Giant magnetothermopower of magnon-assisted transport in ferromagnetic tunnel junctions

Abstract

We present a theoretical description of the thermopower due to magnon-assisted tunneling in a mesoscopic tunnel junction between two ferromagnetic metals. The thermopower is generated in the course of thermal equilibration between two baths of magnons, mediated by electrons. For a junction between two ferromagnets with antiparallel polarizations, the ability of magnon-assisted tunneling to create thermopower $S_{AP}$ depends on the difference between the size ${\Pi }_{\uparrow ,\downarrow }$ of the majority and minority band Fermi surfaces and it is proportional to a temperature dependent factor $(k_{B}T/{\omega }_D{)}^{3/2}$ where ${\omega }_D$ is the magnon Debye energy. The latter factor reflects the fractional change in the net magnetization of the reservoirs due to thermal magnons at temperature $T$ (Bloch's $T^{3/2}$ law). In contrast, the contribution of magnon-assisted tunneling to the thermopower $S_P$ of a junction with parallel polarizations is negligible. As the relative polarizations of ferromagnetic layers can be manipulated by an external magnetic field, a large difference $\Delta S=S_{AP}-S_P\approx S_{AP}\sim -(k_B/e)f({\Pi }_{\uparrow },{\Pi }_{\downarrow })(k_{B}T/{\omega }_D{)}^{3/2}$ results in a magnetothermopower effect. This magnetothermopower effect becomes giant in the extreme case of a junction between two half-metallic ferromagnets, $\Delta S\sim -k_B/e$.

9 pages, 4 eps figures

References in corpus (1)

• Subgap transport in ferromagnet-superconductor junctions due to magnon-assisted Andreev reflection

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