116
C. de Julián Fernández and F. Pineider
activation processes are dominant [98] and the superparamagnetic effect occurs at
room temperature. This behavior has been reported in a wide number of works investigating Au–Fe 3 O 4 heterodimers and Ag–Fe 3 O 4 [45, 89, 93–97, 100]. The blocking
temperature, i.e. the threshold between the blocked and superparamagnetic regimes,
is determined by the size distribution of the particles, their magnetic anisotropy, as
well as the magnetic interparticle interactions. HDs and CS NPs appear to exhibit
different behavior [95, 96, 101]. In the case of HDs, some works [85, 87, 90] have
shown that the magnetic moiety exhibits a spin-glass behavior due to the spin-disorder
in the structure. Different is the case of the CS NPs [96, 100, 101], if the shell
is magnetic and depending on the morphology of this shell—continuous shell or
discontinuous layer of particles—the reversal process could take place by incoherent
rotation modes or by the dipolar competition of the particles. Here, in addition to
the spin-glass behavior, also exchange bias effects have been observed [87, 90, 93]
even if the presence of an antiferromagnetic material counterpart, necessary for the
exchange bias effect, was not detected. Chandra et al. [85] propose this behavior
is possible in the Au@Fe 3 O 4 CS NPs due to the presence of inner or intergranular
spin disordered regions. On the other hand, Zhu et al. [87] propose that the exchange
bias is due to the coupling of weak magnetic layer at the Au–Fe 3 O 4 interface and
the Fe 3 O 4 overlayer. However, these effects are not present in all the investigated
structures: other studies [89, 102] show the classical superparamagnetic behavior in
which interparticle interactions are dominant even if magnetic moiety is composed
by ferromagnetic (Fe 3 O 4 or γ-Fe 2 O 3 ) and antiferromagnetic (α-Fe 2 O 3 ) phases.
The magnetic properties of alloy-based MP nanoparticles, that combine magnetic
and non-magnetic elements, are completely different to those of the hybrid family.
As previously mentioned, the formation of this type of alloys is not straightforward,
but it is even more difficult to obtain magnetic properties at room temperature. The
out-of-equilibrium nature of nanomaterials allows the growth of these alloys, but
also other possible morphologies, such as segregated particles or inhomogeneous
mixtures, are possible. On the other hand, the solid solution of non-magnetic and
magnetic elements gives rise to the decrease of the magnetic moment of the magnetic
elements resulting in the decrease of the total magnetization of the particle and
the weakening of exchange interactions with the consequent decrease of the Curie
temperature. In a few alloys, for example NiCu [103] or FeCuPt [104] alloys, the
solid solution of the two element is possible and the magnetization and the Curie
temperature decrease (non-linearly) as the non-magnetic content increases. In the
case of AuFe [105–107] and AgCo [108, 109] solid solutions, in addition to this
evolution, also spin-glass and diamagnetic behaviors can occur for the large content
of non-magnetic elements. In fact, in these cases the decrease of the magnetization
and One relevant point is thatthe Curie temperature is due to the decrease of the direct
exchange interactions and the increasing role of the indirect exchange interactions
(RKKY) though the s-bands of the non-magnetic elements. In such case the magnetic
behavior can pass from ferromagnetic, to spin-glass, paramagnetic and diamagnetic,
depending of the non-magnetic content but also on the temperature.
The discussions above show that the preservation of both magnetic and plasmonic properties is possible both in hybrid and in alloy nanostructures, but a fine
C. de Julián Fernández and F. Pineider
activation processes are dominant [98] and the superparamagnetic effect occurs at
room temperature. This behavior has been reported in a wide number of works investigating Au–Fe 3 O 4 heterodimers and Ag–Fe 3 O 4 [45, 89, 93–97, 100]. The blocking
temperature, i.e. the threshold between the blocked and superparamagnetic regimes,
is determined by the size distribution of the particles, their magnetic anisotropy, as
well as the magnetic interparticle interactions. HDs and CS NPs appear to exhibit
different behavior [95, 96, 101]. In the case of HDs, some works [85, 87, 90] have
shown that the magnetic moiety exhibits a spin-glass behavior due to the spin-disorder
in the structure. Different is the case of the CS NPs [96, 100, 101], if the shell
is magnetic and depending on the morphology of this shell—continuous shell or
discontinuous layer of particles—the reversal process could take place by incoherent
rotation modes or by the dipolar competition of the particles. Here, in addition to
the spin-glass behavior, also exchange bias effects have been observed [87, 90, 93]
even if the presence of an antiferromagnetic material counterpart, necessary for the
exchange bias effect, was not detected. Chandra et al. [85] propose this behavior
is possible in the Au@Fe 3 O 4 CS NPs due to the presence of inner or intergranular
spin disordered regions. On the other hand, Zhu et al. [87] propose that the exchange
bias is due to the coupling of weak magnetic layer at the Au–Fe 3 O 4 interface and
the Fe 3 O 4 overlayer. However, these effects are not present in all the investigated
structures: other studies [89, 102] show the classical superparamagnetic behavior in
which interparticle interactions are dominant even if magnetic moiety is composed
by ferromagnetic (Fe 3 O 4 or γ-Fe 2 O 3 ) and antiferromagnetic (α-Fe 2 O 3 ) phases.
The magnetic properties of alloy-based MP nanoparticles, that combine magnetic
and non-magnetic elements, are completely different to those of the hybrid family.
As previously mentioned, the formation of this type of alloys is not straightforward,
but it is even more difficult to obtain magnetic properties at room temperature. The
out-of-equilibrium nature of nanomaterials allows the growth of these alloys, but
also other possible morphologies, such as segregated particles or inhomogeneous
mixtures, are possible. On the other hand, the solid solution of non-magnetic and
magnetic elements gives rise to the decrease of the magnetic moment of the magnetic
elements resulting in the decrease of the total magnetization of the particle and
the weakening of exchange interactions with the consequent decrease of the Curie
temperature. In a few alloys, for example NiCu [103] or FeCuPt [104] alloys, the
solid solution of the two element is possible and the magnetization and the Curie
temperature decrease (non-linearly) as the non-magnetic content increases. In the
case of AuFe [105–107] and AgCo [108, 109] solid solutions, in addition to this
evolution, also spin-glass and diamagnetic behaviors can occur for the large content
of non-magnetic elements. In fact, in these cases the decrease of the magnetization
and One relevant point is thatthe Curie temperature is due to the decrease of the direct
exchange interactions and the increasing role of the indirect exchange interactions
(RKKY) though the s-bands of the non-magnetic elements. In such case the magnetic
behavior can pass from ferromagnetic, to spin-glass, paramagnetic and diamagnetic,
depending of the non-magnetic content but also on the temperature.
The discussions above show that the preservation of both magnetic and plasmonic properties is possible both in hybrid and in alloy nanostructures, but a fine