Shot noise in ferromagnetic single-electron tunneling devices

Frequency-dependent current noise in ferromagnetic double junctions with a Coulomb blockade is studied theoretically in the limit of sequential tunneling. Two different relaxation processes are found in the correlations between spin-polarized tunneling currents: low-frequency spin fluctuations and high-frequency charge fluctuations. Spin accumulation in strongly asymmetric junctions is shown to lead to a negative differential resistance. We also show that large spin noise activated in the range of negative differential resistance gives rise to a significant enhancement of the current noise.