Researchers at National Taiwan University reported the first experimental evidence of heat-current-driven transverse accumulation of electrons’ orbital angular momentum.
The study describes the orbital Nernst effect. It was published in Nature Communications.
Hall and Nernst effects are fundamental phenomena in electric and thermal transport. They describe how information carriers like electrons or spins develop a transverse response when driven by an electric field or temperature gradient.
Among the Hall family, spin Hall and orbital Hall effects allow electric current to generate transverse flows of spin and orbital angular momentum. These angular momentums transfer directly or indirectly to magnetic moments in a material, potentially switching magnetization to write information in spintronic memory devices.
Distinguishing orbital from spin angular momentum experimentally remains challenging because both break time-reversal symmetry and share detection geometry. Establishing the orbital Nernst effect is even more demanding because heat flow is harder to control and characterize than electrical current.
To overcome these challenges, the research team designed a multilayer structure that converts heat-generated orbital angular momentum into a measurable electrical signal. They selected naturally oxidized copper film as the orbital current source and yttrium iron garnet as the detection layer.
Identifying the orbital current-dependent thermoelectric signal requires overcoming several experimental complications. The first challenge comes from the charge Seebeck effect, where temperature differences in connecting wires produce background voltage that must be removed.
A second challenge is an unintended out-of-plane temperature gradient perpendicular to the film plane. This contaminates the orbital Nernst signal with spin Seebeck voltages, which researchers separated by examining distinct symmetries under magnetic field reversal.
The final challenge is distinguishing the orbital Nernst effect in naturally oxidized copper film from the conventional spin Nernst effect in platinum. The team compared three sample structures: yttrium iron garnet and platinum, yttrium iron garnet and naturally oxidized copper film, and yttrium iron garnet, platinum, and naturally oxidized copper film.
The platinum-only sample exhibited a thermal signal associated with the spin Nernst effect, whereas the copper-oxide-only sample showed almost no detectable response. When platinum and copper oxide were combined, the measured signal reversed sign, and the estimated total spin current reaching the yttrium iron garnet interface increased to more than twice that of the platinum-only sample.
“The orbital Hall effect has been intensively studied in recent years, while much less is known about its thermal counterpart, the orbital Nernst effect,” said Dr. Danru Qu of Center for Condensed Matter Sciences at National Taiwan University.
“Our work provides experimental evidence that a temperature gradient can generate orbital angular momentum transport. This finding extends our understanding of orbital transport from electrical to thermal driving and may offer new possibilities for spintronic applications,” Qu said.
The research team used yttrium iron garnet, a magnetic insulator that acts as a spin current gate to control reflection and absorption without introducing additional charge transport.
Researchers estimated that the orbital Nernst response of naturally oxidized copper film is approximately four times greater than the spin Nernst response of platinum.
The study was published in Nature Communications by researchers at National Taiwan University.
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