3 Spin-Polarized Plasmonics: Fresh View …
55
binding detection, the feasibility of using SERS to distinguish protein conformational
states, which was shown for human insulin and its analog insulin lispro [21–24]. A
protocol has been developed to detect cell surface markers, CD44 and CD24, in
three breast cancer cell lines [25]. The dielectric functions Ag and Au for this type
of nanostructures were carefully studied as well [26–28].
However, Co nanoparticles (Co-NPs), under certain conditions can support an
excellent plasmon resonance at about 280 nm with a quality factor greater than
Al, In, and comparable to Au in the visible [2]. Importantly, this Co platform is
comprising both magnetic and plasmonic properties. A long lasting search for plasmonic materials in the ultraviolet spectral range does not consider Co as a promising
candidate [29–32]. One of the criteria for a high quality plasmonic material is that the
number of electrons involved in interband transitions must be low, and at the highest
possible frequency. These criteria significantly reduces the number of materials that
are likely to have favorable optical properties, by the simple fact that all materials
with partially occupied d or f states are going to perform poorly across the visible due
to interband transitions [33]. Recent works pushed the plasmonics to high-energy
range using Al [33–35] and In [36].
The Mott model [37, 38] of conductivity in magnetic metals helps to qualitatively
explain observed phenomena for Co nanoparticles. Indeed, the electrical conductivity in metals can be described in terms of two largely independent conducting
channels, corresponding to the spin-up and spin-down electrons. Importantly, the
probability of spin-flip scattering processes in metals is normally small as compared
to the probability of the scattering processes in which the spin is conserved. This
means that the spin-up and spin-down electrons do not mix over long distances and,
therefore, the electrical conduction occurs in parallel for the two spin channels. Also,
the scattering rates in ferromagnetic metals of the spin-up and spin-down electrons
are quite different, whatever the nature of the scattering centers is. These two channels of conductivity with a distinct spin-dependent scattering is the primary origin
of giant magnetoresistance [39].
Here, we discuss the effect of spin polarization on plasmon oscillations of the free
electrons in nanoparticles, which is, crucial in many envisioned applications at the
cross road of magnetism and plasmonics.
3.2 Spin Polarization in Co Nanoparticles
A new type of plasmons is specific for spin-polarized magnetic nanoparticles. One can
expect two independent plasmons which co-exist in a spin-polarized metal nanoparticle following Mott’s model. These two plasmons coexist in a particle at the same
frequency and polarizations of excitation, but for electrons of opposite spin. Internanoparticle interactions completely demolish plasmon quality resonance, which is
the probable reason why it was not observed previously and why the results for bulk
films [1] cannot be used for single domain nanoparticles evaluations. It is known
55
binding detection, the feasibility of using SERS to distinguish protein conformational
states, which was shown for human insulin and its analog insulin lispro [21–24]. A
protocol has been developed to detect cell surface markers, CD44 and CD24, in
three breast cancer cell lines [25]. The dielectric functions Ag and Au for this type
of nanostructures were carefully studied as well [26–28].
However, Co nanoparticles (Co-NPs), under certain conditions can support an
excellent plasmon resonance at about 280 nm with a quality factor greater than
Al, In, and comparable to Au in the visible [2]. Importantly, this Co platform is
comprising both magnetic and plasmonic properties. A long lasting search for plasmonic materials in the ultraviolet spectral range does not consider Co as a promising
candidate [29–32]. One of the criteria for a high quality plasmonic material is that the
number of electrons involved in interband transitions must be low, and at the highest
possible frequency. These criteria significantly reduces the number of materials that
are likely to have favorable optical properties, by the simple fact that all materials
with partially occupied d or f states are going to perform poorly across the visible due
to interband transitions [33]. Recent works pushed the plasmonics to high-energy
range using Al [33–35] and In [36].
The Mott model [37, 38] of conductivity in magnetic metals helps to qualitatively
explain observed phenomena for Co nanoparticles. Indeed, the electrical conductivity in metals can be described in terms of two largely independent conducting
channels, corresponding to the spin-up and spin-down electrons. Importantly, the
probability of spin-flip scattering processes in metals is normally small as compared
to the probability of the scattering processes in which the spin is conserved. This
means that the spin-up and spin-down electrons do not mix over long distances and,
therefore, the electrical conduction occurs in parallel for the two spin channels. Also,
the scattering rates in ferromagnetic metals of the spin-up and spin-down electrons
are quite different, whatever the nature of the scattering centers is. These two channels of conductivity with a distinct spin-dependent scattering is the primary origin
of giant magnetoresistance [39].
Here, we discuss the effect of spin polarization on plasmon oscillations of the free
electrons in nanoparticles, which is, crucial in many envisioned applications at the
cross road of magnetism and plasmonics.
3.2 Spin Polarization in Co Nanoparticles
A new type of plasmons is specific for spin-polarized magnetic nanoparticles. One can
expect two independent plasmons which co-exist in a spin-polarized metal nanoparticle following Mott’s model. These two plasmons coexist in a particle at the same
frequency and polarizations of excitation, but for electrons of opposite spin. Internanoparticle interactions completely demolish plasmon quality resonance, which is
the probable reason why it was not observed previously and why the results for bulk
films [1] cannot be used for single domain nanoparticles evaluations. It is known