The impact of single atomic defects in layered altermagnet CoNb₄Se₈

Altermagnets are characterized by compensated collinear magnetic ordering with momentum dependent spin-splitting of the band structure as a consequence of specific spin-sublattice symmetries. Given their net zero magnetization, their ordering is predicted to be robust to external magnetic field, while still offering spin-polarized behaviors of ferromagnets, making them highly desirable for spintronics. Since magnetic impurities can offer a local probe into the robustness of local electronic and magnetic characteristics of materials, it is interesting to see how they can influence altermagnetic properties. Here, we study magnetic impurities in altermagnetic candidate CoNb₄Se₈ using spin-polarized scanning tunneling microscopy and spectroscopy (SP-STM/S) and density functional theory (DFT). With a ferromagnetic spin-polarized tip, we image magnetic impurity sites as well as clusters of impurities as a function of magnetic field, and we contrast the data to the pure altermagnetic state of the system. Our findings offer real-space insights into the stability and tunability of the altermagnetic characteristics of CoNb₄Se₈.