Recently, the research groups of Professor Lin Miao and Associate Professor Xiaoqian Zhang(the School of Physics, Southeast University), in collaboration with Professor Lixin He's group from the University of Science and Technology of China (USTC), have made significant progress in the band engineering of altermagnetic thin films. The team experimentally observed for the first time a spin-degenerate ground state in altermagnetic CrSb thin films across the entire Brillouin zone (BZ), which is completely different from the spin splitting observed in the bulk phase, revealing the important role of interfacial strain in tuning the magnetic reconstruction of altermagnetic thin films and providing a new paradigm for tailoring altermagnetic spin splitting throughengnieered symmetry. The research findings, titled "Tailoring Altermagnetic Spin Splitting via Strain-Induced Symmetry Reconstruction in CrSb Thin Films," were published online in Advanced Materials on October 23.
Fig 1.
In recent years, the proposal of a new fundamental magnetism—altermagnetism—has attracted widespread attention in the field of physics. Altermagnets possess a collinear antiferromagnetic structure in real space, resulting in a net magnetization of zero; but simultaneously, non-relativistic spin splitting exists in reciprocal space, which is the result of the interaction between spin symmetry and lattice symmetry. Due to the non-relativistic nature of this spin splitting, its energy scale can reach approximately 1 eV, which is an order of magnitude higher than the band splitting induced by spin-orbit coupling (SOC), providing broad application prospects in the fabrication of spintronic devices. Currently, many experimental works have confirmed the spin-split band structure in altermagnetic materials, such as α-MnTe and CrSb, but these works mostly focus on bulk materials. Owing to dimensional effects and interfacial strain, thin film materials typically exhibit unique physical properties not found in bulk materials; however, current research in this direction within the altermagnetic field remains incomplete.
Fig 2.
To address the aforementioned issues, the research team utilized the molecular beam epitaxy (MBE) method to prepare high-quality, large-scale altermagnetic CrSb heterostructure thin films, and combined in-situ RHEED and STM with ex-situ XRD and STEM characterization techniques to confirm the high quality of the single-crystal thin film samples (Fig1). Meanwhile, using synchrotron-based high-resolution ARPES and Spin-ARPES, clear Fermi surface images and band dispersions along different paths in the Brillouin zone were obtained. Theoretical calculation results indicate that CrSb is a g-wave altermagnet with four spin-degenerate nodal planes, exhibiting alternating spin splitting in directions away from the high-symmetry paths. However, Spin-ARPES results showed that the energy bands in the 10 nm CrSb thin films are spin-degenerate throughout the entire Brillouin zone, which is inconsistent with theoretical predictions, and the band dispersion characteristics detected by ARPES do not perfectly match the energy bands calculated by DFT based on the A-AFM phase of bulk CrSb. Therefore, based on this inconsistency between experimental and calculated results, the study proposed a strategy to "tune magnetic configuration transitions using symmetry," and through first-principles calculations, obtained ground-state magnetic configuration phase diagrams under different strain conditions. The calculation results show (Fig 2) that for bulk CrSb, the A-AFM phase is always the lowest-energy ground state, regardless of whether it is under in-plane or out-of-plane stress. In contrast, the magnetic ground state of single-layer (1 ML) CrSb thin films is highly sensitive to the direction and type of applied stress. Specifically, under the combined effects of in-plane compressive strain (IP ~ -2.7%) and out-of-plane tensile strain (OOP ~ +2.4%), the ground state of the system transitions to the G-AFM phase; at this point, the CrSb thin film is situated exactly at the G-AFM / A-AFM phase boundary (marked by a yellow star), and this is further verified by the perfect match between the experimental results and the calculated energy bands of the G-AFM phase. Additionally, the G-AFM phase preserves PT symmetry; according to symmetry constraints, PT symmetry enforces spin degeneracy across the entire Brillouin zone, which is consistent with the Spin-ARPES results. This research establishes a "strain-symmetry reconstruction-spin properties" tuning paradigm, providing an experimental basis for the symmetry engineering of low-dimensional altermagnetic materials.
The School of Physics at Southeast University is the primary completing institution for the paper; Professor Lin Miao and Associate Professor Xiaoqian Zhang from Southeast University, along with Professor Lixin He from USTC, serve as co-corresponding authors, and PhD student Wenting Lin, Associate Professor Xiaoqian Zhang, and Associate Professor Chaokai Li from the School of Physics, alongside Associate Researcher Zhenxiong Shen from USTC, are co-first authors. This research was funded by the National Key R&D Programof China, National Natural Science Foundation of China, Natural Science Foundation of Jiangsu Province, China, and the open research fund of Key Laboratory of Quantum Materials and Devices (Southeast University), Ministry of Education.
Link: https://doi.org/10.1002/adma.202515712.
