Condensed Matter Seminar - Orbital Liquid in Ultrathin Magnetic Films

The 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 discuss surprising experimental observations revealing a crucial role of the electron's orbital moment in ferromagnetism and elucidating a previously unrecognized connection between magnetism and unconventional superconductivity.

Our magnetoelectronic measurements of heterostructures based on ultrathin transition metal ferromagnets revealed two separate magnetic order parameters: one associated with spin ordering at the Curie point Tc1, and another "anomalous" order parameter with a critical point Tc2 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 show that the "anomalous" order parameter is associated with incipient orbital ferromagnetism whose signatures vanish below Tc2 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 Tc2.

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 [2] - a long-range correlated orbital state that lacks ordering due to the geometric orbital frustration, analogous to quantum spin liquids formed by frustrated spins and believed to hold the key to high-temperature superconductivity [3]. 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 the mechanisms of magnetism and for the emerging field of orbitronics.