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 tunnel ing. Two different relaxation processes are found in the correlations betwe en spin-polarized tunneling currents: low-frequency spin fluctuations and h igh-frequency charge fluctuations. Spin accumulation in strongly asymmetric junctions is shown to lend 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. [S 0163-1829(99)09841-0].