Abstract
Resonant tunneling can lead to inverse tunnel magnetoresistance when impurity levels rather than direct tunneling dominate the transport process. We fabricated hybrid magnetic tunnel junctions of CoFe/LiF/EuS/Ti, with an epitaxial LiF energy barrier joined with a polycrystalline EuS spin-filter barrier. Due to the water solubility of LiF, the devices were fully packaged in situ. The devices showed sizeable positive TMR up to 16% at low bias voltages but clearly inverted TMR at higher bias voltages. The TMR inversion depends sensitively on the thickness of LiF, and the tendency of inversion disappears when LiF gets thick enough and recovers its intrinsic properties.
Highlights
A traditional Magnetic tunnel junction consists of two ferromagnetic layers separated by a thin insulator barrier, such as MgO or Al2O3
In a standard tunneling magnetoresistance (TMR) device, we would expect the AP configuration to have higher resistance because of the spin misalignment, the TMR ratio is positive in most cases
As many theoretical and experimental studies have already explored, the sign of TMR can be affected by many factors: 1) the intrinsic spin polarization of certain ferromagnets being minority spin dominated, such as Fe3O4;3 ii) the mobility of sp- or d-electrons being different, rendering the dominant spins in transport different from the dominant spins populating the Fermi level, such as Co coupled with Al2O3 barriers;[4] iii) the effect of bonding near the Fermi level at the metal-oxide interface, such as that between Co and SrTiO3;5 iv) resonant tunneling due to the presence of impurity levels inside the tunnel barrier.[6]
Summary
A traditional Magnetic tunnel junction consists of two ferromagnetic layers (electrodes) separated by a thin insulator barrier, such as MgO or Al2O3. Resonant TMR inversion in LiF/EuS based spin-filter tunnel junctions We fabricated hybrid magnetic tunnel junctions of CoFe/LiF/EuS/Ti, with an epitaxial LiF energy barrier joined with a polycrystalline EuS spin-filter barrier.
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