The achievement represents a big step forward in the development of spin-orbit torque magnetoresistive random-access memory (SOT-MRAM) devices with the potential to replace existing memory technologies.
The research team, led by Pham Nam Hai at the Department of Electrical and Electronic Engineering, Tokyo Institute of Technology (Tokyo Tech), has developed thin films of BiSb for a topological insulator that simultaneously achieves a colossal spin Hall effect and high electrical conductivity.
Their study could accelerate the development of high-density, ultra-low power, and ultra-fast non-volatile memories for Internet of Things (IoT) and other applications.
The BiSb thin films achieve a spin Hall angle of approximately 52, conductivity of 2.5 x 105 and spin Hall conductivity of 1.3×107 at room temperature. Notably, the spin Hall conductivity is two orders of magnitude greater than that of bismuth selenide (Bi2Se3), reported in 2014.
Up until now, the search for suitable spin Hall materials for next-generation SOT-MRAM devices has been confronted with the dilemma of using either heavy metals such as platinum, tantalum and tungsten, that have high electrical conductivity but a small spin Hall effect. Topological insulators investigated to date have a large spin Hall effect but low electrical conductivity.
The BiSb thin films satisfy both requirements and do so at room temperature. This raises the real possibility that BiSb-based SOT-MRAM could outperform the existing spin-transfer torque (STT) MRAM technology.
"As SOT-MRAM can be switched one order of magnitude faster than STT-MRAM, the switching energy can be reduced by at least two orders of magnitude," says Pham. "Also, the writing speed could be increased 20 times and the bit density increased by a factor of ten."
If scaled up successfully, BiSb-based SOT-MRAM could drastically improve upon its heavy metal-based counterparts and even become competitive with dynamic random access memory (DRAM), the dominant technology of today.
BiSb has tended to be overlooked by the research community due to its small band gap and complex surface states. However, Pham says: "From an electrical engineering perspective, BiSb is very attractive due to its high carrier mobility, which makes it easier to drive a current within the material."
The thin films were grown using a high-precision method called molecular beam epitaxy (MBE). The researchers discovered a particular surface orientation named BiSb(012), which is thought to be a key factor behind the large spin Hall effect. Pham points out that the number of Dirac cones[6]0 on the BiSb(012) surface is another important factor, which his team is now investigating.
Pham is currently collaborating with industry to test and scale up BiSb-based SOT-MRAM.
"The first step is to demonstrate manufacturability," he says. "We aim to show it's still possible to achieve a strong spin Hall effect, even when BiSb thin films are fabricated using industry-friendly technologies such as the sputtering method."
"It's been over ten years since the emergence of topological insulators, but it was not clear whether those materials could be used in realistic devices at room temperature. Our research brings topological insulators to a new level, where they hold great promise for ultra-low power SOT-MRAM."
