124
C. de Julián Fernández and F. Pineider
previous studies it has been argued that such differences come from the difficulty
in accurately determining the optical and magneto-optical properties of magnetic
materials composing the nanostructures, since they can be affected by oxidation
processes and by surface and size effects, resulting in a dielectric tensor with can be
very different from the starting bulk material, and very hard to characterize. Vlasin
et al. [213] propose to take into account the diamagnetic MO contribution of the
medium, to adequately perform the theoretical calculations with the MG model.
The amplification of the MO signal has been theoretically calculated with the
Maxwell–Garnett effective field approximation in bimetallic CS and onion-like
nanostructures [201, 214, 215] and observed in Ag@Co [38] and Ag@CoFe CS
NPs [51] and Au top-capped Ni wires [216]. López-Ortega et al. [51] compares the
plasmonics and MO properties of CoFe NPs to those of Ag@CoFe NPs (Fig. 5.3iv).
The bare magnetic NPs do not present SPR and their MO signal is weaker than that
of the MPs CS NPs that have a SPR and Faraday effects at coincident wavelength.
Interestingly, the MO signal exhibits a similar shift in energy as the SPR when the
surrounding medium is changed, indicating the possibility to apply the MO technique
for chemical and biochemical sensors.
Wang et al. [38] investigated the evolution of the plasmon-induced MO signal of
Au@Co CS NPs as function of the Co shell thickness. They observe an increase of
the Faraday signal and of the SPR as Co layer was thinner even if bare Co NPs had
small MO signal and no SPR. This behavior was in agreement with the simulations
employing MG model confirming the expected damping of the SPR induced by the
strong absorption of the magnetic component. However, the authors correlate the
stronger MO activity of the CS NPs with thinner magnetic layer to an amplification
effect of the EMFs induced at the LSPR. In fact, the local enhancement of the EMFs
at the plasmonic resonance is a well-known and exploited optical effect that can
give rise to the amplification of a wide number of optical effects. For example,
Shemer et al. [217] showed that the Kerr effect in Fe 3 O 4 NPs increases up to 3 times
when placed in the proximity of plasmon-active rough metallic surfaces (Fig. 5.3iii).
The proof of concept for this plasmon-amplified MO effect was demonstrated in
composites of Au nanoparticles dispersed in garnet films [218–220]. The majority
of available studies currently involve CS and HDs NPs [221–223] with magnetite
(Fe 3 O 4 ), maghemite (γ-Fe 2 O 3 ) or spinel oxides doped with a second transition metal
cation, due to the easier synthesis and their possible application in biomedicine. Jain
et al. [222] reported the amplification of the Faraday signal of Au-coated maghemite
nanoparticles (Fig. 5.3ii). Li et al. [221] observed an increase of the Faraday rotation
and a change of its sign in Ag–CoFe 2 O 4 HDs, but not at the SPR wavelength. In
fact, the comparison between the MO properties of the bare magnetic moiety and
in the hybrid nanostructures is not straightforward, due to the previously discussed
magneto-structural differences between the two systems.
Several studies investigated the MO properties of mixtures of magnetic and plasmonic NPs, in which direct contact is excluded and mainly near field EMF interactions modify the MO signal. Several studies have shown the amplification [224,
225] or the reduction [68] of MO signal and the change of its sign [224], considering different interparticle distances and hence different dipolar strength. Discrete
C. de Julián Fernández and F. Pineider
previous studies it has been argued that such differences come from the difficulty
in accurately determining the optical and magneto-optical properties of magnetic
materials composing the nanostructures, since they can be affected by oxidation
processes and by surface and size effects, resulting in a dielectric tensor with can be
very different from the starting bulk material, and very hard to characterize. Vlasin
et al. [213] propose to take into account the diamagnetic MO contribution of the
medium, to adequately perform the theoretical calculations with the MG model.
The amplification of the MO signal has been theoretically calculated with the
Maxwell–Garnett effective field approximation in bimetallic CS and onion-like
nanostructures [201, 214, 215] and observed in Ag@Co [38] and Ag@CoFe CS
NPs [51] and Au top-capped Ni wires [216]. López-Ortega et al. [51] compares the
plasmonics and MO properties of CoFe NPs to those of Ag@CoFe NPs (Fig. 5.3iv).
The bare magnetic NPs do not present SPR and their MO signal is weaker than that
of the MPs CS NPs that have a SPR and Faraday effects at coincident wavelength.
Interestingly, the MO signal exhibits a similar shift in energy as the SPR when the
surrounding medium is changed, indicating the possibility to apply the MO technique
for chemical and biochemical sensors.
Wang et al. [38] investigated the evolution of the plasmon-induced MO signal of
Au@Co CS NPs as function of the Co shell thickness. They observe an increase of
the Faraday signal and of the SPR as Co layer was thinner even if bare Co NPs had
small MO signal and no SPR. This behavior was in agreement with the simulations
employing MG model confirming the expected damping of the SPR induced by the
strong absorption of the magnetic component. However, the authors correlate the
stronger MO activity of the CS NPs with thinner magnetic layer to an amplification
effect of the EMFs induced at the LSPR. In fact, the local enhancement of the EMFs
at the plasmonic resonance is a well-known and exploited optical effect that can
give rise to the amplification of a wide number of optical effects. For example,
Shemer et al. [217] showed that the Kerr effect in Fe 3 O 4 NPs increases up to 3 times
when placed in the proximity of plasmon-active rough metallic surfaces (Fig. 5.3iii).
The proof of concept for this plasmon-amplified MO effect was demonstrated in
composites of Au nanoparticles dispersed in garnet films [218–220]. The majority
of available studies currently involve CS and HDs NPs [221–223] with magnetite
(Fe 3 O 4 ), maghemite (γ-Fe 2 O 3 ) or spinel oxides doped with a second transition metal
cation, due to the easier synthesis and their possible application in biomedicine. Jain
et al. [222] reported the amplification of the Faraday signal of Au-coated maghemite
nanoparticles (Fig. 5.3ii). Li et al. [221] observed an increase of the Faraday rotation
and a change of its sign in Ag–CoFe 2 O 4 HDs, but not at the SPR wavelength. In
fact, the comparison between the MO properties of the bare magnetic moiety and
in the hybrid nanostructures is not straightforward, due to the previously discussed
magneto-structural differences between the two systems.
Several studies investigated the MO properties of mixtures of magnetic and plasmonic NPs, in which direct contact is excluded and mainly near field EMF interactions modify the MO signal. Several studies have shown the amplification [224,
225] or the reduction [68] of MO signal and the change of its sign [224], considering different interparticle distances and hence different dipolar strength. Discrete