Experiments on giant thermal magnetogalvanic efects in magnetic tunnel junctions

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Spin-dependent Seebeck effects have been discovered in lateral devices and giant-magnetoresistance (GMR) nanopillar junctions. Thermally driven spin accumulation leads to a magnetothermopower over length on the scale of the spin diffusion. Spin-wave excitations also contribute to thermally driven magnetothermopower over length on the order of millimeters. We will focus on giant-tunneling-magnetoresistance (TMR) devices (Heusler/MgO/Heusler and Co-Fe-B/MgO/Co-Fe-B) with high TMR values of >200%. First, the strong spin asymmetry of the thermal tunneling current leads to a spin-dependent shift of the chemical potential. In joint collaboration with a theory project, high magnetothermopowers (MTEPs) of ~60 μV have been predicted to and show peculiar temperature dependence. Second, inelastic tunneling processes may open tunneling channels in half-metallic junctions, mirroring the different magnon temperatures on both side of the tunnel barrier. To determine the spin-dependent Seebeck coefficients in giant tunneling magnetoresistance (TMR) devices, we will develop techniques to generate temperature gradients in tunnel junctions by resistive and optical heating. In addition, we will lay the foundation to perform not only static heating, which allows only very small temperature gradients of a one tenth of a degree temperature drop at the tunnel barrier, but also dynamic experiments with nanosecond electrical pulses and femtosecond optical excitation. Our aim is to understand, in close collaboration with our theoretical collaborator, the origin of high spin-dependent Seebeck coefficients arising from strong spin asymmetries of the tunneling probability in giant tunneling magnetoresistance devices.