Spin polarized tunneling has been extensively explored in modern spintronic studies. In addition to its leading role in tunnel magnetoresistance (TMR), the state-of-art technology for magnetic sensing and nonvolatile memory, it is also a solution to the conductivity mismatch problem for spin polarized transport into semiconducting spin transistors.
Spin filter (SF) tunneling is one of its latest developments. SF occurs due to the high selectivity in quantum tunneling for spin-up and -down electrons through a magnetic tunnel barrier: spin-up electrons cross the barrier easily as they face a lower barrier height, while spin-down electrons are largely filtered out with the higher barrier. As a result the net tunnel current is highly spin polarized. This approach is fundamentally different from traditional spin sources that require ferromagnetic electrodes (such as Co, Fe, Ni etc.) in order to generate the spin current. Combining two such SF barriers in a tunnel junction leads to large TMR without the necessity of magnetic electrodes. Unlike the conventional magnetic tunnel junctions in which both the spin source and the spin detector are the ferromagnetic electrodes, magnetoresistance in these SF tunnel junctions completely originates from the internal tuning of tunnel barrier heights. We demonstrate the first realization of such unconventional tunnel junction using EuS based SF barriers and nonmagnetic electrodes. The novel non-monotonic and asymmetric bias behavior in magnetoresistance can be qualitatively modeled in the framework of WKB approximations. This demonstration of principle clearly shows the potential of SF effect as a novel spin source for spintronic development. In recent work, we further proceed to show its feasibility in direct spin readout when nonequilibrium spin population has been established by way of indirect exchange between Al conduction electrons and localized Eu 4f electrons. A voltage readout at zero current directly matches the chemical potentials of the two spin channels.