Universality in Voltage-driven Nonequilibrium Phase Transitions

We consider the nonequilibrium ferromagnetic transition of a mesoscopic sample of a resistive Stoner ferromagnet coupled to two paramagnetic leads. The transition is controlled by either the lead temperature T or the transport voltage V applied between the leads. We calculate the T and V dependence of the magnetization. For systems with a flat density of states we find as an exact result that even at finite bias the magnetization does not depend on the position along the sample axis, although the charge density and other quantities do vary. This may be relevant for possible spintronics applications. In addition, we establish a generalized control parameter in terms of T and V which allows for a universal description of the temperature- and voltage-driven transition.