Local probe evidence supporting altermagnetism in Co 1 / 4 NbSe 2

Muon spin rotation ( μ + SR ) was used to investigate the magnetic properties of cobalt-intercalated 2 H − NbSe 2 ( Co 1 / 4 NbSe 2 ). Co 1 / 4 NbSe 2 is predicted to be an altermagnet, and while neutron diffraction has proposed its magnetic structure, the microscopic details of this remain unclear. Therefore, a local probe investigation of the magnetism in Co 1 / 4 NbSe 2 is essential. Our results reveal a sharp second-order transition to a full-volume-fraction, homogeneous magnetic order below T N = 168 K . The moments are aligned antiparallel along the c axis, consistent with neutron diffraction and altermagnetism. μ + SR reveals that the state remains stable under a c -axis magnetic field up to 0.78 T , with magnetization measurements suggesting that this robust regime extends to at least 5 T. Within the time sensitivity of μ + SR , no precursor slow magnetic fluctuations were detected above T N , which is important for interpreting the band splitting in the paramagnetic state reported by photoemission studies.