Tunneling Magnetoresistance and Spin-Dependent Diode Performance in Fully Epitaxial Magnetic Tunnel Junctions with Rock-salt Type ZnO/MgO

We fabricate fully epitaxial Fe/ZnO/MgO/Fe magnetic tunnel junctions (MTJs) with a bilayer tunnel barrier, in which ZnO has a metastable rock-salt crystal structure. We observe a high magnetoresistance ratio up to 96% at room temperature (RT) and find that these MTJs have asymmetric current-voltage characteristics, and their rectifying performances are largely dependent on the magnetization alignments of the Fe electrodes. Diode responsibilities at a zero-bias voltage ($\beta_{0}$), which is an important performance index for harvesting applications, are observed up to 1.3 A/W at RT in the antiparallel alignment of the magnetizations while maintaining rather low resistance-area (RA) products (a few tens of k${\Omega\mu}$m$^2$). Even with the same top and bottom electrodes (Fe), the obtained $\beta_{0}$ values are comparable to those of reported high-performance tunnel diodes consisting of amorphous bilayer tunnel barriers with polycrystalline dissimilar electrodes. This strongly suggests that the epitaxial ZnO/MgO bilayer tunnel barrier is effective for enhancing the $\beta_{0}$ without significant increase in the RA. In addition, we demonstrate that a zero-bias anomaly in thetunnel conductance, which originates from the magnon excitations at the Fe/barrier interfaces, plays a crucial role in observed spin-dependent diode performance. The results indicate that a fully epitaxial MTJ with a bilayer tunnel barrier is a promising candidate to establish a high-performance high-frequency rectifying system.