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MIT scientists have revealed a novel type of magnetism that might someday be utilized to create quicker, denser, and more energy-efficient “spintronic” memory chips.
The newly identified magnetic state is a fusion of two primary forms of magnetism: the ferromagnetism seen in commonplace refrigerator magnets and compass needles, and antiferromagnetism, where substances possess magnetic traits at the microscale yet do not show macroscopic magnetization.
Currently, the MIT group has unveiled a fresh iteration of magnetism, dubbed “p-wave magnetism.”
Researchers have long noted that electrons in standard ferromagnets maintain a uniform orientation of “spin,” resembling numerous small compasses all pointing the same way. This alignment of spin generates a magnetic field, which imparts a ferromagnet with its intrinsic magnetism. In antiferromagnets, electrons associated with magnetic atoms also possess spin, but these spins alternate, causing electrons surrounding adjacent atoms to align their spins in opposite directions. When combined, the resulting equal and opposing spins neutralize each other, and the antiferromagnet does not present macroscopic magnetization.
The team uncovered the novel p-wave magnetism in nickel iodide (NiI2), a two-dimensional crystalline substance that they produced in their laboratory. Similar to a ferromagnet, the electrons display a favored spin direction, and akin to an antiferromagnet, balanced populations of opposing spins lead to a net cancellation. However, the spins on the nickel atoms adopt a distinctive pattern, generating spiral-like arrangements within the material that are mirror images of one another, much like how one hand reflects the other.
Moreover, the researchers discovered that this spiral spin arrangement enabled them to perform “spin switching”: Depending on the direction of the spiraling spins in the material, they could apply a modest electric field in an associated direction to easily switch a left-handed spiral of spins to a right-handed spiral, and vice versa.
The capacity to toggle electron spins is fundamental to “spintronics,” which represents a proposed alternative to traditional electronics. Through this methodology, data can be encoded in the form of an electron’s spin, rather than its electrical charge, potentially allowing for orders of magnitude more data to be compressed onto a device while consuming significantly less power to write and read that information.
“We demonstrated that this new type of magnetism can be controlled electrically,” states Qian Song, a research scientist at MIT’s Materials Research Laboratory. “This breakthrough opens the door for a new category of ultrafast, compact, energy-efficient, and nonvolatile magnetic memory devices.”
Song and his associates published their findings on May 28 in the journal Nature. MIT co-authors include Connor Occhialini, Batyr Ilyas, Emre Ergeçen, Nuh Gedik, and Riccardo Comin, along with Rafael Fernandes from the University of Illinois Urbana-Champaign and collaborators from various institutions.
Connecting the dots
This discovery builds on earlier research conducted by Comin’s group in 2022. During that investigation, the team examined the magnetic features of the same substance, nickel iodide. At the microscopic level, nickel iodide appears as a triangular lattice of nickel and iodine atoms. Nickel serves as the primary magnetic component, as the electrons present on nickel atoms display spin, while those on iodine atoms do not.
In those tests, the group noted that the spins of the nickel atoms were organized in a spiral configuration throughout the material’s lattice, and that this arrangement could spiral in two distinct orientations.
At that time, Comin was unaware that this unique atomic spin pattern could facilitate precise switching of spins in neighboring electrons. This potential was later suggested by collaborator Rafael Fernandes, who, along with other theorists, was fascinated by a recently proposed concept for a new, unconventional “p-wave” magnet, wherein electrons moving in opposite directions within the material would have their spins aligned in contrasting directions.
Fernandes and his colleagues recognized that if the spins of atoms in a material create the geometric spiral arrangement observed by Comin in nickel iodide, that would embody a “p-wave” magnet. Consequently, when an electric field is introduced to alter the “handedness” of the spiral, it should likewise switch the spin orientation of the electrons traveling along that direction.
In essence, such a p-wave magnet could enable straightforward and controllable switching of electron spins, in a manner potentially useful for spintronic applications.
“It was an entirely novel concept at that time, and we opted to test it experimentally because we recognized that nickel iodide was an excellent candidate to demonstrate this type of p-wave magnet effect,” Comin notes.
Spin current
For their recent study, the team produced single-crystal flakes of nickel iodide by first depositing the powders of the respective elements onto a crystalline substrate, which they positioned in a high-temperature furnace. This process prompts the elements to arrange themselves into layers, each microscopically organized in a triangular lattice of nickel and iodine atoms.
“What emerges from the oven are samples several millimeters wide and thin, akin to cracker bread,” Comin explains. “We then exfoliate the material, peeling off even smaller flakes, each only a few microns wide and several tens of nanometers thick.”
The researchers aimed to confirm whether the spiral configuration of the nickel atom spins would compel electrons traveling in opposing directions to possess opposing spins, as Fernandes predicted a p-wave magnet should display. To investigate this, the team directed a beam of circularly polarized light onto each flake — light that generates an electric field which rotates in a specific direction, either clockwise or counterclockwise.
They proposed that if the electrons moving through the spin spirals align their spin in the corresponding direction, then incoming light, polarized in the same orientation, should resonate and generate a characteristic signal. This signal would verify that the spins of the traveling electrons align due to the spiral configuration and that the material indeed exhibits p-wave magnetism.
Indeed, this is what the group discovered. In experiments involving multiple nickel iodide flakes, the researchers directly observed that the orientation of the electron’s spin correlated with the handedness of the light used to excite those electrons. This is a definitive indication of p-wave magnetism, now observed for the first time.
Going further, they assessed whether they could toggle the spins of the electrons by applying an electric field, or a small voltage, in various directions within the material. They found that when the electric field direction aligned with the direction of the spin spiral, the effect switched electrons along that path to spin in the same direction, yielding a current of identically spinning electrons.
“With such a current of spins, you can perform intriguing tasks at the device level; for example, you could flip magnetic domains that could be utilized for controlling a magnetic bit,” Comin clarifies. “These spintronic phenomena are more efficient than traditional electronics since you are merely manipulating spins instead of moving charges. That means you aren’t affected by dissipation effects that cause heat, which is fundamentally the reason computers heat up.”
“We only require a small electric field to manage this magnetic switching,” Song adds. “P-wave magnets could conserve five orders of magnitude of energy, which is substantial.”
“We are thrilled to witness these groundbreaking experiments affirm our prediction of p-wave spin polarized states,” says Libor Šmejkal, director of the Max Planck Research Group in Dresden, Germany, who is one of the authors of the theoretical work that proposed the idea of p-wave magnetism but was not part of the current study. “The demonstration of electrically switchable p-wave spin polarization also underscores the promising applications of unconventional magnetic states.”
The team identified p-wave magnetism in nickel iodide flakes only at ultracold temperatures around 60 kelvins.
“That’s below liquid nitrogen, which isn’t necessarily practical for applications,” Comin remarks. “However, now that we’ve discovered this new magnetic state, the next challenge is finding a material with these properties at room temperature. Then we can integrate this into a spintronic device.”
This research received support, in part, from the National Science Foundation, the Department of Energy, and the Air Force Office of Scientific Research.
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