Controllable Fast, Tiny Magnetic Bits

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These skyrmions are as small as 10 nanometers, which is the current world record for room temperature skyrmions. The researchers demonstrated current driven domain wall motion of 1.3 kilometers per second, using a mechanism that can also be used to move skyrmions, which also sets a new world record.

Except for the synchrotron work, all the research was done at MIT. “We grow the materials, do the fabrication and characterize the materials here at MIT,” Caretta says.

Magnetic Modeling

These skyrmions are one type of spin configuration of electron spins in these materials, while domain walls are another. Domain walls are the boundary between domains of opposing spin orientation. In the field of spintronics, these configurations are known as solitons, or spin textures. Since skyrmions are a fundamental property of materials, mathematical characterization of their energy of formation and motion involves a complex set of equations incorporating their circular size, spin angular momentum, orbital angular momentum, electronic charge, magnetic strength, layer thickness, and several special physics terms that capture the energy of interactions between neighboring spins and neighboring layers, such as the exchange interaction.

One of these interactions, which is called the Dzyaloshinskii-Moriya interaction (DMI), is of special significance to forming skyrmions and arises from the interplay between electrons in the platinum layer and the magnetic layer. In the Dzyaloshinskii-Moriya interaction, spins align perpendicular to each other, which stabilizes the skyrmion, Lemesh says. The DMI interaction allows for these skyrmions to be topological, giving rise to fascinating physics phenomena, making them stable, and allowing for them to be moved with a current.

“The platinum itself is what provides what’s called a spin current which is what drives the spin textures into motion,” Caretta says. “The spin current provides a torque on the magnetization of the ferro or ferrimagnet adjacent to it, and this torque is what ultimately causes the motion of the spin texture. We’re basically using simple materials to realize complicated phenomena at interfaces.”

In both papers, the researchers performed a mix of micromagnetic and atomistic spin calculations to determine the energy required to form skyrmions and to move them.

“It turns out that by changing the fraction of a magnetic layer, you can change the average magnetic properties of the whole system, so now we don’t need to go to a different material to generate other properties,” Lemesh says. “You can just dilute the magnetic layer with a spacer layer of different thickness, and you will wind up with different magnetic properties, and that gives you an infinite number of opportunities to fabricate your system.”

Precise Control

"Precise control of creating magnetic skyrmions is a central topic of the field,” says Jiadong Zang, an assistant professor of physics at the University of New Hampshire, who was not involved in this research, regarding the Advanced Materials paper. “This work has presented a new way of generating zero field skyrmions via current pulse. This is definitely a solid step towards skyrmion manipulations in nanosecond regime.”

Commenting on the Nature Nanotechnology report, Christopher Marrows, a professor of condensed matter physics at the University of Leeds in the United Kingdom says: “The fact that the skyrmions are so small but can be stabilized at room temperature makes it very significant.”

Marrows, who also was not involved in this research, noted that the Beach group had predicted room temperature skyrmions in a Scientific Reports paper earlier this year and said the new results are work of the highest quality. “But they made the prediction and real life does not always live up to theoretical expectations, so they deserve all the credit for this breakthrough,” Marrows says.

Zang, commenting on the Nature Nanotechnology paper, adds: “A bottleneck of skyrmion study is to reach a size of smaller than 20 nanometers [the size of state-of-art memory unit], and drive its motion with speed beyond one kilometer per second. Both challenges have been tackled in this seminal work.

“A key innovation is to use ferrimagnet, instead of commonly used ferromagnet, to host skyrmions,” Zang says. “This work greatly stimulates the design of skyrmion-based memory and logic devices. This is definitely a star paper in the skyrmion field.”

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