Unconventional views on orbitronics supported by experimental results

Abstract: Emerging orbitronics assumes long-range orbital current transport, analogous to spin currents. However, recent theory and experiments challenge this view, showing rather local characters for orbital polarization and orbit-spin conversions. We study angular momentum generated by ferromagnetic resonance and thermal gradients in Ni/(Pt)Ti/Au heterostructures. The observed charge current produced is independent of Ti thickness up to 60 nm, incompatible with orbital transport in Ti. Instead, its magnitude depends on both Ti interfaces, evidencing spin-mediated transport in between after and before local orbit-spin interconversions.

• The paper reveals that orbital-to-charge conversion is dominated by local interfacial effects rather than long-range orbital transport in Ti.
• The authors use ferromagnetic resonance and thermal gradient methods to show that charge currents remain independent of Ti thickness, setting an orbital diffusion upper limit of ~1 nm.
• The findings call for a paradigm shift in orbitronic device design, emphasizing precise interface engineering for effective spin-orbit and orbital-to-spin conversion.

Revisiting Orbital Angular Momentum Transport in Metals: Evidence for Interfacial Orbitronics

Introduction

The study "Unconventional views on orbitronics supported by experimental results" (2603.28075) challenges the prevailing paradigm in orbitronics regarding long-range orbital angular momentum (OAM) transport in metals. While initial orbitronic models, inspired by spintronics, posited that orbital currents can propagate over distances comparable to or greater than spin currents, this work combines rigorous experimental and first-principles theoretical analysis in Ti-based heterostructures to argue for a distinctly local—predominantly interfacial—nature of orbital-to-charge conversion. The observed independence of charge and thermal currents on Ti thickness, along with strong dependence on interfacial engineering, directly contradicts claims of macroscopic orbital diffusion lengths in clean light metals.

Theoretical Background and Controversy

Historically, significant theoretical efforts have predicted a large orbital Hall conductivity ($\sigma_{OH}$) in early $3d$ transition metals such as Ti, surpassing spin Hall conductivity ($\sigma_{SH}$) by up to an order of magnitude [Go2024; Tanaka2008]. These models, and several experimental claims, have supported the hypothesis of bulk-mediated, long-range OAM transport, with reported orbital diffusion lengths ($l_{of}$) ranging between 3 and 60 nm for Ti [Hayashi2023, Choi2023, Sun2025, Santos2025, santos2026probing]. In contrast, recent quantum and ab initio theoretical analyses have shown that in centrosymmetric and time-reversal invariant nonmagnetic lattices, OAM current and orbital accumulation relax over a few atomic layers ($l_{of} \leq 1$ nm) even in disorder-limited regimes [Kelly2024; Valet2025; ValetPRL2025; Ning2025]. Interfacial effects and local interconversion via the (inverse) orbital Edelstein effect (IOEE/OEE) become dominant in this scenario [Go2018; Sergio2023].

Experimental Strategy

To resolve this debate, the authors fabricate a series of high-quality Ni/Ti/Au and Ni/Pt/Ti/Au heterostructures, with engineered interfaces and variable Ti thickness up to 60 nm. They employ two angular momentum injection schemes: spin/orbital pumping via ferromagnetic resonance (SOP-FMR) and a thermal gradient-induced spin/orbital Seebeck effect (SOSE). Both approaches are chosen for their ability to distinguish between interfacial conversion processes and potential bulk OAM propagation.

Key Experimental Findings

Ti Thickness Independence: Evidence for Interfacial Conversion

The central empirical result is the independence of both the charge current ($I_c$) generated via SOP-FMR and the thermally-induced charge current ($i_{thermo}$) on Ti layer thickness, across all heterostructure variants. Even as the Ti thickness is increased from 2 to 60 nm, $I_c$ remains invariant, in stark contrast to expectations from bulk IOHE or ISHE, where the signal should scale as $\tanh(t_{Ti}/2l_{of})$ and saturate at several orbital diffusion lengths. The experimental data, exhibiting plateaus for $t_{Ti} > 4$ nm, set an upper bound $3d$0 nm for Ti. The charge current magnitude strongly depends on both the bottom Ni/Ti and the top Ti/X (X = Au, Pt, W, Al) interface, not the Ti bulk. Capping with Pt or Au produces significant modulation in conversion efficiency, consistent with their high SOC and interface-specific conversion properties.

