A tunnel junction between a ferromagnet and a normal metal: magnon-assisted contribution to thermopower and conductance

Abstract

We develop a theoretical model of magnon-assisted transport in a mesoscopic tunnel junction between a ferromagnetic metal and a normal (nonmagnetic) metal. The current response to a bias voltage is dominated by the contribution of elastic processes rather than magnon-assisted processes and the degree of spin polarization of the current, parameterized by a function P(Π ↑(↓),Π N), 0⩽ P⩽1, depends on the relative sizes of the majority Π ↑ and minority Π ↓ band Fermi surface in the ferromagnet and of the Fermi surface of the normal metal Π N. On the other hand, magnon-assisted tunneling gives the dominant contribution to the current response to a temperature difference across the junction. The resulting thermopower is large, S∼−( k B/ e)( k B T/ ω D) 3/2 P(Π ↑(↓),Π N), where the temperature dependent factor ( k B T/ ω D) 3/2 reflects the fractional change in the net magnetization of the ferromagnet due to thermal magnons at temperature T (Bloch's T 3/2 law) and ω D is the magnon Debye energy.