Spintronics is a budding path in the quest for a future beyond CMOS. Where today’s electronics harness an electron’s charge, spintronics plays with another key property: an electron’s spin, which is its intrinsic angular momentum that points either “up” or “down.” Spintronic devices use much less power than their CMOS counterparts—and keep their data even when the device is switched off.
Spin is already used in memory, such as magnetoresistive RAM (MRAM). However, manipulating tiny magnetic effects for the purpose of performing logical operations is more difficult. In 2018, for instance, MIT researchers experimented with hydrogen ions as a possible basis for spintronic transistors. Their efforts were very preliminary; the team admitted that there was still a long way to go before a fully functioning spintronic transistor that could store and process information might be developed.
Now, researchers at imec and Intel have created a spintronic logic device that can be fully controlled with electric current rather than magnetic fields. PhD candidate Eline Raymenants presented the team’s work at the recent IEEE International Electron Devices Meeting (IEDM).
An electron’s spin generates a magnetic moment. When many electrons with identical spins are close together, their magnetic moments can align and join forces to form a larger magnetic field. Such a region is called a magnetic domain, and the boundaries between domains are called domain walls. A material can consist of many such domains and domain walls, assembled like a magnetized mosaic.
Devices can encode 0s and 1s in those domains. A domain pointing “up” could represent a 0, where a “down” represents a 1. The Intel-imec device uses domains placed in a single-file line of nanoscale wire. The device then uses current to shift those domains and their walls along the wire, like cars along a train track.
The track meets the switch at a magnetic tunnel junction (MTJ). It’s similar to the read-heads of today’s hard disks, but the researchers have implemented a new type of MTJ that’s optimized to move the domain walls more quickly. The MTJs read information from the track and act as logic inputs. At IEDM, the researchers presented a proof of concept: several MTJs feeding an AND gate.
The same MTJ is also where information is written into the track. To do this, the Intel-imec device uses the same technology that’s used in MRAM today. The device passes a spin-polarized current—most of whose electrons have spins in one direction—through a magnetic domain. That current can realign the magnetic field’s direction, creating or editing domain walls in the process.
It’s similar to racetrack memory, an experimental form of data storage that was first proposed over a decade ago. Racetrack memory also writes information into magnetic domains and uses current to shuttle those domains along a nanoscale wire, or “racetrack.” But the Intel-imec device takes advantage of advances in materials, allowing domain walls to move down the line far more quickly. This, the researchers say, is key for allowing logic.
Researchers so far have largely focused on optimizing those materials, according to Van Dai Nguyen, a researcher at imec. “But to build the full devices,” he says, “that’s been missing.”
The Intel-imec team is not alone. Earlier in 2020, researchers at ETH Zurich created a logic gate using domain-wall logic. Researchers at MIT also recently demonstrated a domain-wall-based artificial neuron. Like the Intel-imec researchers and like racetrack memory, these devices also use current to shift domains down the line.
But the Zurich and MIT devices rely on magnetic fields to write information. For logic, that’s not ideal. “If you build a logic circuit,” says Iuliana Radu, Director of Exploratory Computing at imec, “you’re not going to put…a huge magnet that you change direction or switch on and off to implement the logic.” Full electrical control, Radu says, will also allow the Intel-imec device to be connected to CMOS circuits.
The researchers say their next steps will be to show their device in action. They’ve designed a majority gate, which returns a positive result if the majority of its inputs are positive. Radu, however, says that they have yet to really explore this design. Only then will the researchers know how their spintronic logic will fare against the CMOS establishment.
This post was corrected on 21 January 2021 to clarify attributions.