Innovative technologies that can accommodate the burgeoning volume of digital information the world generates, without compromising on computing performance or sustainability, are increasingly needed. Such innovations require the development of memory and storage solutions that strike a balance between cost-effectiveness, energy efficiency, steadfast stability, and scalability.

Spintronics-based memory devices, which leverage a magnetic property of electrons known as electron spin, provide an attractive solution to meet these demands. Unlike conventional electronics, which primarily rely on the manipulation of electron charge, spintronics devices use electron spin, an intrinsic angular momentum of electrons that determines their alignment. Manipulating the electron spin can be used to encode and process information. Such devices show tremendous potential to create more efficient, faster, and low-power memory technologies.

2D materials for spintronic memory device

Seeking such solutions is Professor Yang Hyunsoo from NUS Electrical and Computer Engineering. An expert in spintronics-based memory devices, terahertz spintronics and unconventional computing, Prof Yang turned to two-dimensional (2D) materials in his search for materials that could form the foundation of future spintronics-based technologies.

In a recent article, published recently in Nature Materials, Prof Yang explored the potential of specific 2D materials in spintronic-based memory devices.

2D materials, are atomically thin materials, whose properties can be easily tuned by external factors, exist in various forms, including metallic, insulating, ferromagnetic, antiferromagnetic, and semiconducting. These materials can also be stacked in any order and combination, which makes them potentially ideal for memory technologies, where the interfaces between ultrathin materials can be refined to enable required functionalities. For example, 2D materials such as graphene and transition metal dichalcogenides exhibit unique electronic and spintronic properties, offering potential solutions to overcome some of the limitations associated with traditional materials.

Spin Hall conductivity is the ability of a material to efficiently induce a spin current under the effect of an external electric current, while z spin is the component of the electron spin in the out-of-plane (z) direction.

Prof Yang found that individually, Tungsten Ditelluride, or WTe₂, exhibits z spins but with a small spin Hall conductivity, while PtTe₂, or Platinum Ditelluride, shows a large spin Hall conductivity without z spins. When combined however, the bilayer exhibits a 17-times larger in-plane spin Hall conductivity and eight times larger out-of-plane spin Hall conductivity compared to WTe2 monolayer.

The team also explored a phenomenon known as Spin-orbit Torque (SOT) switching. This involves the transfer of the angular momentum between the electron's spin and its orbital motion in a material, resulting in a torque that can manipulate the orientation of the magnetic moment in ferromagnetic materials.

To manipulate a ferromagnetic material with perpendicular magnetisation for high-density data storage applications, an external assisting magnetic field is often required. However, if a material exhibits z spins, it can overcome the limitation of requiring an external magnetic field, hence lowering the energy required to power the device.

The SOT efficiency and field-free switching of perpendicular magnetisation are determined by the y spins and z spins of a material under the influence of an external current. In their study, Prof Yang discovered that the high out-of-plane spin Hall conductivity observed is attributed to the conversion from y spin in PtTe₂ to z spin in WTe₂, induced by the crystal asymmetry of WTe₂. This creates an ample supply of z spins for efficient field-free SOT switching of perpendicular magnetisation. This out-of-plane spin Hall conductivity is also the highest out of other low-symmetry materials and antiferromagnets, indicating that the PtTe₂/WTe₂ bilayer is a promising 2D heterostructure that allows an all-electric room temperature manipulation of the perpendicular magnetisation at low power consumption without any external magnetic field. It was also found that the associated field-free switching of perpendicular magnetisation was 67 times lower in power consumption than that of the Pt-based control sample (Pt/CoFeB heterostructure).

The realisation of 2D heterostructures with high spin Hall conductivity via z spins without the use of external magnetic fields significantly enhances the SOT performance in spintronic devices. Prof Yang anticipates that such SOT heterostructure engineering may expand to other materials systems in the future and may lead to next-generation memory applications that are fast, efficient and consume significantly less power than traditional memory devices.