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C. de Julián Fernández and F. Pineider
materials exhibit a rich phenomenology correlated to size and surface effects that
will be present also in the MPs nanomaterials. In addition to size effects, the properties of the MP nanoparticles are affected by structural and electronic correlations
between the two types of elements or materials, the final properties being different
than those of bare magnetic or plasmonic materials and hence giving rise to novel
properties. Moreover, very promising new phenomena and applications appear due
to the synergy between the two moieties. For these reasons, nowadays MP materials are object of intense investigation in materials science. In addition, these new
materials have given rise to the development of novel applications of each class
of material—photonics for plasmonics and recording for magnetism—and multifunctional applications in the fields in which both materials are promising like in
biomedicine and catalysis [1–5].
Focusing on the term plasmonics, [1, 6] this describes optical phenomena
produced by the excitation of collective electron oscillations, the surface plasmon
resonances (SPR), that are induced by an electromagnetic field (EMF) propagating
in a metal/dielectric interface. In the case of nanoparticles, the SPR is denominated
localized surface plasmon resonance (LSPR), but SPR can be generated in other
morphologies as disks, thin films (propagating SPR), holes, gratings and a wide
number of structures [1–9]. Plasmonics opens in photonics the possibility of new
ways to manipulate the light, new optical properties of matter, and the possibility of
engineering light at the nanoscale [1–3].
The possibility of obtaining a single material with both strong plasmonic and
magnetic properties is nowadays an unsolved issue. Plasmonic phenomena require
the presence of free electrons, hence metals with a partially-filled s-band at the Fermi
level [1, 8–10]. On the opposite, magnetism is mainly correlated to bound electrons
in the d and f bands [11]. Hence both properties are correlated to materials with very
different electronic features that are not compatible in the same electronic structure.
In fact, the main plasmonic materials, Au and Ag, are diamagnetic while most of
magnetic metals like Fe, Co and Ni exhibit weak plasmon resonances.
Figure 5.1 shows a selection of the wide variety of MP NPs that have been grown
so far [12–15]. Between all, magnetic metal NPs should be most simple MPs system
that can exhibit magnetic and plasmonic properties simultaneously in a single entity.
Also NPs composed by a solid solution of the magnetic and plasmonic elements—
like AuFe, AgFe, AgCo should be good candidates. However, as we will discuss
later, the electronic structure of the single phase and the solid-solution NPs is such
that one or both plasmonic and magnetic properties are typically damped. The design
of hetero-structures in which plasmonic and magnetic moieties are separated but in
direct physical and electronic contact is a second approach to design MP systems. A
wide variety of morphologies have been created, such as core@shell (CS), flowers,
heterodimers (HSs), hybrid-cigars, stars, nanodomes, etc. These MP systems have
nanometric dimension, hence the size and surface effects have a key role in their properties. Their sizes are smaller or of the order of the characteristic lengths of plasmonic
and magnetic phenomena, i.e. visible light wavelength, electron mean free path and
domain wall length. Then, the magnetic and plasmonic properties should be affected
C. de Julián Fernández and F. Pineider
materials exhibit a rich phenomenology correlated to size and surface effects that
will be present also in the MPs nanomaterials. In addition to size effects, the properties of the MP nanoparticles are affected by structural and electronic correlations
between the two types of elements or materials, the final properties being different
than those of bare magnetic or plasmonic materials and hence giving rise to novel
properties. Moreover, very promising new phenomena and applications appear due
to the synergy between the two moieties. For these reasons, nowadays MP materials are object of intense investigation in materials science. In addition, these new
materials have given rise to the development of novel applications of each class
of material—photonics for plasmonics and recording for magnetism—and multifunctional applications in the fields in which both materials are promising like in
biomedicine and catalysis [1–5].
Focusing on the term plasmonics, [1, 6] this describes optical phenomena
produced by the excitation of collective electron oscillations, the surface plasmon
resonances (SPR), that are induced by an electromagnetic field (EMF) propagating
in a metal/dielectric interface. In the case of nanoparticles, the SPR is denominated
localized surface plasmon resonance (LSPR), but SPR can be generated in other
morphologies as disks, thin films (propagating SPR), holes, gratings and a wide
number of structures [1–9]. Plasmonics opens in photonics the possibility of new
ways to manipulate the light, new optical properties of matter, and the possibility of
engineering light at the nanoscale [1–3].
The possibility of obtaining a single material with both strong plasmonic and
magnetic properties is nowadays an unsolved issue. Plasmonic phenomena require
the presence of free electrons, hence metals with a partially-filled s-band at the Fermi
level [1, 8–10]. On the opposite, magnetism is mainly correlated to bound electrons
in the d and f bands [11]. Hence both properties are correlated to materials with very
different electronic features that are not compatible in the same electronic structure.
In fact, the main plasmonic materials, Au and Ag, are diamagnetic while most of
magnetic metals like Fe, Co and Ni exhibit weak plasmon resonances.
Figure 5.1 shows a selection of the wide variety of MP NPs that have been grown
so far [12–15]. Between all, magnetic metal NPs should be most simple MPs system
that can exhibit magnetic and plasmonic properties simultaneously in a single entity.
Also NPs composed by a solid solution of the magnetic and plasmonic elements—
like AuFe, AgFe, AgCo should be good candidates. However, as we will discuss
later, the electronic structure of the single phase and the solid-solution NPs is such
that one or both plasmonic and magnetic properties are typically damped. The design
of hetero-structures in which plasmonic and magnetic moieties are separated but in
direct physical and electronic contact is a second approach to design MP systems. A
wide variety of morphologies have been created, such as core@shell (CS), flowers,
heterodimers (HSs), hybrid-cigars, stars, nanodomes, etc. These MP systems have
nanometric dimension, hence the size and surface effects have a key role in their properties. Their sizes are smaller or of the order of the characteristic lengths of plasmonic
and magnetic phenomena, i.e. visible light wavelength, electron mean free path and
domain wall length. Then, the magnetic and plasmonic properties should be affected