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We have grown Fe(0 0 1)/GaO x (0 0 1)/MgO(0 0 1)/Fe(0 0 1) magnetic tunnel junctions (MTJs) with or without in situ annealing after the deposition of GaO x layer and performed structural characterizations by focusing on the formation process of the single-crystalline GaO x . It was found that, even without the in situ annealing, the as-grown GaO x grown on the MgO was mostly single-crystalline except near the surface region (amorphous). The crystallization temperature of the amorphous region was reduced from 500 °C down to 250 °C by depositing the Fe upper electrode (poly-crystalline). It was clarified that the crystallization of the amorphous region near the Fe/GaO x interface caused the realignments of the crystal grains in the poly-crystalline Fe upper electrode, and, as a result, the fully epitaxial Fe/GaO x /MgO/ Fe structure is eventually formed. All the MTJs showed high tunneling magnetoresistance ratios (about 100%) at room temperature, which was almost independent of the formation temperature of the single-crystalline GaO x .

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 (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 kΩm 2 ). Even with the same top and bottom electrodes (Fe), the obtained 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 0 without significant increase in the RA. In addition, we demonstrate that a zero-bias anomaly in the 2 / 34 tunnel 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.

We have grown an epitaxial MgO/Ga 2 O 3 heterostructure on a MgO(001) substrate by molecular beam epitaxy. Crystallographic studies revealed the out-of-plane and in-plane crystal orientations between the MgO overlayer and the Ga 2 O 3 layer, which were MgO(001) k β-Ga 2 O 3 (001) and MgO[100] k β-Ga 2 O 3 ½02 1, respectively. The valence band offset at the MgO/β-Ga 2 O 3 interface was determined to be 0.19 eV (type-II band alignment) by X-ray photoelectron spectroscopy, resulting in a large conduction band offset of 2.7-3.2 eV. These results indicate that MgO is a promising potential barrier for electrons in an epitaxial MgO/Ga 2 O 3 multilayered structure.