First principles design of multifunctional spintronic devices based on super narrow borophene nanoribbons.

Borophene, as a new material with various configurations, has attracted significant research attention in recent years. In this study, the electronic properties of a series of χ-type borophene nanoribbons (BNRs) are investigated using a first-principles approach. The results show that the width and edge pattern of the nanoribbons can effectively tune their electronic properties. Notably, a super-narrow χ-type BNR is found to be ferromagnetic and exhibits half-metallic properties. Based on these findings, a χ-type borophene nanojunction is proposed, and its spintronic transport properties are investigated using the non-equilibrium Green's function method combined with density functional theory. The results demonstrate that the nanojunction exhibits excellent spin filtering capabilities under moderate bias voltages when two electrodes are spin parallel. Furthermore, when the spin configurations of the two electrodes are changed from parallel to antiparallel, both the spin-up and spin-down currents demonstrate significant rectifying effects with reversed rectifying directions, and the rectification ratios reach up to 10. Consequently, opposite spin filtering polarizations are obtained for spin-up and spin-down currents. More intriguingly, such bipolar spin filtering and spin rectifying effects can also be achieved by compressing the width of either electrode by 5%, while maintaining the spin parallel configuration of two electrodes. Additionally, the resistance of the device is largely modulated by altering the magnetic configurations of the electrodes or narrowing the width of either electrode, leading to a giant magnetoresistance effect and a piezoresistance effect. These findings open up new avenues for future applications of borophene in spintronic nanodevices.