  The single-spin memory effect is considered within a minimal polaron model
describing a single-level quantum dot interacting with a vibron and weakly
coupled to ferromagnetic leads. We show that in the case of strong
electron-vibron and Coulomb interactions the rate of spontaneous quantum
switching between two spin states is suppressed at zero bias voltage, but can
be tuned through a wide range of finite switching timescales upon changing the
bias. We further find that such junctions exhibit hysteretic behavior enabling
controlled switching of a spin state. Spin lifetime, current and spin
polarization are calculated as a function of the bias voltage by the master
equation method. We also propose to use a third tunneling contact to control
and readout the spin state.
