  We explore inelastic cotunneling through a strongly Coulomb-blockaded quantum
dot attached to two ferromagnetic leads in the weak coupling limit using a
generic quantum Langevin equation approach. We first develop a Bloch-type
equation microscopically to describe the cotunneling-induced spin relaxation
dynamics, and then develop explicit analytical expressions for the local
magnetization, current, and its fluctuations. On this basis, we predict a novel
zero-bias anomaly of the differential conductance in the absence of a magnetic
field for the anti-parallel configuration, and asymmetric peak splitting in a
magnetic field. Also, for the same system with large polarization, we find a
negative zero-frequency differential shot noise in the low positive
bias-voltage region. All these effects are ascribed to rapid spin-reversal due
to underlying spin-flip cotunneling.
