Abstract
Here it is experimentally shown that Co nanoparticles with a single-domain crystal structure support a plasmon resonance at approximately 280 nm with better quality than gold nanoparticle resonance in the visible. Magnetic nature of the nanoparticles suggests a new type of these plasmons. The exchange interaction of electrons splits the energy bands between spin-up electrons and spin-down electrons. It makes it possible for coexistence of two independent channels of conductivity as well as two independent plasmons in the same nanoparticle with very different electron relaxation. Indeed, the density of empty states in a partially populated d-band is high, resulting in a large relaxation rate of the spin-down conduction electrons and consequently in low quality of the plasmon resonance. In contrast, the majority electrons with a completely filled d-band do not provide final states for the scattering processes of the conduction spin-up electrons, therefore supporting a high quality plasmon resonance. The scattering without spin flip is required to keep these two plasmons independent.
Highlights
The Mott model[1,2] of conductivity in magnetic metals introduces three main points
It is a common belief that the quality of the plasmon resonance of magnetic nanoparticles such as Co is quite low, which follows, in particular, from the experimental data for permittivity of bulk cobalt by Johnson and Christy (J&C)[4]
Our experiments prove that Co nanoparticles with a single-domain crystal structure support a sharp plasmon resonance at about 280 nm with the resonance quality comparable to gold nanoparticles
Summary
The Mott model[1,2] of conductivity in magnetic metals introduces three main points. First, 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, which are distinguished according to the projection of their spins along the quantization axis. 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[3]. Understanding the effect of spin polarization on plasmon oscillations of the free electrons in nanoparticles is, essentially, unexplored and crucial in many envisioned applications at the cross road of magnetism and plasmonics. Our experiments prove that Co nanoparticles with a single-domain crystal structure support a sharp plasmon resonance at about 280 nm with the resonance quality comparable to gold nanoparticles This type of plasmons is quite different from known plasmons in noble metals. The majority electrons with a completely filled d-band does not affect the relaxation rate and plasmon resonance of the conduction spin-up electrons within magnetic nanoparticles. Note that the plasmon resonance of Co is in the deep ultraviolet spectral range, which is the range for bio-molecule resonances and attractive for bio-medical applications as well as for its magnetic nature
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