The new magnetic state is a mash-up of two main forms of magnetism: the ferromagnetism of everyday fridge magnets and compass needles, and antiferromagnetism, in which materials have magnetic properties at the microscale yet are not macroscopically magnetized.
Now, the MIT team has demonstrated a new form of magnetism, termed "p-wave magnetism."
Physicists have long observed that electrons of atoms in regular ferromagnets share the same orientation of "spin," like so many tiny compasses pointing in the same direction. This spin alignment generates a magnetic field, which gives a ferromagnet its inherent magnetism. Electrons belonging to magnetic atoms in an antiferromagnet also have spin, although these spins alternate, with electrons orbiting neighboring atoms aligning their spins antiparallel to each other. Taken together, the equal and opposite spins cancel out, and the antiferromagnet does not exhibit macroscopic magnetization.
The team discovered the new p-wave magnetism in nickel iodide (NiI2), a two-dimensional crystalline material that they synthesized in the lab. Like a ferromagnet, the electrons exhibit a preferred spin orientation, and, like an antiferromagnet, equal populations of opposite spins result in a net cancellation. However, the spins on the nickel atoms exhibit a unique pattern, forming spiral-like configurations within the material that are mirror-images of each other, much like the left hand is the right hand's mirror image.
What's more, the researchers found this spiral spin configuration enabled them to carry out "spin switching": Depending on the direction of spiraling spins in the material, they could apply a small electric field in a related direction to easily flip a left-handed spiral of spins into a right-handed spiral of spins, and vice-versa.
The ability to switch electron spins is at the heart of "spintronics," which is a proposed alternative to conventional electronics. With this approach, data can be written in the form of an electron's spin, rather than its electronic charge, potentially allowing orders of magnitude more data to be packed onto a device while using far less power to write and read that data.
"We showed that this new form of magnetism can be manipulated electrically," says Qian Song, a research scientist in MIT's Materials Research Laboratory. "This breakthrough paves the way for a new class of ultrafast, compact, energy-efficient, and nonvolatile magnetic memory devices."
Song and his colleagues published their results May 28 in the journal Nature. MIT co-authors include Connor Occhialini, Batyr Ilyas, Emre Ergecen, Nuh Gedik, and Riccardo Comin, along with Rafael Fernandes at the University of Illinois Urbana-Champaign, and collaborators from multiple other institutions.
In those experiments, the team observed that the spins of those nickel atoms were arranged in a spiral pattern throughout the material's lattice, and that this pattern could spiral in two different orientations.
At the time, Comin had no idea that this unique pattern of atomic spins could enable precise switching of spins in surrounding electrons. This possibility was later raised by collaborator Rafael Fernandes, who along with other theorists was intrigued by a recently proposed idea for a new, unconventional, "p-wave" magnet, in which electrons moving along opposite directions in the material would have their spins aligned in opposite directions.
Fernandes and his colleagues recognized that if the spins of atoms in a material form the geometric spiral arrangement that Comin observed in nickel iodide, that would be a realization of a "p-wave" magnet. Then, when an electric field is applied to switch the "handedness" of the spiral, it should also switch the spin alignment of the electrons traveling along the same direction.
In other words, such a p-wave magnet could enable simple and controllable switching of electron spins, in a way that could be harnessed for spintronic applications.
"It was a completely new idea at the time, and we decided to test it experimentally because we realized nickel iodide was a good candidate to show this kind of p-wave magnet effect," Comin says.
"What comes out of the oven are samples that are several millimeters wide and thin, like cracker bread," Comin says. "We then exfoliate the material, peeling off even smaller flakes, each several microns wide, and a few tens of nanometers thin."
The researchers wanted to know if, indeed, the spiral geometry of the nickel atoms's spins would force electrons traveling in opposite directions to have opposite spins, like what Fernandes expected a p-wave magnet should exhibit. To observe this, the group applied to each flake a beam of circularly polarized light - light that produces an electric field that rotates in a particular direction, for instance, either clockwise or counterclockwise.
They reasoned that if travelling electrons interacting with the spin spirals have a spin that is aligned in the same direction, then incoming light, polarized in that same direction, should resonate and produce a characteristic signal. Such a signal would confirm that the traveling electrons' spins align because of the spiral configuration, and furthermore, that the material does in fact exhibit p-wave magnetism.
And indeed, that's what the group found. In experiments with multiple nickel iodide flakes, the researchers directly observed that the direction of the electron's spin was correlated to the handedness of the light used to excite those electrons. Such is a telltale signature of p-wave magnetism, here observed for the first time.
Going a step further, they looked to see whether they could switch the spins of the electrons by applying an electric field, or a small amount of voltage, along different directions through the material. They found that when the direction of the electric field was in line with the direction of the spin spiral, the effect switched electrons along the route to spin in the same direction, producing a current of like-spinning electrons.
"With such a current of spin, you can do interesting things at the device level, for instance, you could flip magnetic domains that can be used for control of a magnetic bit," Comin explains. "These spintronic effects are more efficient than conventional electronics because you're just moving spins around, rather than moving charges. That means you're not subject to any dissipation effects that generate heat, which is essentially the reason computers heat up."
"We just need a small electric field to control this magnetic switching," Song adds. "P-wave magnets could save five orders of magnitude of energy. Which is huge."
"We are excited to see these cutting-edge experiments confirm our prediction of p-wave spin polarized states," says Libor Smejkal, head of the Max Planck Research Group in Dresden, Germany, who is one of the authors of the theoretical work that proposed the concept of p-wave magnetism but was not involved in the new paper. "The demonstration of electrically switchable p-wave spin polarization also highlights the promising applications of unconventional magnetic states."
The team observed p-wave magnetism in nickel iodide flakes, only at ultracold temperatures of about 60 kelvins.
"That's below liquid nitrogen, which is not necessarily practical for applications," Comin says. "But now that we've realized this new state of magnetism, the next frontier is finding a material with these properties, at room temperature. Then we can apply this to a spintronic device."
This research was supported, in part, by the National Science Foundation, the Department of Energy, and the Air Force Office of Scientific Research.
Research Report:Electrical switching of a p-wave magnet
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