Spin based Information Processing and Storage
We proposed valley-coupled spintronic devices based on monolayer WSe2 that utilize Valley-Spin Hall (VSH) effect to switch nano-magnets. The unique features of the proposed device are (a) the ability to switch magnets with perpendicular magnetic anisotropy (PMA) and (b) an integrated gate that can modulate the charge/spin current flow. The former attribute results in high energy efficiency (compared to the Giant-Spin Hall (GSH) effect based devices with in-plane MA (IMA) magnets). The latter feature averts the need for additional access transistor in a memory array leading to high integration density.
VSH effect in 2D Transition-Metal Dichalcogenide (TMD) based Non-volatile Memories
Spin-based storage has emerged as a promising alternative to standard charge-based memories due to zero array stand-by leakage, non –volatility and high integration density. However, low write efficiency and self-conflicting design requirements for read and write make it challenging to design low power robust spin-memories. At ICDL, we are exploring novel techniques at the device, circuit and array level to counter the issues associated with spin-based storage. This includes the device-circuit-array co-design for memories employing voltage based magnetization switching phenomena, development of variation-tolerant design methodologies for memories with separate read-write paths and techniques to enhance the magneto resistance. We are also exploring spin-based logic based on multi-terminal spin devices.
The quest for a universal memory which offers high speed, energy efficiency, stability, endurance, distinguishability, integration density and non-volatility still continues, as several of the existing candidates, though promising, suffer from one or the other design issues. As one of the solutions, we explored 2D TMD spin-based memory design, which offers many other intriguing possibilities for various flavors of non-volatile memory design, such as lower write time and switching/write power even with higher integration density compared to standard CMOS. We proposed two flavours of VSH magnetic RAMs namely: (a) Single ended VSH-MRAM and (b) Differential DVSH-MRAM. The proposed memory designs showcased high integration density align with ultra low power memory operations compared to the traditional GSH-MRAMs.
VSH effect based beyond CMOS Logic Design
Spin-based logic has garnered an immense attention due to the possibilities of 1) ultra-low voltage operation (~10mV), which is much below the fundamental limit associated with the standard CMOS technology, 2) non-volatility that allows the device to retain its information even when the power goes off and 3) area-efficiency. With the extremely energy-efficient computation, spin-based devices can be utilized to implement the CMOS-like Boolean logic functions.
Moreover, recently, it has been experimentally shown that the monolayer TMD materials, e.g. MoS2 and WSe2, can induce Valley Spin Hall (VSH) effect, which is derived from the intrinsic inversion asymmetry and strong spin-orbit coupling. This gives rise to an out-of-plane magnetization, i.e. PMA, which enables lower critical switching current than IMA to tune the magnetization of the neighboring ferro-magnet. By exploiting the unique features offered by TMD materials, we explore novel design techniques by coupling TMD-based spin generator with nano-magnets to improve the computational efficiency of the spin-based logic.
 S.K. Thirumala et al., " WSe2 based Valley-Coupled-Spintronic Devices for Low Power Non-Volatile Memories" Device Research Conference, 2019.