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H. Khurshid et al.
or nanograins, as has been described in the literature [13, 16, 61]. These nanograins
tend to dominate the magnetic behavior of the hollow nanoparticles.
In order to better understand this, we can focus on the particular case of the
evolution from core/shell to hollow of Fe/γ-Fe 2 O 3 MNPs [16]. In this work, we
analyzed the “hollowing” process of two core/shell nanoparticles, with different
sizes, around 8 and 12 nm, respectively. For the smaller core/shell MNPs, the Fe
core occupies ~6% of the total volume of the MNP, while for the bigger ones, it
occupies ~23%. Therefore, the influence of the magnetic core is more relevant in
the case of the bigger MNPs. For the 12 nm core/shell MNPs, the obtained overall
magnetic behavior is similar to an ensemble of interacting MNPs with a collective
freezing into a super spin glass state at low temperatures [16]. As the morphology
changes into core/void/shell and then hollow, dipolar interactions between the MNPs
decrease, the magnetic moment per nanoparticle greatly decreases, and the effective
anisotropy increases due to the growing number of surface disordered spins. This
leads to a frustrated cluster glass-like behavior at low temperatures. However, for the
smaller sizes (8 nm), both the core/shell and the hollow MNPs exhibit a very similar
spin glass like magnetic behavior mediated by the grains in the shell, independently
of the morphology of the MNPs.
Even more interesting is the evolution of the EB effect in these MNPs. For the
bigger 12 nm MNPs, EB greatly increases as the particle becomes hollow, going
from 1500 Oe for the core/shell MNP to a maximum value of 7000 Oe for the
hollow ones. This great increase in the EB value can be related to the increase in
the shell thickness when the MNPs become hollow (see Fig. 6.8). The shell can be
understood as composed of two kind of spins, those occupying the interior of the shell
(magnetically reversible) and those placed on the (inner and outer) exterior surfaces
of the shell (magnetically irreversible) [13]. In the case of the 12 nm core/shell MNPs,
the EB arises mainly due to the interaction between the disordered spins in the shell
and the ordered spins in the core (1 interface, as depicted in Fig. 6.8a). However,
as the MNPs become hollow and the shell thickness increases, two new interfaces
are formed, between the ordered atoms in the interior of the shell and the disordered
atoms on the exterior surfaces (Fig. 6.8b). This can give rise to an enhanced EB effect
in comparison with the core/shell MNPs [52, 16]. This EB enhancement requires of a
minimum shell thickness in order to be appreciable: in the case of the 8 nm core/shell
and hollow MNPs, the shell is thinner and the volume occupied by the interior spins
is significantly reduced, leading to the disappearance of the increase in the EB effect
with changing morphology.
Therefore, analyzing the change of the morphology and magnetic behavior of
these MNPs as they evolve from core/shell to core/void/shell and finally to hollow can
provide a deeper insight into the surface and finite-size effects in magnetic nanoparticle systems, with special relevance to the EB effect. The combination of MNP
diameter and shell thickness plays a crucial role in determining the final magnetic
behavior of these MNPs, and by carefully tuning these two parameters, the magnetic
response can be manipulated according to the desired application.
H. Khurshid et al.
or nanograins, as has been described in the literature [13, 16, 61]. These nanograins
tend to dominate the magnetic behavior of the hollow nanoparticles.
In order to better understand this, we can focus on the particular case of the
evolution from core/shell to hollow of Fe/γ-Fe 2 O 3 MNPs [16]. In this work, we
analyzed the “hollowing” process of two core/shell nanoparticles, with different
sizes, around 8 and 12 nm, respectively. For the smaller core/shell MNPs, the Fe
core occupies ~6% of the total volume of the MNP, while for the bigger ones, it
occupies ~23%. Therefore, the influence of the magnetic core is more relevant in
the case of the bigger MNPs. For the 12 nm core/shell MNPs, the obtained overall
magnetic behavior is similar to an ensemble of interacting MNPs with a collective
freezing into a super spin glass state at low temperatures [16]. As the morphology
changes into core/void/shell and then hollow, dipolar interactions between the MNPs
decrease, the magnetic moment per nanoparticle greatly decreases, and the effective
anisotropy increases due to the growing number of surface disordered spins. This
leads to a frustrated cluster glass-like behavior at low temperatures. However, for the
smaller sizes (8 nm), both the core/shell and the hollow MNPs exhibit a very similar
spin glass like magnetic behavior mediated by the grains in the shell, independently
of the morphology of the MNPs.
Even more interesting is the evolution of the EB effect in these MNPs. For the
bigger 12 nm MNPs, EB greatly increases as the particle becomes hollow, going
from 1500 Oe for the core/shell MNP to a maximum value of 7000 Oe for the
hollow ones. This great increase in the EB value can be related to the increase in
the shell thickness when the MNPs become hollow (see Fig. 6.8). The shell can be
understood as composed of two kind of spins, those occupying the interior of the shell
(magnetically reversible) and those placed on the (inner and outer) exterior surfaces
of the shell (magnetically irreversible) [13]. In the case of the 12 nm core/shell MNPs,
the EB arises mainly due to the interaction between the disordered spins in the shell
and the ordered spins in the core (1 interface, as depicted in Fig. 6.8a). However,
as the MNPs become hollow and the shell thickness increases, two new interfaces
are formed, between the ordered atoms in the interior of the shell and the disordered
atoms on the exterior surfaces (Fig. 6.8b). This can give rise to an enhanced EB effect
in comparison with the core/shell MNPs [52, 16]. This EB enhancement requires of a
minimum shell thickness in order to be appreciable: in the case of the 8 nm core/shell
and hollow MNPs, the shell is thinner and the volume occupied by the interior spins
is significantly reduced, leading to the disappearance of the increase in the EB effect
with changing morphology.
Therefore, analyzing the change of the morphology and magnetic behavior of
these MNPs as they evolve from core/shell to core/void/shell and finally to hollow can
provide a deeper insight into the surface and finite-size effects in magnetic nanoparticle systems, with special relevance to the EB effect. The combination of MNP
diameter and shell thickness plays a crucial role in determining the final magnetic
behavior of these MNPs, and by carefully tuning these two parameters, the magnetic
response can be manipulated according to the desired application.