Significant enhancement of Rashba spin-orbit interaction using metastable heavy metal interface

Spin-orbit torque (SOT) is a fundamental phenomenon in spintronics, facilitating efficient control of magnetic states in advanced device architectures. The discovery of a large spin Hall angle in metastable heavy metals has significantly enhanced the potential of SOT-based devices. However, the inherently low electrical conductivity of these materials poses a critical obstacle to achieving efficient device performance. To overcome this limitation, we systematically investigate interfacial Rashba spin-orbit interactions at the interfaces between metastable heavy metals and highly conductive normal metals through a combination of theoretical and experimental approaches. First-principles calculations demonstrate that interfaces involving β-phase tungsten (β-W) and tantalum (β-Ta) exhibit substantially enhanced Rashba interactions compared to their α-phase counterparts. This enhancement is attributed to modifications in the interfacial electric field. Furthermore, we provide the first clear evidence of a positive correlation between the Rashba parameter and the interfacial work-function. These findings highlight the potential of engineering metastable interfacial structures to optimize spin-orbit coupling, enabling precise spin current control. This advancement offers a promising pathway for the development of high-performance spintronic devices and the realization of next-generation spin-based electronics.