Room-temperature spin filtering in a<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="normal">Co</mml:mi><mml:msub><mml:mi mathvariant="normal">Fe</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>4</mml:mn></mml:msub><mml:mo>∕</mml:mo><mml:mi mathvariant="normal">Mg</mml:mi><mml:msub><mml:mi mathvariant="normal">Al</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>4</mml:mn></mml:msub><mml:mo>∕</mml:mo><mml:msub><mml:mi mathvariant="normal">Fe</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>magnetic tunnel barrier

By growing ultrathin and high quality $\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}∕\mathrm{Mg}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}∕{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ epitaxial multilayers, we have demonstrated a room temperature spin filter junction, which could open the way for highly polarized spin injection, single-qubit spintronic measurements, magnetic field sensing, and other applications. In these devices the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ layer functions as a source of spin polarized electrons while the $\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}∕\mathrm{Mg}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ double barrier functions as a spin filter. The current-voltage curves depend on the relative orientation of the magnetization of the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ and $\mathrm{Co}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ layers and display a large magnetoresistive effect. The inferred net spin polarizations produced by the junctions typically exceeded 70%.