  We present a complete theory of the spin torque phenomena in a ultrasmall
nanomagnet coupled to non-collinear ferromagnetic electrodes through tunnelling
junctions. This model system can be described by a simple microscopic model
which captures many physical effects characteristic of spintronics: tunneling
magneto resistance, intrinsic and transport induced magnetic relaxation,
current induced magnetization reversal and spin accumulation. Treating on the
same footing the magnetic and transport degrees of freedom, we arrive at a
closed equation for the time evolution of the magnetization. This equation is
very close to the Landau-Lifshitz-Gilbert equation used in spin valves
structures. We discuss how the presence of the Coulomb blockade phenomena and
the discretization of the one-body spectrum gives some additional features to
the current induced spin torque. Depending on the regime, the dynamic induced
by the coupling to electrode can be viewed either as a spin torque or as a
relaxation process. In addition to the possibility of stabilizing uniform spin
precession states, we find that the system is highly hysteretic: up to three
different magnetic states can be simultaneously stable in one region of the
parameter space (magnetic field and bias voltage).We also discuss how the
magneto-resistance can be used to provide additional information on the
non-equilibrium peaks present in the nanomagnet spectroscopy experiments.
