Observation of orbital liquid in ultrathin magnetic films

Electron's orbital moment has recently emerged as an important degree of freedom which can be generated, transported, and used to control magnetic systems [1]. However, its role in magnetism remains poorly understood. I will present experimental and theoretical evidence for the crucial role of electron’s orbital moment in ferromagnetism.

Magnetoelectronic measurements of heterostructures based on ultrathin transition metal ferromagnets reveal two separate magnetic order parameters: one associated with spin ordering at the Curie point T_C1, and another “anomalous” order parameter with a critical point T_c2 about 50K above the Curie temperature. Remarkably, magneto-optical measurements are not sensitive to the anomalous contribution, suggesting that the origin of the latter is qualitatively different from spin magnetism. X-ray magnetic circular dichroism (XMCD) measurements reveal that the “anomalous” order parameter is associated with incipient orbital ferromagnetism whose signatures vanish below T_c2 without the onset of ferromagnetic orbital ordering. Electric current applied to micropatterned structures in this regime reveals that orbital magnetism is “hidden” but does not disappear below T_c2.

I will show that these anomalous behaviors are captured by a simple Hubbard model of orbital correlations among nearest neighbor sites in an ultrathin ferromagnetic layer, leading to the conclusion that orbital moments form an orbital liquid – a long-range correlated orbital state that lacks ordering due to the geometric orbital frustration, analogous to quantum spin liquids formed by frustrated spins [2]. In the studied orbital liquid, orbital moments are ferromagnetically coupled, which would be impossible for spin liquid due to spin conservation. I will discuss the implications of these results for our understanding of magnetism and for the emerging field of orbitronics.



[1] D. Go, et al. “Orbitronics: Orbital currents in solids”, EPL 135, 37001 (2021).

[2] S. Ivanov, J. Peacock, S. Urazhdin “Orbital correlations in ultrathin films of late transition metals”, Phys. Rev. Mater. 7, 01440 (2023)