Large magnetoresistance in hybrid spin filter devices

Spin electronic ("spintronic") devices, based on utilizing the spin as well as the charge of electrons, open up an entirely new class of electronics. Such devices could include nonvolatile magnetic memories, re-programmable logic, and quantum computers. One thing hampering the development of spin electronic devices so far is the lack of sufficiently polarized (nearing 100% spin polarization) current sources for spin injection into semiconductors. So-called "half-metallic ferromagnets" would circumvent this problem, but true half-metals have proven extremely difficult to realize in practice. However, the phenomenon of spin filtering may also be exploited to create near 100% polarization. Here we propose and demonstrate a different approach, combining spin filter tunnel barriers and spin-dependent tunneling, similar to a device proposed by Worledge et al. (2000). The combination of a non-magnetic electrode with a spin filter tunnel barrier is used to effectively mimic a half-metallic tunneling electrode and achieve nearly 100% spin polarization. Using this, "artificial half-metal" bilayer, we additionally use a second magnetic electrode, creating a nonmagnetic metal/ferromagnetic insulator/ferromagnetic metal (M-FI-F) device. We utilize EuS as the magnetic insulator, with Gd ferromagnetic and Al nonmagnetic electrodes. The, tunnel current in this case depends on the relative magnetization orientation of the EuS filter and the Gd "analyzer," in analogy to a half-metallic ferromagnet/insulator/ferromagnet tunnel junction. The spin filtering in this configuration yields a previously unobserved magnetoresistance effect, exceeding 100%, suggesting a filtering efficiency close to 100%. The present scheme would also circumvent impedance mismatch problems with semiconducting counter electrodes, and thus potentially allow spin injection from even a non-magnetic metal into a semiconductor.