Three-dimensional Rashba spin splitting dominated by out-of-plane spin polarization

Rashba-type spin-orbit coupling is a long-term active topic due to its superiority of magnetic-field-free control of the electron spin. However, numerous Rashba-related works focus on improving the magnitude of the spin splitting that exhibits the traditional in-plane spin polarization. Very few studies concern manipulating the direction of the spin splitting, such as the generation of out-of-plane spin polarization especially in nonmagnetic materials. Intriguingly, this research field has recently become very appealing owing to its great prospects in high-density and energy-efficient spintronic applications. Here, we report a prominent out-of-plane spin polarization in the two-dimensional Dion-Jacobson (DJ) perovskite and reveal that it is the physical origin of the large Rashba spin splitting observed in both experiments and theoretical calculations. Consequently, the spin-split band structures of the studied system must be interpreted by a proposed three-dimensional Rashba model, with the spin polarization mainly along the out-of-plane direction. Moreover, the temperature-dependent Rashba spin splitting shows that both static and dynamical Rashba effects contribute to the total spin splitting, where the out-of-plane spin polarization always dominates the overall splitting. Such a robust generation of this unconventional spin component indicates the significant influence of the in-plane asymmetric crystal field on the spin-orbit interactions. It is worth emphasizing that it is this out-of-plane spin splitting, not the commonly considered in-plane ones, responsible for the reported large band splitting in DJ perovskites. These results present that the studied system may serve as a type of spin source material to explore the attractive out-of-plane spin polarization, which is highly desired for high-density magnetic switching as well as field-free spin-based computing. It also suggests that crystal symmetry engineering can be a promising strategy to manipulate the spin polarization.