Magnetic tunnel junctions consisting of a periodic grating barrier and two half-metallic electrodes

Abstract

We have developed a spintronic theory for magnetic tunnel junctions consisting of a single-crystal barrier and two half-metallic ferromagnetic electrodes. Radically different from the conventional theories, the barrier is now regarded as an optical diffraction grating, and treated by the traditional optical scattering method, i.e. Bethe theory and two-beam approximation. After tunneling, the electrons can thus possess high coherence. In the case that the electrodes are both half-metallic, the conventional theories give an infinite tunneling magnetoresistance (TMR). By contrast, in the Bethe theory and two-beam approximation, there can exist the scattering channels of nonconservation of energy. Therefore, the TMR can still be far away from infinity, which is in accordance with experiments. Also, we find that, due to the half-metallicity of the electrodes, the parallel conductance oscillates with temperature whereas the antiparallel conductance will increase other than oscillate with temperature. That is in agreement with experiments, too. Finally, two applications of the present theory are discussed with regard to the material design and engineering: one is how to choose appropriate materials for the barrier to realize infinite TMR; the other is a criterion for judging whether a material is half-metallic or not.

Tunnel junctions with moiré superlattice as barrier

“…According to the standard methods in diffraction physics, i.e. Bethe theory and two-beam approximation [44][45][46][47][48], the transmitted wave function ψ(r) can be obtained as follows,…”

“…Previously, a spintronic theory for the magnetic tunnel junctions (MTJs) with single-crystal barrier has been developed by us [44][45][46][47]. The theory is founded on the traditional optical scattering theory [48].…”

“…This coherence is the main mechanism responsible for the MTJs with single-crystal barrier. So far, the theory has successfully explained the basic properties of the MgO-based MTJs [44][45][46][47].…”

“…Physically, a moiré superlattice is nearly insulating along the interlayer, or longitudinal, direction [49], therefore, when discussing the longitudinal transportation of electrons, it can be regarded as a tunneling barrier, and obviously, the so-called interlayer conductance is a kind of tunneling conductance [40]. Besides, as stated above, every periodic atomic layer within a moiré superlattice can be considered as a diffraction-grating in optics [44][45][46][47]. Therefore, a moiré superlattice is essentially in physics the same as a single-crystal tunneling barrier, the previous tunneling theory for MTJs [44][45][46][47] can be transplanted to handle moiré superlattices.…”

“…Besides, as stated above, every periodic atomic layer within a moiré superlattice can be considered as a diffraction-grating in optics [44][45][46][47]. Therefore, a moiré superlattice is essentially in physics the same as a single-crystal tunneling barrier, the previous tunneling theory for MTJs [44][45][46][47] can be transplanted to handle moiré superlattices. That constitutes the main object of this paper.…”