  The magnetization detection and switching of an ultrasmall Stoner nanograin
in a non-local spin valve (NLSV) device is studied theoretically. With the help
of the rate equations, a unified description can be presented on the same
footing for the NLSV signal that reads out the magnetization, and for the
switching process. The setup can be viewed as that the grain is connected to
two non-magnetic leads via sequential tunneling. In one lead, the chemical
potentials for spin-up and -down electrons are split due to the spin injection
in the NLSV. This splitting (or the spin bias) is crucial to the NLSV signal
and the critical condition to the magnetization switching. By using the
standard spin diffusion equation and parameters from recent NLSV device, the
magnitude of the spin bias is estimated, and found large enough to drive the
magnetization switching of the cobalt nanograin reported in earlier
experiments. A microscopic interpretation of NLSV signal in the sequential
tunneling regime is thereby raised, which show properties due to the ultrasmall
size of the grain. The dynamics at the reversal point shows that there may be a
spin-polarized current instead of the anticipated pure spin current flowing
during the reversal due to the electron accumulation in the floating lead used
for the readout of NLSV signal.
