Room Temperature Strong Orbital Moments in Perpendicularly Magnetized Magnetic Insulator

The balance between the orbital and spin magnetic moments in a magnetic system is the heart of many intriguing phenomena. Here, experimental evidence of a large orbital moment is shown, which competes with its spin counterpart in a ferrimagnetic insulator thulium iron garnet, Tm 3 Fe 5 O 12 . Leveraging element‐specific X‐ray magnetic circular dichroism (XMCD), it is established that the dominant contribution to the orbital moment originates from 4 f orbitals of Tm. Besides the large Tm orbital moment, intriguingly, the results also reveal a smaller but evident non‐zero XMCD signal in the O K edge, suggesting additional spin‐orbit coupling and exchange interactions with the nearest neighbor Fe atoms. The unquenched orbital moment is primarily responsible for a significant reduction in g ‐factor, typically 2 in transition metals, as determined independently using ferromagnetic resonance spectroscopy. The findings reveal a non‐linear reduction in the g ‐factor from 1.7 at 300 K to 1.56 at 200 K in Tm 3 Fe 5 O 12 thin films. These results provide critical insights into the role of the f orbitals in long‐range magnetic order and stimulate further exploration in orbitronics.