Magnon Based Logic Device
Fig. 1 Comparison between charge-based memory and magnon-based memory
Motivation and Background: Limitations of current memory technologies
To cope with data explosion with the advent of 4th industrial revolution, information technology industries demand faster and more energy-efficient memory devices for mobile applications, artificial intelligence, internet of things, and big-data applications. However, the conventional memory devices are now facing the limitations for both scale down and functionality: DRAM/SRAM is suffering from the miniaturization as well as the large standby power consumption due to their inherent volatility, while HDD/SSD has a limitation in the operation speed. To overcome these limitations, novel memory devices, named as the storage class memory, have been brought up for an innovative memory-storage hierarchy. This new class of memory includes the phase changing memory (PRAM), resistive memory (RRAM), ferroelectric memory (FeRAM), and spin-transfer-torque magnetic memory (STT-MRAM), all of which exhibit a fast speed with non-volatility. Among them, the STT-MRAM is technically most mature. Major semiconductor companies, including Samsung Electronics, TSMC, Global foundry, Intel, and UMC, are participating in developing the STT-MRAM, and now the STT-MRAM occupies some portion of the embedded memory markets.
To expand the market portion, MRAM is now pursuing to reduce the operation energy further by employing new physical mechanisms such as the spin-orbit torque (SOT)-induced switching. However, the writing / reading process still requires a charge current flow, inevitably leading to a large energy consumption as well as device degradation due to Joule heating. In addition, the use of tunnel barrier in the device structure, which requires an atomically-precise thickness control, frequently causes reliability issues in manufacturing and device operation. Therefore, there is a great demand for the ‘beyond MRAM devices’ which operates by fundamentally different mechanisms without using the charge current.
Our long-term vision: magnon-based MRAM using magnetic insulators
When localized electron spins in a magnetic insulator are collectively excited by an external stimulus such as heat, light, or microwave, a pure spin current is generated in the form of propagating spin waves. In the quasiparticle picture, the quanta of those spin waves are called the ‘magnons’. If we utilize magnons, instead of spin-polarized conduction electrons, for transferring information (momenta) or energy, the transport process occurs free from Joule heating loss. The main dissipation channel of magnons in magnetic insulators is spin-lattice coupling, which is much weaker than Joule heating. Moreover, the wave nature of magnons provides a long propagation distance up to millimeters. These characteristics make magnons ideal information carriers, based on which some electronic components for future magnonic circuits are being developed. We envision that a novel memory based on such magnons could be a next generation platform that can address the shortcomings (Joule heating and tunnel barrier reliability) of current charge-based memories (Fig. 1).