Unconventional spin glass state in elemental neodymium in the absence of extrinsic disorder

Spin glasses are a fascinating but highly complex magnetic state of matter, traditionally investigated in materials with a random distribution of exchange interactions. Intricately linked to spin frustration and disorder, spin glasses exhibit no long-range order and exude aging phenomena, distinguishing them from quantum spin liquids. Here we report a new type of non-collinear spin glass state, a so-called spin-Q glass, surprisingly observable in crystalline metallic neodymium. Using spin-polarized scanning tunnelling microscopy combined with ab initio calculations and atomistic spin-dynamics simulations, we visualize the variations in atomic-scale non-collinear order and its response to magnetic field and temperature. We quantify the aging phenomena relating the glassy behaviour to the crystalline symmetry and the energy landscape. This not only resolves the long-standing debate of the magnetism of neodymium, but also suggests that glassiness may arise in other magnetic solids lacking extrinsic disorder, providing new platforms for brain-inspired hardware and topological quantum excitations.