Overview of new “dials” to control the magnetism of a material

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New research on an atomically thin semiconductor demonstrates how a material’s magnetism can be controlled using small amounts of strain. Posted in Nature’s nanotechnology, this study provides key insights for applications ranging from new spintronic devices to faster hard drives. This research was conducted by graduate student Zhuoliang Ni and led by assistant professor Liang Wu in collaboration with Penn’s Charlie Kane and Eugene Mele, as well as researchers from the University of Tennessee, Knoxville, Texas A&M University, the University of Freiburg and Oak Ridge National Laboratory.

Wu’s lab mainly focuses on experiments with topological materials. But, with recent studies on the photogalvanic effects of two metal alloys and the discovery of exotic particles in cobalt monosilicide, the lab’s latest paper on manganese phosphorus triselenide (MnPSe3), a semiconductor material, digs deeper into the concepts around symmetry, a physical or mathematical characteristic of a system that does not change when subjected to certain transformations. Symmetry is a key idea in physics, from conservation laws to particle behavior, and is essential for understanding materials that have controllable or switchable magnetic states such as MnPSe3.

There are different types of magnets. For ferromagnetic materials, the electrons all spin in the same direction and imbue the material with a spontaneous magnetism that allows them to stick to certain types of metals. In contrast, antiferromagnetic materials, like MnPSe3, have a pattern with an equal number of electrons with up and down spins in an antiparallel arrangement. This cancels out their overall magnetic moments, meaning they don’t have an external stray field like ferromagnetic materials; however, they still have electrons with varying spin orientations.

Existing hard drives rely on ferromagnetic materials, where changes in electron spin directions represent the bits, or zeros and ones, that make up memory, but there is interest in developing memory devices from antiferromagnetic materials. For example, information stored in ferromagnetic devices can be lost if another magnetic field is present. These devices are also limited in how quickly they can operate by the time it takes to manually change a bit, in the nanosecond range. Antiferromagnetic materials, on the other hand, are able to change spin orientation much faster, in the picosecond range, and are also much less sensitive to external magnetic fields.

But while antiferromagnetic materials have some advantages, working with this type of material, especially two-dimensional material, is technically challenging, Wu says. In order to study this material, Ni and Wu first had to develop a way to measure tiny signals without delivering too much power that would damage the atomically thin material. “By using a photon counter, we were able to reduce the noise,” Wu explains. “This was the technical breakthrough that allowed us to detect antiferromagnetism in the monolayer.”

Using their new imaging approach, the researchers found they could “switch” the material to be in an antiferromagnetic phase at low temperatures. They also found that the material had fewer states, similar to the bits used in computer memory, than expected. The researchers observed only two states even though, based on its rotational symmetry, it was predicted to have six states.

Wu turned to Kane and Mele to offer a theory that might help explain these unexpected results, and through this collaboration he realized the significant impact that lateral deformation, such as stretching or shearing, could have. on its symmetry. “A perfect sample has threefold rotational symmetry, but if something tugs at it, it’s not the same if you rotate it 120°,” Kane explains. “Once Liang suggested there might be tension, it was immediately apparent as a theorist that two of the six areas had to be chosen.”

After follow-up experiments that confirmed their hypothesis, the researchers were further surprised at how powerful a small amount of stress could be in altering material properties. “In the past, people used constraint to change directions of rotation, but in our case, what’s important is that a tiny amount of constraint can control rotation, and that’s because the role strain is really fundamental in the phase transition in our case,” says Wu.

With this new idea, the researchers say this study could be a starting point to better control antiferromagnetic properties using small strain changes. Strain is also a much easier property to control in this class of materials, which currently require a massive magnetic field – on the order of several tesla – to change the spin direction of electrons and could be a kind of dial or button which could change the magnetic order. , or the spin model of the electron.

“The lack of stray fields in antiferromagnetic materials means you don’t have a macroscopic thing you can use to manipulate the momentum,” Mele explains, “But there is a degree of internal freedom that allows you to do that. by coupling directly to the control. »

To study this material in more depth, Ni is working on several follow-up experiments. This includes seeing if electric fields and pulses can change spin direction and evaluating the use of terahertz pulses, the natural resonant frequency of antiferromagnetic materials, to control both electron spin direction and switching speed. “We can eventually use terahertz to control spins,” Ni says of this system, which is also an expertise regimen for the Wu lab. “Terahertz is much faster than gigahertz, and for antiferromagnetic spins , it’s possible we could use terahertz to control ultra-fast switching from one state to another.”

“Antiferromagnetic materials offer exciting new opportunities to create faster spintronic devices for information processing as well as new ways to efficiently generate terahertz radiation, which is part of the electromagnetic spectrum for wireless communications beyond 5G,” says Joe Qiu, program manager for Solid-State Electronics and Electromagnetics at the Army Research Office, which funded the study. “All of these technologies are important for future military electronic systems.”

Charlie Kane is the Christopher H. Browne Professor Emeritus of Physics in the Department of Physics and Astronomy in the School of Arts and Sciences at the University of Pennsylvania.

Eugene Mele is the Christopher H. Browne Professor Emeritus of Physics in the Department of Physics and Astronomy at Penn’s School of Arts & Sciences.

Zhuoliang Ni is a graduate student in the Department of Physics and Astronomy at Penn’s School of Arts & Sciences.

Liang Wu is an assistant professor in the Department of Physics and Astronomy at Penn’s School of Arts & Sciences.

This research was supported by the Army Research Office (grants W911NF1910342 and W911NF2020166), the National Science Foundation (grants DMR-1720530 and EAGER 1838456), and a Simons Investigator grant from the Simons Foundation.

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