tS
2 ), where
A S is the exchange constant of the soft component. For magnetic fields larger than
H N the domain wall moves toward the interface where the spins are pinned to the
hard magnetic component (see Fig. 4.6). When H is further increased above H irr ,
the domain wall will be compressed to the interface till the energy necessary for
a displacement of the domain wall into the hard magnetic phase is reached. The
irreversible field is usually smaller than the switching field of the hard phase but has
the same order of magnitude. Below H irr a reversible behavior of the magnetization
is observed which resembled spring behavior, which originated the name of the
process. Finally, when t s δ c the magnetization inversion process corresponds to
two independent phases.
The predicted evolution from rigid-coupled to exchange spring and beyond the
exchange coupled behavior is nicely illustrated in the hard/soft Fe 0.65 Pt 0.35 -core/Coshell nanoparticles case [4]. The system was synthesized by one-pot microwave
chemical route which allows the control of the Co shell thickness from 0.6 to
2.7 nm over the hard fcc Fe 0.65 Pt 0.35 of 5 nm of diameter. From the low-temperature
hysteresis loop, the size dependence of the magnetic parameters was obtained as
shown in Fig. 4.6. As expected, the saturation magnetization of the system systematically increases with the shell thickness, following a volumetric power law M S ∝
r
3 . Conversely, the coercivity field follows non-monotonic size dependence. For a
thinner Co layer, the H C increases, a behavior ascribed to the improving of the hard
magnetic properties due to the strong exchange coupling between the interface Co
spins and the FePt core. When the thickness of the shell increases, the coercivity
decreases due to the weaker pinning action exerted by the FePt hard magnetic phase
over the outer Co layers, within this size range exchange spring process is observed.
For a thicker shell, the magnetic behavior is dominated by the soft FM Co, as a consequence H C diminishes and tends to an asymptotic value. An important consequence
of the described behavior is the enhancement of the energy product which increases
from 1.10(8) MGOe for FePt nanoparticles up to 3.82(5) MGOe when the core is
encapsulated by a 1 nm thickness Co shell.
Finally, as the advanced chemical methods enabled to obtain monodispersed
nanoparticle systems, large area of self-assembled nanoparticles can be fabricated. These systems look to optimize the core/shell nanoparticles as building
blocks for advanced permanent magnetic applications reaching promising results
at room temperature, as the FePt–Fe3O4 nanocomposites with an energy product
of 20.1MGOe [65], FePt/Co [37], or rare-earth free core/shell nanoparticles
CoFe 2 O 4 /CoFe 2 [56].
Much has been advanced in this time driven by new techniques of manufacture
and characterization, and a large number of different materials has been fabricated
E. L. Winkler and R. D. Zysler
average of the parameters of both phases, H SW = 2(K H V H + K S V S )/(M H V H +
M S V S ), where K, M and V are the magnetic anisotropy, saturation magnetization,
and volume, respectively, and the subindex S and H correspond to the soft and hard
component.
When the thickness of the soft phase is larger than the critical thickness, t s >
δ c , the magnetization of both phases remains parallel until the Bloch-type domain
wall nucleates in the soft phase at the magnetic field H N = π
2 AS/(2M S tS
2 ), where
A S is the exchange constant of the soft component. For magnetic fields larger than
H N the domain wall moves toward the interface where the spins are pinned to the
hard magnetic component (see Fig. 4.6). When H is further increased above H irr ,
the domain wall will be compressed to the interface till the energy necessary for
a displacement of the domain wall into the hard magnetic phase is reached. The
irreversible field is usually smaller than the switching field of the hard phase but has
the same order of magnitude. Below H irr a reversible behavior of the magnetization
is observed which resembled spring behavior, which originated the name of the
process. Finally, when t s δ c the magnetization inversion process corresponds to
two independent phases.
The predicted evolution from rigid-coupled to exchange spring and beyond the
exchange coupled behavior is nicely illustrated in the hard/soft Fe 0.65 Pt 0.35 -core/Coshell nanoparticles case [4]. The system was synthesized by one-pot microwave
chemical route which allows the control of the Co shell thickness from 0.6 to
2.7 nm over the hard fcc Fe 0.65 Pt 0.35 of 5 nm of diameter. From the low-temperature
hysteresis loop, the size dependence of the magnetic parameters was obtained as
shown in Fig. 4.6. As expected, the saturation magnetization of the system systematically increases with the shell thickness, following a volumetric power law M S ∝
r
3 . Conversely, the coercivity field follows non-monotonic size dependence. For a
thinner Co layer, the H C increases, a behavior ascribed to the improving of the hard
magnetic properties due to the strong exchange coupling between the interface Co
spins and the FePt core. When the thickness of the shell increases, the coercivity
decreases due to the weaker pinning action exerted by the FePt hard magnetic phase
over the outer Co layers, within this size range exchange spring process is observed.
For a thicker shell, the magnetic behavior is dominated by the soft FM Co, as a consequence H C diminishes and tends to an asymptotic value. An important consequence
of the described behavior is the enhancement of the energy product which increases
from 1.10(8) MGOe for FePt nanoparticles up to 3.82(5) MGOe when the core is
encapsulated by a 1 nm thickness Co shell.
Finally, as the advanced chemical methods enabled to obtain monodispersed
nanoparticle systems, large area of self-assembled nanoparticles can be fabricated. These systems look to optimize the core/shell nanoparticles as building
blocks for advanced permanent magnetic applications reaching promising results
at room temperature, as the FePt–Fe3O4 nanocomposites with an energy product
of 20.1MGOe [65], FePt/Co [37], or rare-earth free core/shell nanoparticles
CoFe 2 O 4 /CoFe 2 [56].
Much has been advanced in this time driven by new techniques of manufacture
and characterization, and a large number of different materials has been fabricated