We report large tunnel magnetoresistance (TMR) ratios of up to 219% at 300 K and 366% at 3.7 K obtained for a high-quality fully epitaxial double-barrier magnetic tunnel junction (MTJ) composed of Fe/MgO/Fe/γ-Al2O3/Nb-doped SrTiO3. The obtained TMR ratios are among the highest values reported in Fe/MgO/Fe structures. This result may be attributed to the small in-plane wave vectors of the tunneling electrons injected from the Nb-doped SrTiO3 electrode with a small carrier density, demonstrating good compatibility between the Fe-based MTJ and SrTiO3.

Developing technology to realize oxide-based nanoscale planar integrated circuits is in high demand for next-generation multifunctional electronics. Oxide circuits can have a variety of unique functions, including ferromagnetism, ferroelectricity, multiferroicity, superconductivity, and mechanical flexibility. In particular, for spin-transistor applications, the wide tunability of the physical properties due to the presence of multiple oxide phases is valuable for precise conductivity matching between the channel and ferromagnetic electrodes. This feature is essential for realistic spin-transistor operations. Here, a substantially large magnetoresistance (MR) ratio of up to ≈140% is demonstrated for planar-type (La,Sr)MnO 3 (LSMO)-based spin-valve devices. This MR ratio is 10-100 times larger than the best values obtained for semiconductor-based planar devices, which have been studied over the past three decades. This structure is prepared by implementing an artificial nanolength Mott-insulator barrier region using the phase transition of metallic LSMO. The barrier height of the Mott-insulator region is only 55 meV, which enables the large MR ratio. Furthermore, a successful current modulation, which is a fundamental functionality for spin transistors, is shown. These results open up a new avenue for realizing oxide planar circuits with unique functionalities that conventional semiconductors cannot achieve.

In conventional magnetic tunnel junctions (MTJs), the tunnel magnetoresistance (TMR) monotonically decreases with increasing bias voltage, which limits the bias voltage range for the operation of MTJs. In our study, using double-barrier MTJs composed of Fe/MgO/Fe/γ-Al2O3 grown on a Nb-doped SrTiO3 substrate, we demonstrate unconventional bias dependences of the TMR, in which the TMR ratio increases with increasing bias voltage. We reveal that this behavior originates from the sharp giant resistance peak near zero bias likely induced by the Coulomb blockade effect via Fe impurities in γ-Al2O3, which are diffused from the Fe layer. The observed TMR ratio is 23% at a bias voltage of −4 V at 3.5 K, which is a very high value in this large bias voltage range. Our results offer a novel way to improve the bias voltage dependence of TMR.