Giant electron-spingfactors in a ferromagnetic nanoparticle

Abstract

We utilize single-electron tunneling spectroscopy to measure the discrete energy levels in a nanometer-scale cobalt particle at T=60mK, and find effective single-electron spin g-factors ≈ 7.3. These large g-factors do not result from the typical orbital contribution to g-factors, since the orbital angular momentum is quenched. Instead, they are due to non-trivial many-body excitations. A kink in the plot of conductance vs. voltage and magnetic field is a signature of degenerate total spin on the particle. Spin-Orbit interactions cause the new particle eigenstates to have 'spin' that is an admixture of pure spin states. Fluctuations in the discrete energy level spacing allow for the total change in 'spin' on the particle during a single-electron tunneling event to be ∆S ′ = 3/2, leading to a g-factor around 6.

Magnetic response of energy levels of superconducting nanoparticles with spin-orbit scattering

“…The excited states |Θ ′ Ne contributing to Eq. (19) are the two-quasiparticles excitations with the excitation energies E Ne Θ ′ − E Ne 0 given by the sums of the quasiparticle energies (31). The curvature of the even ground state becomes (see Appendix B)…”

“…given by the sums of the quasiparticle energies (31). The curvature of the even ground state becomes (see Appendix B)…”

“…Large g-factors were recently observed in ferromagnetic cobalt nanoparticles. 31 Spin-orbit scattering preserves time-reversal symmetry and therefore does not suppress pairing correlations. In this work we explore the effect of an attractive pairing interaction on the magnetic-field response of manyparticle energy levels of a grain in the presence of strong spin-orbit scattering, where the exchange interaction is negligible.…”