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Spin Filtering: towards total spin polarization
Spin filtering (SF) takes place due to the discriminative tunneling probabilities for spin-up and spin-down electrons through a magnetictunnel barrier and can result in highly spin polarized tunnel current. This is in contrast to junctions with conventional magnetic electrodes which give rise to the spin polarized tunnel current through a nonmagnetic barrier. Thus combining two SF barriers in a magnetic tunnel junction (MTJ) can lead to a large magnetoresistance, by tuning the magnetization direction of the two SF layers without the necessity of magnetic electrodes. To demonstrate this phenomenon we use the chalcogenides material EuS, which is a well-established SF material and its magnetic properties are shown to be tunable with deposition temperature [1]. In tunnel junctions consisting of double EuS SF barriers, different coercive fields of the two adjacent SFs are achieved by depositing one EuS layer at room temperature and quench-condensing the other at liquid nitrogen temperature. A thin Al2O3 spacer layer is introduced in between the two SF layersto prevent direct magnetic coupling. Tunnel magnetoresistance (TMR) up to 60% with well-defined spin-parallel and -antiparallel states is obtained this way, and the resistance change originates completely within the SF/I/SF composite tunnel barrier, with no ferromagnetic electrodes involved. The novel nonmonotonic and asymmetric bias behavior in magnetoresistance can be qualitatively modeled in the framework of WKB approximations [2].
[1] Miao, Moodera, APL 94, 18xxxx (May, 2009).
[2] Miao, Müller, Moodera, PRL 102, 076601 (2009).
Contact person: Dr. Guoxing Miao
Illustration of the behavior of double SF MTJs with (in nm): 1.5EuS / 0.6Al2O3 / 3EuS. (a) TMR bias dependence; (b) R vs. H loops; (c) R vs. T in P-state.
