Chiral phonons generate spin currents without the need for magnetic materials

Researchers from North Carolina State University and the University of North Carolina at Chapel Hill used chiral phonons to convert waste heat into spin information – without the need for magnetic materials. The finding could lead to new classes of less expensive, energy-efficient spintronic devices that can be used in applications ranging from computational storage to power grids.

Spintronic devices are electronic devices that use an electron’s spin instead of its charge to generate electricity used in data storage, communications, and computing. Spin-caloritronic devices — so-called because they use thermal energy to generate spin current — hold promise because they can convert waste heat into spin information, making them extremely energy-efficient. However, current spin caloritronic devices must contain magnetic materials to generate and control the spin of the electron.

“We used chiral phonons to generate a spin current at room temperature without the need for magnetic materials,” says Dali Sun, associate professor of physics and member of the Organic and Carbon Electronics Lab (ORaCEL) at North Carolina State University.

“By applying a thermal gradient to a material containing chiral phonons, you can direct their angular momentum and create and control a spin current,” says Jun Liu, associate professor of mechanical and aerospace engineering at NC State and an ORaCEL member.

Both Liu and Sun are co-corresponding authors of the research that appears in Nature Materials.

Chiral phonons are groups of atoms that move in a circular motion when excited by a source of energy, in this case heat. As the phonons move through a material, they propagate this circular motion, or angular momentum, through it. Angular momentum serves as the source of the spin, and chirality determines the direction of the spin.

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“Chiral materials are materials that cannot be superimposed on their mirror image,” says Sun. “Think of your right and left hands – they are chiral. You cannot put a left handed glove on a right hand or vice versa. This “handedness” allows us to control the direction of rotation, which is important if you plan to use these devices as storage.”

Researchers demonstrated spin currents generated by chiral phonons in a two-dimensional layered hybrid organic-inorganic perovskite by using a thermal gradient to inject heat into the system.

“A gradient is needed because temperature differences in the material – from hot to cold – drive the movement of the chiral phonons through the material,” says Liu. “The thermal gradient also allows us to use trapped waste heat to generate spin current.”

Researchers hope the work will result in spintronic devices that are cheaper to manufacture and can be used in a wider variety of applications.

“By eliminating the need for magnetism in these devices, you open the door wide in terms of access to potential materials,” says Liu. “And that also means increased profitability.”

“Using waste heat instead of electrical signals to generate spin current makes the system energy efficient — and the devices can operate at room temperature,” says Sun. “This could lead to a much larger variety of spintronic devices than we currently have available.”

The research was supported by the National Science Foundation and the US Department of Energy. Wei You, a professor of chemistry at the University of North Carolina at Chapel Hill and a member of ORaCEL, is also a co-corresponding author of the study.

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Note to editors: An abstract follows.

“Chiral Phonon Activated Spin Seebeck Effect”

DOI: 10.1038/s41563-023-01473-9

Authors: Kyunghoon Kim, Eric Vetter, Cong Yang, Ziqi Wang, Rui Sun, Andrew Comstock, Dali Sun, Jun Liu, North Carolina State University; Liang Yan, Wei You, University of North Carolina at Chapel Hill; Yu Yang, Xiao Li, Jun Zhou, Lifa Zhang, Nanjing Normal University, Nanjing, China
Published: February 13, 2023 in Nature Materials

The use of the interaction between spin and heat flows is at the heart of the field of spin caloritronics. Chiral phonons, which possess angular momentum resulting from the broken symmetry of a nonmagnetic material, create the potential to generate spin currents at room temperature in response to a thermal gradient, eliminating the need for a ferromagnetic contact. Here we show the observation of spin currents generated by chiral phonons in a two-dimensional layered hybrid organic-inorganic perovskite implanted with chiral cations when subjected to a thermal gradient. The generated spin current shows a strong dependence on the chirality of the film and external magnetic fields, the coefficient of which is orders of magnitude larger than that generated by the reported spin-Seebeck effect. Our results indicate the potential of chiral phonons for spin caloritronic applications and offer a new way to generate spin without magnetic materials.