Electrons possess electrical charge but also magnetic spin and orbital angular momentum. While charge and spin currents have driven advancements in electronics and spintronics, respectively, generating “orbital currents” has remained a challenge. An international research team has now successfully observed “orbital pumping,” a new phenomenon in which orbital currents are generated from the precession of magnetization in magnetic materials. The discovery could provide the foundation for “orbitronic” technologies based on orbital currents.

For decades, electronic devices have been developed by utilizing the electrical property of electrons, namely its charge. Then came spintronics, a technology that leveraged the electron’s “spin angular momentum,” a quantum property, to generate “spin currents” (flow of spins) that led to high-performance, low-power electronic devices. However, the electron also possesses an “orbital angular momentum” that could potentially provide the orbital counterpart to spin current—orbital current—and enable new developments in electronics. However, despite increasing research interests on this front, generating orbital currents has remained a challenge.

In a recent study made available online on June 27, 2024, and published in Volume 7, Issue 8 of the journal Nature Electronics on August 01, 2024, an international research team led by Associate Professor Kazuya Ando from Keio University, Japan reported the first experimental observation of “orbital pumping,” the orbital counterpart of spin pumping, a technique in which spin currents are produced from the motion of magnetization. The team successfully observed and demonstrated that the precession of the magnetization resulted in the emission of an orbital current in magnetic materials.

In their study, the researchers employed a bilayer system consisting of nickel (Ni) as the ferromagnetic material and titanium (Ti) as the non-magnetic material and utilized microwave-induced magnetization to generate orbital currents in the system. The team varied the thickness of the Ti layer and studied its dependence on charge current generation. They investigated the system’s response by applying an in-plane magnetic field and using a vector network analyzer to measure microwave absorption intensity. By examining the structure, magnetic field, and material dependence of the generated signal, the team successfully detected orbital pumping and confirmed that orbital currents could be induced from the magnetization of ferromagnetic materials, marking a remarkable advancement in our understanding of orbital dynamics and the physics of orbital currents.

“We believe that the discovery of orbital pumping will open up new avenues for electronic technologies based on orbital currents,” says Dr. Ando. “While much attention has been focused on spin currents in recent years, our study represents an unexplored frontier of electron transport that could significantly contribute to the development of devices, such as non-volatile memory, high-frequency oscillators, and neuromorphic systems.”

For the Ni/Ti bilayer, the team further checked the validity of the experiment by performing a pumping experiment in which the out-of-plane microwave magnetic field was dominant. By examining additional bilayer combinations, including cobalt (Co)/Ti and iron (Fe)/Ti, they observed that the orbital pumping efficiency was strongly influenced by the correlation between spin and orbital angular momenta near the Fermi surface. “Material properties, particularly the strength of spin–orbit coupling, are essential for the efficient generation of orbital currents,” explains Dr. Ando.

Additionally, the researchers found that the orbital current generated by the orbital pumping in the Ni/Ti bilayer was converted into a charge current through a mechanism known as the “inverse orbital Hall effect” (IOHE) in the Ti layer. Notably, this happened without relying on the usual spin–orbit coupling mechanism. The IOHE mechanism was thus a crucial aspect of this study, as it electrically detected the orbital pumping phenomenon.

In summary, the successful observation of orbital pumping marks a significant milestone in the field of electron transport and could pave the way for a new era of electronics beyond conventional charge and spin-based technologies. Just as spin pumping has been instrumental in advancing spintronics, orbital pumping could lead to “orbitronic” technologies and devices with reduced power consumption and improved speeds.

***

Reference

Title of original paper: Observation of orbital pumping

Journal: Nature Electronics

DOI: https://doi.org/10.1038/s41928-024-01193-1