Magnon-assisted transport and thermopower in ferromagnet-normal metal tunnel junctions

Abstract

We develop a theoretical model of magnon-assisted transport in a mesoscopic tunnel junction between a ferromagnetic metal and a normal (non-magnetic) 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 $P({\Pi }_{\uparrow (\downarrow )},{\Pi }_N)$, $0\le P\le 1$, depends on the relative sizes of the majority ${\Pi }_{\uparrow }$ and minority ${\Pi }_{\downarrow }$ band Fermi surface in the ferromagnet and of the Fermi surface of the normal metal ${\Pi }_N$. 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, $S\sim -(k_B/e)(k_{B}T/{\omega }_D{)}^{3/2}P({\Pi }_{\uparrow (\downarrow )},{\Pi }_N)$, where the temperature dependent factor $(k_{B}T/{\omega }_D{)}^{3/2}$ 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 ${\omega }_D$ is the magnon Debye energy.

7 pages, 2 eps figures