Absence of Bulk Orbital Hall and Spin Hall Effects in Ti

The authors present corroborating data that show negligible ISHE and IOHE in Ti. The minimal variation of the effective magnetic damping constant ($3d$1) with Ti thickness further reinforces the absence of bulk, diffusive transport of angular momentum via OAM in pure Ti. First-principles calculations show minimal OAM current generation within the bulk, but significant interfacial conversion capacities, particularly at the Ti/Au and Ti/Pt interfaces.

Multistep Interfacial Conversion Mechanism

Systematic interface engineering reveals a multi-interface angular momentum transfer pathway. OAM is initially generated (pumped) at the Ni/Ti interface, with local conversion to spin current via O-to-S conversion. The spin current then propagates across Ti (owing to a long spin diffusion length $3d$2 nm) and is reconverted to OAM at the top interface (Ti/Au, Ti/Pt, etc.) and subsequently to a measurable charge current via IOEE. The observed $3d$3 is strongly modulated by interface properties, showing highest efficiency for Ni/Ti/Au and Ni/Pt/Ti/Pt, and lowest for Ni/Ti/W, consistent with the sign and magnitude of spin and orbital Hall conductivities of the cap metals. The effective interfacial conversion length is measured as $3d$4 pm, an order of magnitude smaller than values in Pt-based ISHE or double Rashba interfaces [C-Rojas2019; Anadon2025].

Suppression of Long-Range OAM Transport in Clean Crystalline Ti

By carefully avoiding rectification artifacts and using ultraclean, quasi-epitaxial Ti with metallic capping, the work rules out signal enhancement by disorder, oxidation, or extrinsic side-effects—factors implicated in some previous reports of anomalously large $3d$5 [Yactayo2026; Liu2025-disorder; Belashchenko2023]. The observed OAM relaxation is strictly local, with all "long-range" orbital effects traceable to interfacial conversions and subsequent spin current propagation, not intrinsic OAM drift-diffusion in the bulk.

Contradictory Claims in Context

This study forcefully contradicts prior experimental interpretations of long OAM diffusion lengths in Ti [Choi2023; Hayashi2023] by demonstrating that in structurally controlled, metallic Ti the orbital signal is confined to interfaces. The findings are isomorphic to contemporary claims of pure spin-current-induced torques in metals with large theoretical $3d$6 (e.g., Ta, Zr), where the measured efficiency is always consistent with spin Hall effect alone [Liu2025].

Theoretical and Practical Implications

The results require a fundamental shift in orbital transport models for clean metals: OAM is neither a bulk-propagating carrier nor can it enable genuine long-range order unless mediated by spin transport. This has direct implications for device design in orbitronics; interface engineering and materials selection for optimal local O-to-S and S-to-O conversion are paramount, while controlling spin diffusion properties in the bulk remains essential. In practical terms, this enables rational analysis of device stacks for efficient OAM-to-charge conversion or spin-orbit torque generation and highlights the need for robust interface synthesis.

Recent theoretical work supports this interfacial paradigm, predicting local orbital relaxation and emphasizing the dominance of extrinsic and disorder-mediated effects in cases of apparent long-range OAM transport [Kelly2024; Valet2025; Ning2025; Liu2025-disorder]. The presented model also rationalizes unexpected variations in previous experiments by linking signal magnitude to nuances in interface quality, capping, and device microstructure.

Outlook and Future Directions

The conclusion that OAM transport in clean metallic systems is inherently short-range refocuses future orbitronics research on two main avenues:

• Interfacial orbital and spin conversion: Maximizing IOEE and related effects through interface alloying, atomic-scale engineering, and selection of high SOC interfaces.
• Disorder and proximity engineering: Controlled introduction of disorder or the use of oxide, chiral, or topological cap layers may extend OAM-based phenomena nonlocally, but only extrinsically and with significant complexity in interpretation.

Theoretical developments should refine ab initio and kinetic descriptions for interfacial conversion, including the impact of crystal fields, Rashba and topological effects, symmetry, and hybridization on conversion efficiency. Practically, these insights will inform the design of spin-orbitronic and orbitronic elements, including data storage and logic devices, operating far from the drift-diffusion limit.

Conclusion

This study provides robust direct evidence that, contrary to earlier models, orbital current and accumulation in clean Ti are confined to interfacial regions, with the effective orbital relaxation length on the order of 1 nm or less. Long-distance angular momentum transport observed in Ni/Ti/X and Ni/Pt/Ti/X heterostructures emerges from spin current propagation, not OAM drift-diffusion. The interfacial O-to-S and S-to-O conversion mechanisms, modulated by cap layer selection, are the essential ingredients enabling observable signals. These results constrain theoretical models of OAM transport and set the agenda for the design and interpretation of orbitronic devices based on local conversion and spin-mediated transport (2603.28075).