We predict a new zero-bias anomaly in the differential conductance through a
quantum dot coupled to two ferromagnetic leads with antiparallel magnetization.
The anomaly differs in origin and properties from other anomalies in transport
through quantum dots, such as the Kondo effect. It occurs in Coulomb-blockade
valleys with an unpaired dot electron. It is a consequence of the interplay of
single- and double-barrier cotunneling processes and their effect on the spin
accumulation in the dot. The anomaly becomes significantly modified when a
magnetic field is applied.Comment: 4 pages, 3 figure