New Magnetic Device Makes Microelectronic Chips More Sustainable

Microelectronic chips are at the foundation of our daily lives with prevalence in numerous industries, including but not limited to artificial intelligence (AI), healthcare, automotive, wireless, space, and others. Crafted to combat the post-pandemic shortage of microelectronic chips and the need for increased domestic research and manufacturing of microelectronics, the recent passage of the CHIPS and Science Act only underscored the strategic importance of microelectronics. 

Northwestern Engineering’s Pedram Khalili is keenly aware of microelectronic chips’ importance. He has pioneered the development of magnetic random-access memory (MRAM), which has a combination of speed, endurance, and nonvolatile operation that makes it an attractive alternative compared to both existing and other emerging memories.  

Khalili’s latest research makes it possible to build MRAM devices based on an entirely new class of magnetic materials, which could make the resulting microelectronic chips more environmentally friendly.

Published recently in the journal Advanced Materials, “Electrically Controlled All-Antiferromagnetic Tunnel Junctions on Silicon with Large Room-Temperature Magnetoresistance” highlights a new, electrically controlled material structure called all-antiferromagnetic tunnel junction (ATJ). The structure could have applications ranging from MRAM chips to terahertz electronics.

The amount of memory on today’s semiconductor chips directly translates into higher performance and better energy efficiency when they run data-intensive tasks. Denser on-chip memory can therefore directly address an important problem: energy footprint of computing is a major sustainability challenge. Information, computing, and communication technologies require exponentially growing energy resources and are expected to consume around 20 percent of the world's electricity by 2030. 

To combat this, Khalili and his lab invented electrically controlled ATJs, using antiferromagnetic (AFM) materials – materials that don’t emanate a strong magnetic field even though they are magnetically organized on a tiny scale. Not only can these materials store information without generating a magnetic field, they can also potentially work faster, fit more data in a smaller space, and avoid compromising nearby stored data.

“Our ATJs combine electrical writing and reading within the same structure, thus having both ingredients needed for a useful memory or computing device,” said Khalili, an associate professor of electrical and computer engineering.

Previously, it was thought that reading information from AFM materials would be inefficient, precisely because of the lack of the magnetic field emanating from them, also called their magnetization. In other words, it was believed that the microscopic opposing magnetic poles of AFM materials should not change the material's electrical properties, regardless of the poles’ directions. However, the Northwestern team found a way to make the electrical resistance of these materials vary significantly, thus enabling efficient information processing.

The researchers achieved this advancement by using special AFM materials where the magnetic poles cancel each other out, not just in pairs, but in threes, at angles of about 120 degrees. This noncollinear setup leads to unique electrical properties not seen in simpler arrangements. By creating a tiny structure where electrons quantum mechanically jump (tunnel) through a thin barrier between two pieces of this special material, they found that the electrical resistance changes depending on whether the poles in those two pieces point in the same or opposite directions. This change in resistance – referred to as tunneling magnetoresistance or TMR – is like what happens in traditional ferromagnetic materials but was previously thought impossible in AFMs.

Crucially, the Northwestern team has shown that this new method works on silicon, which is the standard material used in electronics, meaning the discovery could be directly applied to current technology manufacturing processes.

“We demonstrate that these ATJs can be grown on conventional silicon substrates using sputter deposition, which is an established and widely used technique in semiconductor manufacturing. This should make it possible to deposit such ATJs on the large wafer sizes required for advanced chip production, using existing tools in semiconductor foundries” said Khalili, who is also a faculty affiliate of the Paula M. Trienens Institute for Sustainability and Energy.

The importance of this discovery goes far beyond memory: a wide range of devices that use magnetic materials rely on efficient translation of changes in magnetic configurations into changes in electrical resistance. The demonstration of ATJs with large room-temperature TMR, therefore, opens the door to applications in memory, terahertz oscillators and detectors, and perhaps other devices as well.