Tokyo Tech: Using Berry Phase Monopole Engineering for High-Temperature Spintronic Devices

Spin-orbit torque (SOT), an important phenomenon for developing fast and low-power spintronic devices, can be enhanced through Berry phase monopole engineering at high temperatures, shown in a new study by Tokyo Tech (Tokyo Institute of Technology) researchers.

In this study, the temperature dependence of the intrinsic spin Hall effect of TaSi₂ was investigated. The results suggest that Berry phase monopole engineering is an effective strategy for achieving high-temperature SOT spintronic devices.

Spintronic devices are electronic devices that utilize the spin of electrons (an intrinsic form of angular momentum possessed by the electron) to achieve high-speed processing and low-cost storage. In this regard, spin-transfer torque is a key phenomenon that enables fast and low-power spintronic devices. Recently, however, SOT has emerged as a promising alternative to spin-transfer torque.

Many studies have investigated the origin of SOT, showing that in non-magnetic materials, a phenomenon called the spin Hall effect (SHE) is key to achieving SOT. In these materials, the existence of a ‘Dirac band’ structure, a specific arrangement of electrons in terms of their energy, is important to achieving large SHE. This is because the Dirac band structure contains ‘hot spots’ for the Berry phase, a quantum phase factor responsible for the intrinsic SHE. Thus, materials with suitable Berry phase hot spots are key to engineering the SHE.

In this context, the material tantalum silicide (TaSi₂) is of interest as it has several Dirac points near the Fermi level in its band structure, suitable for practicing Berry phase engineering. To demonstrate this, a team of researchers, led by Pham Nam Hai, associate professor, department of electrical and electronic engineering, Tokyo Institute of Technology (Tokyo Tech), Japan, recently investigated the influence of Dirac band hot spots on the temperature dependence of SHE in TaSi₂.

“Berry phase monopole engineering is an interesting avenue of research as it can give rise to efficient high-temperature SOT spintronic devices such as the magneto-resistive random-access memory,” explains Dr. Hai about the importance of their study.

Their findings were published in the journal Applied Physics Letters

Through various experiments, the team observed that the SOT efficiency of TaSi₂ remained almost unchanged from 62K to 288K, which was similar to the behavior of conventional heavy metals. However, upon increasing the temperature further, the SOT efficiency suddenly increased and nearly doubled at 346K. Moreover, the corresponding SHE also increased in a similar fashion. Notably, this was quite different from the behavior of conventional heavy metals and their alloys. Upon further analysis, the researchers attributed this sudden increase in SHE at high temperatures to Berry phase monopoles.

“These results provide a strategy to enhance the SOT efficiency at high temperatures via Berry phase monopole engineering,” highlights Hai.

Indeed, their study highlights the potential of Berry phase monopole engineering to effectively use the SHE in non-magnetic materials, and provides a new pathway for the development of high-temperature, fast, and low-power SOT spintronic devices.

Article: Enhanced spin Hall effect at high temperature in non-centrosymmetric silicide TaSi₂ driven by Berry phase monopoles

Applied Physics Letters has published an article written by Ken Ishida, Takanori Shirokura, and Pham Nam Hai, Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, Japan.

Abstract: “We demonstrate the concept of Berry phase monopole engineering of the spin Hall effect in non-centrosymmetric silicide TaSi₂. We show that while the effective damping-like spin–orbit torque (SOT) efficiency θ DL eff of TaSi₂ is nearly unchanged from 62 to 288 K (−0.049 to −0.069), θ DL eff suddenly increases at high temperatures and becomes nearly double (−0.12) at 346 K. The corresponding intrinsic spin Hall conductivity σ DL eff significantly increases at high temperatures, which can be attributed to the increasing contribution from the four degenerate points near the Fermi level via thermal excitation. Our results provide a strategy to enhance θ DL eff at high temperatures via Berry phase monopole engineering and pave the way for SOT spintronic devices working at high temperatures.“