Time-domain observation of ballistic orbital-angular-momentum currents with giant relaxation length in tungsten

Abstract

The emerging field of orbitronics exploits the electron orbital momentum L. Compared to spin-polarized electrons, L may allow the transfer of magnetic information with considerably higher density over longer distances in more materials. However, direct experimental observation of L currents, their extended propagation lengths and their conversion into charge currents has remained challenging. Here, we optically trigger ultrafast angular-momentum transport in Ni|W|SiO2 thin-film stacks. The resulting terahertz charge-current bursts exhibit a marked delay and width that grow linearly with the W thickness. We consistently ascribe these observations to a ballistic L current from Ni through W with a giant decay length (~80 nm) and low velocity (~0.1 nm fs−1). At the W/SiO2 interface, the L flow is efficiently converted into a charge current by the inverse orbital Rashba–Edelstein effect, consistent with ab initio calculations. Our findings establish orbitronic materials with long-distance ballistic L transport as possible candidates for future ultrafast devices and an approach to discriminate Hall-like and Rashba–Edelstein-like conversion processes.

Ultrafast THz probing of nonlocal orbital current in transverse multilayer metallic heterostructures

“…We argue that such inconsistency in the literature is arising because of the fact that conventionally, the experimental outcomes have been interpreted solely in terms of the spin current, neglecting the effect of orbital contribution altogether, though the latter can be gigantic even in the light metals with weak SOC 13 , 14 . The orbital current diffusion length can be several times larger than the spin counterpart in some heavy metals 7 , 25 , 48 , 53 , 59 . The orbital current due to spin-orbit conversion within the heavy metal W-insertion layer is given as 48 , according to which, higher thickness and stronger , both contribute to enhance the orbital current (see Supplementary Section S16 ).…”

“…Due to a large positive value of in the NiFe, a fraction of the ultrafast spin current is converted into an ultrafast orbital current (J L ) of same polarity through the L-S conversion, given 8 , 48 as, . Therefore, an ultrafast optically induced orbital current sets in 48 , 53 , which possess similar symmetry properties to the spin current but can exhibit relatively different transport dynamics 9 , 54 , 55 . Furthermore, as the ultrafast excitation of spin and orbital magnetization has been reported 51 , 56 – 58 to exhibit a similar evolution, the emergence of orbital current can also be comprehended through the analogy with the already established spin current formation 53 .…”

“…Therefore, an ultrafast optically induced orbital current sets in 48 , 53 , which possess similar symmetry properties to the spin current but can exhibit relatively different transport dynamics 9 , 54 , 55 . Furthermore, as the ultrafast excitation of spin and orbital magnetization has been reported 51 , 56 – 58 to exhibit a similar evolution, the emergence of orbital current can also be comprehended through the analogy with the already established spin current formation 53 . Consequently, as indicated in Fig.…”

Bulk and Interface Effects Based on Rashba-like States in Ti and Ru Nanoscale-Thick Films: Implications for Orbital-Charge Conversion in Spintronic Devices

“…Like SHE, the orbital counterpart comprises contributions from both bulk states (OHE) and surface states, denoted by the Orbital Rashba-Edelstein-like Effect (OREE-like), each governed by distinct underlying mechanisms. Although many theoretical studies have explored OHE and OREE, along with their inverse effects, − their generation and experimental detection have only been achieved recently. − Despite their identification and investigation, the controlled injection and accurate detection of orbital currents, specifically the flow of an OAM on a nanometric scale, pose significant challenges. Spin currents and orbital currents exhibit a fundamental distinction: while spin current directly transfers torque to magnetization, orbital current does not have this ability.…”

Unveiling of Terahertz Emission from Ultrafast Demagnetization and the Anomalous Hall Effect in a Single Ferromagnetic Film

“…In the case of sub-picosecond dynamics of the laser-induced electronic and/or magnetic dipole, the frequency of emitted electromagnetic radiation falls into the THz range. [32,33] In 2004, Beaurepaire et al first discovered that ul-trafast demagnetization (UDM) of Ni can generate THz radiation, and it is proportional to the second time derivative of the magnetization and is described by the threetemperature model. [1,2] In 2020, Zhang et al demonstrated a method for ultrafast THz magnetometry which directly accessed the ultrafast magnetization dynamics in a single-layer Fe film.…”