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18 August 2021
Unusual properties of a usual antiferromagnet:
Prof. Junwei Liu’s team discovers a new spin-valley locking mechanism
Researchers in the Department of Physics proposed a new mechanism for creating and manipulating a net magnetization in an antiferromagnetic material and for achieving this desirable property by static and dynamic control schemes. The discovery of the new mechanism was made by Prof. Junwei Liu and members of his research group in collaboration with Prof Jin-feng Jia’s group at Shanghai Jiao Tong University and Prof Wang Yao’s group at Hong Kong University. This discovery will significantly expand existing capabilities to control the magnetization ground state of antiferromagnets. Furthermore, the dynamic control that is made possible by this new mechanism had never been possible before. These breakthroughs have significant importance for future developments in information storage and processing technology. This groundbreaking work was published in Nature Communications 26, 2846 (2021).

The new proposed mechanism, called C-paired spin-valley locking (C-paired SVL), can couple an electron’s spin and valley degrees of freedom (DOFs) in antiferromagnets. In contrast to conventional SVL in nonmagnetic materials, such as transition metal dichalcogenides, spin-orbit coupling (SOC) is no longer a necessary condition for producing valley-contrasted spin splitting. This contrast arises instead via the C-paired SVL mechanism from the exchange coupling of itinerant electrons to AFM order. The resulting spin splitting can be very large. Moreover, valleys of opposite spin polarization in an antiferromagnet are paired by a crystal symmetry, instead of the time-reversal symmetry in conventional SVL, to form the emergent quantum degree of freedom. Therefore, both spin and valley DOFs are much easier to access by versatile approaches, such as strain or electric field, that lift the constraint from the corresponding crystal symmetry.

In details, one can use a strain to break symmetry between different valleys and hence induce a net static valley polarization and magnetization, and can use an electric field to induce a spin current dynamically that carries a net magnetization, which is an essential ingredient in spintronics device applications. Moreover, the magnetization can be easily manipulated by tuning the direction of strain or electric field. In addition, compared to the conventional spin current in an antiferromagnet with either non-collinear magnetic order or strong spin-orbital couplings, the spin decoherence time facilitated by the C-paired SVL mechanism can be expected to be much longer. This highly desirable property is obtained because spin is still a conversed quantity due to the absence of SOC and the spin-valley locking can significantly suppress the spin decoherence since spin flipping also requires a change of valley index and vice versa.

Compared to devices currently in use that manipulate magnetization in ferromagnets to store and manipulate data, devices made of antiferromagnets should be able to achieve much higher storage and processor density and much faster operation speed. However, prior to the discovery of the new C-paied SVL mechanism by Prof. Liu and collaborators, this advantage could not be practically exploited because of the difficulty to create and control magnetization in antiferromagnets. Compared to mechanisms known earlier, which are applicable only to materials with non-collinear antiferromagnetic order or strong SOC, the new mechanism significantly broadens the range of materials that can be manipulated to include the more common collinear antiferromagnets. It also provides unprecedented opportunities to integrate various controls of magnetization and nonvolatile information storage in a single materials, which is highly desirable for versatile research and device applications.

Hai-Yang Ma, Mengli Hu, Nana Li, Jianpeng Liu, Wang Yao, Jin-Feng Jia and Junwei Liu, Multifunctional antiferromagnetic materials with giant piezomagnetism and noncollinear spin current, Nature Communications 26, 2846 (2021).