  We discuss a spin-transfer torque device, where the role of the soft
ferromagnetic layer is played by a magnetic particle or a magnetic molecule, in
weak tunnel contact with two spin polarized leads. We investigate if the
magnetization of the particle can be manipulated electronically, in the regime
where the critical current for magnetization switching is negligibly weak,
which could be due to the reduced particle dimensions. Using master equation
simulations to evaluate the effects of spin-orbit anisotropy energy
fluctuations on spin-transfer, we obtain reliable reading and writing of the
magnetization state of such magnetic particle, and find that the device relies
on a critical voltage rather than a critical current. The critical voltage is
governed by the spin-orbit energy shifts of discrete levels in the particle.
This finding opens a possibility to significantly reduce the power dissipation
involved in spin-transfer torque switching, by using very small magnetic
particles or molecules.
