Tunneling Via Individual Electronic States in Ferromagnetic Nanoparticles

We measure electron tunneling via discrete energy levels in ferromagnetic cobalt particles less than 4 nm in diameter, using non-magnetic electrodes. Due to magnetic anisotropy, the energy of each tunneling resonance shifts as an applied magnetic field rotates the particle's magnetic moment. We see both spin-increasing and decreasing tunneling transitions, but we do not observe the spin degeneracy at small magnetic fields seen previously in non-magnetic materials. The tunneling spectrum is denser than predicted for independent electrons, possibly due to spin-wave excitations.