Fluctuations of g factors of discrete levels in ferromagnetic nanoparticles*

It has been known that the interplay between electron-electron interactions and spin-orbit scattering can cause a wide distribution of g factors in tunneling spectra of metallic nanoparticles, including g-factors much larger than 2 if electron-electron interactions are strong. Here, we present our studies of single Co nanoparticles in Al/Al$_2$O$_3$/(Co nanoparticles)/Al$_2$O$_3$/Al tunnel junctions using electron tunneling spectroscopy at mK-temperatures. The g factor of discrete energy levels exhibits significant difference between minority-spin and majority-spin levels. We have clearly observed large g factors ($\approx$ 6) in one sample at magnetic field greater than 4T, suggesting $\Delta S=3/2$ in the tunneling transition, $S$ is the magnitude of the spin. We will present the latest results on tunneling junctions containing Ni, Permalloy or Gd nanoparticles, which have weaker magnetic anisotropy fluctuations.