Layered room-temperature altermagnet shows promise for advanced spintronics

Traditionally, magnetic materials have been divided into two main categories: ferromagnets and antiferromagnets. Over the past few years, however, physicists have uncovered the existence of altermagnets, a new type of magnetic material that exhibits features of both antiferromagnets and ferromagnets.

Altermagnets are magnetic materials that have no net magnetization (i.e., their atomic magnetic moments cancel each other out), like antiferromagnets. Yet they also break spin degeneracy (i.e., the usual energy equality between spin-up and spin-down electrons), similarly to ferromagnets.

Researchers at Songshan Lake Materials Laboratory, Southern University of Science and Technology, the Hong Kong University of Science and Technology and other institutes in China recently set out to realize a layered altermagnet that can generate non-collinear spin current. The room-temperature metallic altermagnet they unveiled was outlined in a paper published in Nature Physics.

“Traditionally, there are two ways to generate spin-polarized states in quantum materials,” Chaoyu Chen, senior author of the paper, told Phys.org. “One is based on nonmagnetic materials with strong spin-orbital coupling and inversion symmetry breaking. The other one is based on ferromagnets in which the electrons are polarized by the magnetic moments.

“By contrast, antiferromagnets are believed to have no spin polarization. Recently, it was theoretically proposed that in some antiferromagnets with certain symmetry, called altermagnets, the electrons are also polarized.”

Many studies over the past few years have tried to realize and investigate the physical underpinnings of altermagnets. This is because altermagnetic materials could be highly promising for the development of new technologies, particularly spintronics—devices that utilize the spin of electrons and their electric charge to store, transfer and process information.

“Altermagnets combine the advantage of the ferromagnet, such as spin polarization, anomalous Hall transport and spin-splitting torque, and the antiferromagnet, like vanishing stray field and terahertz magnon dynamics,” said Chen.

“This not only opens a new chapter in the research field of fundamental physics but also enables potential spintronic devices for information processing and storage. Consequently, there is currently a global surge in looking for altermagnetic materials.”

Most of the altermagnets realized and studied over the past few years, including MnTe, CrSb and Mn₅Si₃, have a three-dimensional (3D) crystal structure, not a layered one. Moreover, none of these altermagnetic material candidates was found to attain pure spin current, due to limitations associated with their symmetry or an undesired magnetic order.

“The past decade has witnessed the rising of two-dimensional (2D) and layered materials such as graphene, transition metal dichalcogenides, with great potential in exploring novel physics and developing ultracompact electronic and optoelectronic devices,” said Chen.

“Consequently, it is our motivation to look for layered altermagnets that can produce non-collinear spin current. V₂Te₂O and V₂Se₂O were predicted as a two-dimensional altermagnet in 2021, and hence we focus on V₂Te₂O-related compounds.”

To demonstrate that the material they realized, namely Rb_1-δV₂Te₂O, has a room-temperature magnetic order, Chen and his colleagues measured its magnetic susceptibility. In addition, they used a technique known as angle-resolved photoemission spectroscopy (ARPES) to show that the material has a spin-split band structure at both low temperatures and room temperature.

They then used spin-resolved ARPES to prove the spin polarization of the material’s electronic structure. Finally, they employed techniques called scanning tunneling microscopy and spectroscopy to confirm that the scattering of electrons between two opposite spins in Rb_1-δV₂Te₂O is “forbidden” (i.e., cannot occur).

“We realized a layered altermagnet with promising spin current generation, which is remarkable given that all previously introduced altermagnet candidates are not layered and not capable of generating non-collinear spin current,” said Chen.

“The layered nature of Rb_1-δV₂Te₂O not only promises advances in searching for new quantum phases such as topological superconductivity, and Chern/axion insulator, but also enables various advantages in 2D materials including but not limited to the realization of novel superconducting/magnetic phases via proximity effect, tunable electronic properties through gating, strain and potential to form twisted superlattices.”

In the future, the altermagnet identified by this team of researchers could be investigated further and potentially used to develop new spintronic devices. Moreover, the recent work by Chen and his colleagues could inspire other research groups to attempt the engineering of similar room-temperature altermagnets.

“We now plan to fabricate spin transport devices and directly measure the spin current based on Rb_1-δV₂Te₂O, which will guide the potential application in spintronics,” added Chen.

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