Observation of orbital liquid in ultra-thin magnetic films*

We experimentally demonstrate the existence of an orbital liquid, which similarly to quantum spin liquids is manifested by long-range correlations of orbital moments that lack ordering due to geometric frustration. Magnetoelectronic measurements of heterostructures based on ultrathin CoFeB films reveal two magnetic order parameters: one associated with spin ordering at the Curie point, and another "anomalous" order parameter with a critical point about 50K above the Curie point. Remarkably, magneto-optical Kerr effect measurements are not sensitive to the latter contribution, suggesting that its origin is qualitatively different from spin magnetism. X-ray magnetic circular dichroism measurements reveal that the "anomalous" order parameter is associated with orbital magnetism whose signatures vanish at low temperatures without the onset of orbital ordering. Our findings are consistent with ferromagnetic orbital correlations due to the Hund's interaction, which do not result in orbital ferromagnetism due to the geometric frustration. The results elucidate the role of orbital degrees of freedom in magnetism and suggest new possibilities for controlling orbital moments in magnetic materials for orbitronic applications.