Breakthrough discovery in high bias spintronics

In most spintronics and related devices, the giant tunneling magnetoresistance (TMR) effects form the basis of electron transfer mechanisms. This effect is a consequence of the spin-resolved electron symmetry tunneling and has been observed in Fe/MgO/Fe and similar magnetic tunnel junctions (MTJs). The most notable contribution of this effect is that it enables room temperature functionality in spin electronics, widening its application range across different fields. Unfortunately, the breakdown of the electron symmetry filtering under applied bias has remained the greatest challenge in most spintronics. It is mainly attributed to the effects of interface diffusion, surface states and roughness. Its worst impact is the significant reduction of (TMR) above 0.5V, which strongly limits the application range of MTJs.

Different strategies have been developed to circumvent this problem. For instance, ferromagnetic electrode/barrier optimization and sequential tunneling in double-barrier MTJs have been used to compensate for the inherent shortcoming of the MTJs. Another method that has increasingly attracted significant research attention is improving the conductance property of specific biases through resonant tunneling via quantum well states. This is carried out in thin layers of MTJs. Unfortunately, previous findings have shown that this approach is largely effective in low-temperature operating conditions. The associated technical problems in the symmetry confinement are ferromagnetic monolayers are another challenge. Moreover, similar approaches capable of producing (TMR) modulation also have numerous limitations. Thus, effective approaches for improving the tunnel magnetoresistance under electric bias at room temperature are highly desirable.

To over the above challenges, PhD candidate César González-Ruano and Professor Farkhad Aliev from Universidad Autónoma de Madrid in collaboration with Professor Coriolan Tiusan and Professor Michel Hehn from Nancy Universitè developed a breakthrough and fundamentally different strategy for improving the room-temperature magnetoresistance in hybrid MTJs under electric bias. This approach utilized spin-orbit coupling (SOC) controlled interfacial states in vanadium to allow tunneling at low biases. The main aim of the study was to facilitate the development of energy-efficient room temperature spintronic devices. Their work is currently published in the journal, Advanced Electronic Materials.

The research team demonstrated that the newly introduced pathway enabled a strong room temperature TMR boost under the applied bias in V/MgO/Fe/MgO/Fe/Co hybrids. Unlike V(001), the SOC-controlled interfacial states played a critical role in enhancing tunneling to Fe(001) at low biases. An output parameter value exceeding that of a single barrier was reported at biases above 0.5V. Additionally, the high room temperature above 0.8V was the highest to be ever reported. Furthermore, the strong increase of the TMR with bias was experimentally modeled using two nonlinear series resistances. It was observed that the low bias conductance of the first element (V/MgO/Fe) was primarily improved by the SOC-controlled interfacial states and the symmetry of the protected symmetry states. In contrast, the conductance of the second element (Fe/MgO/Fe/Co) depended on the alignment of its magnetic states.

In summary, a novel and breakthrough hybrid magnetic tunnel junctions comprising standard MTJs sequentially coupled with a strong nonlinear part was proposed to boost the TMR effect under applied bias. This configuration enabled a robust (TMR) enhancement with a 0.5V bias in a wide range of temperatures that were characterized by unprecedented behavior for high output voltage for room temperature spintronics. This unprecedented behavior was illustrated using a simplified model that consisted of two nonlinear series devices and magnetic-state-dependent sequential tunneling. The results demonstrated the significance of electron-symmetry-protected surface states found within metallic states. In a statement to Advances in Engineering, Professor Farkhad Aliev stated that their findings would contribute to the exploration of previously unexplored spintronic device schemes, which will enhance their applicability towards higher biases.

The work has been supported by Spanish National Science Foundation (RTI2018-095303-B-C55) and Madrid Autonomy Region (NANOMAGCOST-CM Ref. P2018/NMT-4321) grants. Professor Aliev has been supported by Echegaray professorship excellence grant.