Session G28 - Focus Session: Magnetic Nanoparticles II.
FOCUS session, Tuesday morning, March 04
Room 18B, Austin Convention Center
We present a theory of spin excitations in ferromagnetic metal nanoparticles aimed at achieving an unified quantum description of both collective and quasiparticle physics. We focus on the role played by spin-orbit interaction, which is responsible for the magnetic anisotropy, for the energy gap E_res in the collective excitation spectrum and the coupling between collective excitations and particle-hole pairs. For nanoparticles containing a few hundred atoms, we find one isolated spin collective mode below the lowest particle-hole excitation energy, at E_res \approx 0.1 meV, whose spectral weight exhausts the total oscillator strength of the dynamical susceptibility. As the level spacing of the quasi-particle spectrum approaches E_res, the collective mode couples strongly with discrete particle-hole excitations, as evinced by the fragmentation of the collective mode peak in the susceptibility. In this regime the distinction between the two types of excitations is no longer meaningful, with important consequences for the interpretation of recent single-electron tunneling experiments.