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C. de Julián Fernández and F. Pineider
An alternative simple approach to develop MP materials is using single particles composed by a solid solution of magnetic and plasmonic elements. The most
important plasmonic elements, the Ag and Au, are poorly soluble in Fe and Co
in the bulk, but their alloying has been experimentally demonstrated in nanoparticles [25–29]. Size and surface effects allow overcoming the thermodynamic equilibrium immiscibility [29–31] but the synthesis of alloy MP NPs is nowadays a
challenge. As a consequence of the mixing of both elements, electronic hybridization occurs between the 5d6s bands (for gold) and the 3d4s bands of the plasmonic
and magnetic metals, respectively, which gives rise to larger electronic densities at
Fermi level and a broad spectral overlap of the inter and intraband transitions [26,
27, 29]. This leads to a significant damping of the plasmon resonance. At difference
with single metal and hybrid nanostructures, the SPR of the alloys can either blue
shift or red shift [29] depending on the metal composition. The optical properties of
these alloys are different than those corresponding to the weighted combination of
the constants of the containing elements, as demonstrated by Amendola et al. [27],
who simulated the SPR spectrum of AuFe nanoparticles first considering the dielectric properties measured from AuFe films, then using theoretical values obtained
combining elemental weighted dielectric constants of the Au and Fe. Significantly
better agreement with the experiment was found using the former set of optical
constants.
A second class of systems are the hybrid heterostructures constituted by a plasmonic and a magnetic moiety that are in direct contact. In this case the optical
properties of the two moieties are intertwined by their proximity (dipolar fields),
but also influenced by their mutual structural arrangement (epitaxy) and electronic
hybridization at the contact interface [13, 31–36]. In general the plasmonic properties depend critically on the geometry and the materials that are selected. When
the magnetic moiety is metallic, the LSPRs of the two moieties could be excited
separately, since they are separated in energy. However, the SPR of the plasmonic
moiety can be damped by the absorption of the counterpart (Fig. 5.3v), but also electronic excitations coupling between the two moieties can occur [37–41]. For instance,
Sachan et al. [40] employed Energy Electron Loss experiments to observe locally
the plasmonic excitation of the two moieties of CoFe–Ag HDs (Fig. 5.3 iii). They
determined that both moieties exhibit distinct LSPR modes that are split in energy,
the LSPR of the magnetic moiety being red shifted with respect to the one of Ag.
Interestingly, isolated CoFe NPs do not exhibit LSPR. Moreover, a plasmon resonance was observed at the CoFe–Ag interface corresponding to a hybridized state.
In the case that the magnetic part is an oxide, this material behaves as a dielectric
medium (i.e. real part of ε m > 1) with also large absorption (ε 2 > 2). See for example,
in Fig. 5.2 the dielectric functions of the most investigated oxide, the magnetite
(Fe 3 O 4 ), and the promising oxide Yttrium iron garnet (YIG, Y 3 Fe 5 O 12 ) [42, 43].
Regarding morphology, in the case that the plasmonic nanoparticles are surrounded
totally (in CS NPs) or partially (as in HDs) by the oxide, plasmon resonance could
be analyzed by using a mean field approach (the Maxwell–Garnett, MG, model, for
example) considering an absorbent medium or a multilayer structure [35, 44, 45].
As can be seen in Fig. 5.3iv, both in CS and HD morphologies, plasmon resonance
C. de Julián Fernández and F. Pineider
An alternative simple approach to develop MP materials is using single particles composed by a solid solution of magnetic and plasmonic elements. The most
important plasmonic elements, the Ag and Au, are poorly soluble in Fe and Co
in the bulk, but their alloying has been experimentally demonstrated in nanoparticles [25–29]. Size and surface effects allow overcoming the thermodynamic equilibrium immiscibility [29–31] but the synthesis of alloy MP NPs is nowadays a
challenge. As a consequence of the mixing of both elements, electronic hybridization occurs between the 5d6s bands (for gold) and the 3d4s bands of the plasmonic
and magnetic metals, respectively, which gives rise to larger electronic densities at
Fermi level and a broad spectral overlap of the inter and intraband transitions [26,
27, 29]. This leads to a significant damping of the plasmon resonance. At difference
with single metal and hybrid nanostructures, the SPR of the alloys can either blue
shift or red shift [29] depending on the metal composition. The optical properties of
these alloys are different than those corresponding to the weighted combination of
the constants of the containing elements, as demonstrated by Amendola et al. [27],
who simulated the SPR spectrum of AuFe nanoparticles first considering the dielectric properties measured from AuFe films, then using theoretical values obtained
combining elemental weighted dielectric constants of the Au and Fe. Significantly
better agreement with the experiment was found using the former set of optical
constants.
A second class of systems are the hybrid heterostructures constituted by a plasmonic and a magnetic moiety that are in direct contact. In this case the optical
properties of the two moieties are intertwined by their proximity (dipolar fields),
but also influenced by their mutual structural arrangement (epitaxy) and electronic
hybridization at the contact interface [13, 31–36]. In general the plasmonic properties depend critically on the geometry and the materials that are selected. When
the magnetic moiety is metallic, the LSPRs of the two moieties could be excited
separately, since they are separated in energy. However, the SPR of the plasmonic
moiety can be damped by the absorption of the counterpart (Fig. 5.3v), but also electronic excitations coupling between the two moieties can occur [37–41]. For instance,
Sachan et al. [40] employed Energy Electron Loss experiments to observe locally
the plasmonic excitation of the two moieties of CoFe–Ag HDs (Fig. 5.3 iii). They
determined that both moieties exhibit distinct LSPR modes that are split in energy,
the LSPR of the magnetic moiety being red shifted with respect to the one of Ag.
Interestingly, isolated CoFe NPs do not exhibit LSPR. Moreover, a plasmon resonance was observed at the CoFe–Ag interface corresponding to a hybridized state.
In the case that the magnetic part is an oxide, this material behaves as a dielectric
medium (i.e. real part of ε m > 1) with also large absorption (ε 2 > 2). See for example,
in Fig. 5.2 the dielectric functions of the most investigated oxide, the magnetite
(Fe 3 O 4 ), and the promising oxide Yttrium iron garnet (YIG, Y 3 Fe 5 O 12 ) [42, 43].
Regarding morphology, in the case that the plasmonic nanoparticles are surrounded
totally (in CS NPs) or partially (as in HDs) by the oxide, plasmon resonance could
be analyzed by using a mean field approach (the Maxwell–Garnett, MG, model, for
example) considering an absorbent medium or a multilayer structure [35, 44, 45].
As can be seen in Fig. 5.3iv, both in CS and HD morphologies, plasmon resonance