Theory of tunneling spectroscopy in ferromagnetic nanoparticles

We present a theory of the low-energy excitations of a ferromagnetic metal nanoparticle. In addition to the particle-hole excitations, which occur in a paramagnetic metal nanoparticle, we predict a branch of excitations involving the magnetization-orientation collective coordinate. Tunneling matrix elements are in general sizable for several different collective states associated with the same band configuration. We point out that the average change in ground state spin per added electron differs from noninteracting quasiparticle expectations, and that the change in the spin polarization, due to Zeeman coupling, is strongly influenced by Coulomb blockade physics.