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Breakthrough in 2D Magnetic Materials for Energy-Efficient Computing: MIT Study

The flow of electric current in the lower crystalline plate (represented by WTe2) breaks the mirror symmetry (broken glass), and the material itself breaks the opposite mirror symmetry (cracked glass). The resulting spin current has a vertical polarization that changes the magnetic state of the upper 2D ferromagnenet. Credit: Image courtesy of researchers

The future of spintronics: manipulation of spins in atomic layers without external magnetic fields. Credit: Deblina Sarkar

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“Our device enables strong magnetization switching without the need for an external magnetic field, opening up unprecedented opportunities for ultra-low power and sustainable computing technology for big data and artificial intelligence,” said lead author Devlina Sarkar, Asst. AT&T Career Development at AT&T. MIT Media Lab and Center for Neurobiological Engineering, and leader of the Nanocybernetic Research Group at Biotrak. “In addition, the atomic layer structure of our device provides unique capabilities including improved interface and gate voltage tunability, as well as flexible and transparent spintronics technologies.”

Sarkar is joined on the first author document by Shivam Kajala, a graduate student in Sarkar’s research group at Media Lab; Thanh Nguyen, a graduate student in the Department of Nuclear Science and Engineering (NSE); Nguyen Tuan Hung, MIT Visiting Scholar at NSE and Assistant Professor at Tohoku University in Japan; and Mingda Lee, NSE associate professor.

break the mirror symmetry

When an electric current flows through heavy metals such as platinum or tantalum, the electrons are separated in the materials based on their spin components, a phenomenon known as the spin Hall effect, says Kajale. How this separation occurs depends on the material, especially on its symmetries.

“Conversion of electric currents to spin currents in heavy metals is at the heart of the control of electromagnets,” Kajala notes. “The microscopic structure of materials in traditional use, such as platinum, has a kind of mirror symmetry that restricts the spin currents to a single in-plane spin polarization.”

Kajale explains that two mirror symmetries must be broken to produce an “out-of-plane” spin component that can be transferred to a magnetic layer to cause field-free switching. “An electric current can ‘break’ the mirror symmetry along one plane in platinum, but its crystal structure prevents the mirror symmetry from breaking in the second plane.”

In their previous experiments, the researchers used a small magnetic field to break the second mirror plane. To get rid of the need for magnetic pressure, Kajla and Sarkar and his colleagues instead looked for a material with a structure that could break the second mirror plane without outside help. This led them to another 2D material, tungsten difluoride. The tungsten difluoride used by the researchers has an orthorhombic crystal structure. The material itself has one broken mirror plane. Thus, by applying a current along its low symmetry axis (parallel to the plane of the refracted mirror), the spin current has an out-of-plane spin component that can directly induce a change in the interface of the ultrathin tungsten difluoride magnet.

“Because it is also a two-dimensional van der Waals material, it can also ensure that when we harden the two materials together, we get seamless interfaces and a good flow of electron spins between the materials,” says Kajala.

to be more energy efficient

Computer memory and processors built from magnetic materials consume less energy than traditional silicon devices. And the van der Waals magnets can offer higher energy efficiency and better scalability compared to bulk magnetic material, the researchers note.

The density of the electric current used to change the magnet is the same as the amount of energy dissipated during the transfer. Lower density means a much more energy efficient material. “The new design has one of the lowest current densities in van der Waals magnetic materials,” says Kajala. “This new design is an order of magnitude lower in terms of the switching current required in bulk materials. This represents two orders of magnitude improvement in energy efficiency.”

The research team is now looking at similar low symmetry van der Waals materials to see if they can further reduce the current density. They also hope to collaborate with other researchers to find ways to manufacture the 2D magnetic switch devices on a commercial scale.

This work was done, in part, using the facilities at MIT.nano. It was funded by Media Lab, the US National Science Foundation and the US Department of Energy.

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