5 Magneto-Plasmonic Nanoparticles
125
Dipole Approximation calculations predict in Ag and CoFe 2 O 4 packed NPs a nonmonotonic dependence of the MO signal on the interparticle distance, the maximum
being at 20 nm interparticle distance [226].
The overlapping of the SPR with a MO transition has been also taken into account.
Campo et al. [68] investigated the effect of the SPR on the different MO transitions
of CoFe 2 O 4 NPs dispersed in a solvent with a small concentration of Au NPs (<1%).
This oxide exhibits three bands associated to different interband and intraband optical
transitions. The broad SPR peak of the composite overlaps with the interband chargetransfer (CT at 1.8 eV) transition and a crystal field (CF at 2.0 eV) transition of the
Co
2+ ions, which result in distinct MO spectral features. While the MO signal at the
CF transition decreases, the one of the CT transitions that fully overlaps with the
SPR, evolve to several weak MO transitions. On the other hand, plasmon-induced
amplification of the MO signal of single-molecule magnet molecules (terbium(III)
bis-phthalocianinate) deposited of Au discs was recently observed [227]. In this
case the SPR of the substrate discs almost overlaps with the MO transitions related
to the main absorption band of the molecule. A moderate, but very clear fivefold
enhancement of the MO signal from the magnetic molecule was observed in this
case.
One relevant point is that also non-magnetic plasmonic nanostructures exhibit MO
effects at the SPR [203, 228, 229]. In fact, the interaction of light with free electrons
in presence of a magnetic field perpendicular to the oscillating electric field of light
gives rise to magneto-optical effects due to the Lorenz force [198, 230]. This results
in Kerr or Faraday rotations due to the modification of the propagation plane. The
magnetic field-induced change of the electron oscillation momentum implies that
the SPR energy also varies and hence the plasmon frequency shifts in the presence
of an applied magnetic field [203]. When using circularly polarized light, circular
plasmonic modes with right and left polarization are split in energy in presence of a
magnetic field parallel to the propagation direction giving rise to MCD nearby the
SPR [229] (Fig. 5.3 iv). Nowadays different Kerr, Faraday and MCD effects have
been demonstrated in Au [231], Ag [232, 233] and even in plasmonic semiconductors
[234, 235] on nanoparticles, discs, rods and other morphologies. The MO effect of
plasmonics nanoparticles varies linearly with the magnetic field, and it requires strong
magnetic fields to be significant [229, 236]. In fact, it is significantly smaller than
that of magnetic materials due to their different electronic origin.
5.5 Perspectives
In previous chapters we showed several examples of the multifunctionality of the
MP nanostructures. However, a wide number of difficulties must be overcome to
obtain high performance MP materials and devices. Such difficulties are linked to
the nature of the involved materials and to the difficulties in the chemical design of the
researched morphologies. From the optical point of view, the large optical losses of
the most investigated magnetic materials: magnetite and maghemite, and Fe, Co, Ni,
125
Dipole Approximation calculations predict in Ag and CoFe 2 O 4 packed NPs a nonmonotonic dependence of the MO signal on the interparticle distance, the maximum
being at 20 nm interparticle distance [226].
The overlapping of the SPR with a MO transition has been also taken into account.
Campo et al. [68] investigated the effect of the SPR on the different MO transitions
of CoFe 2 O 4 NPs dispersed in a solvent with a small concentration of Au NPs (<1%).
This oxide exhibits three bands associated to different interband and intraband optical
transitions. The broad SPR peak of the composite overlaps with the interband chargetransfer (CT at 1.8 eV) transition and a crystal field (CF at 2.0 eV) transition of the
Co
2+ ions, which result in distinct MO spectral features. While the MO signal at the
CF transition decreases, the one of the CT transitions that fully overlaps with the
SPR, evolve to several weak MO transitions. On the other hand, plasmon-induced
amplification of the MO signal of single-molecule magnet molecules (terbium(III)
bis-phthalocianinate) deposited of Au discs was recently observed [227]. In this
case the SPR of the substrate discs almost overlaps with the MO transitions related
to the main absorption band of the molecule. A moderate, but very clear fivefold
enhancement of the MO signal from the magnetic molecule was observed in this
case.
One relevant point is that also non-magnetic plasmonic nanostructures exhibit MO
effects at the SPR [203, 228, 229]. In fact, the interaction of light with free electrons
in presence of a magnetic field perpendicular to the oscillating electric field of light
gives rise to magneto-optical effects due to the Lorenz force [198, 230]. This results
in Kerr or Faraday rotations due to the modification of the propagation plane. The
magnetic field-induced change of the electron oscillation momentum implies that
the SPR energy also varies and hence the plasmon frequency shifts in the presence
of an applied magnetic field [203]. When using circularly polarized light, circular
plasmonic modes with right and left polarization are split in energy in presence of a
magnetic field parallel to the propagation direction giving rise to MCD nearby the
SPR [229] (Fig. 5.3 iv). Nowadays different Kerr, Faraday and MCD effects have
been demonstrated in Au [231], Ag [232, 233] and even in plasmonic semiconductors
[234, 235] on nanoparticles, discs, rods and other morphologies. The MO effect of
plasmonics nanoparticles varies linearly with the magnetic field, and it requires strong
magnetic fields to be significant [229, 236]. In fact, it is significantly smaller than
that of magnetic materials due to their different electronic origin.
5.5 Perspectives
In previous chapters we showed several examples of the multifunctionality of the
MP nanostructures. However, a wide number of difficulties must be overcome to
obtain high performance MP materials and devices. Such difficulties are linked to
the nature of the involved materials and to the difficulties in the chemical design of the
researched morphologies. From the optical point of view, the large optical losses of
the most investigated magnetic materials: magnetite and maghemite, and Fe, Co, Ni,