Absence of orbital current torque in Ta/ferromagnet bilayers

Lijun Zhu; Qianbiao Liu

REVIEW 3 major objections 4 minor 1 cited by

Absence of orbital current torque in Ta/ferromagnet bilayers

T0 review · 3 major / 4 minor · reviewed 2026-08-10 · deepseek-v4-flash

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Pith's one-line read The spin-orbit torque from tantalum in Ta/ferromagnet bilayers is uniformly negative and matches the spin Hall effect alone; the positive torque reported for Ta/Ni is a self-induced ST-FMR artifact of the Ni layer.

desk verdict A potentially important negative result on orbital torque in Ta/FM, but the central subtraction step lacks a reported current-density normalization that could flip the sign. read the letter →

arxiv 2501.10260 v2 pith:MWAYMYXU submitted 2025-01-17 cond-mat.mtrl-sci

Qianbiao Liu , Lijun Zhu This is my paper · ORCID

classification cond-mat.mtrl-sci

keywords spin-orbittorqueorbitalHalleffectcurrenttantalumspin-torqueferromagneticresonanceself-inducedST-FMRspinferromagnetbilayer

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper aims to settle whether the orbital Hall effect of tantalum produces a detectable orbital-current torque on an adjacent ferromagnetic layer. By measuring spin-torque ferromagnetic resonance across five different ferromagnets and a wide thickness range, the authors find that the torque generated by Ta has essentially the same negative efficiency in every case, matching the sign and magnitude expected from Ta's spin Hall effect alone. They argue that the positive torque previously reported for Ta/Ni, often cited as evidence for orbital-current torque, is an artifact of a strong thickness-dependent self-induced ST-FMR signal inside the Ni layer itself. If correct, this removes a key experimental pillar of orbital-torque claims and restores the standard spin Hall picture for Ta/ferromagnet bilayers.

What carries the argument

The key mechanism is the self-induced bulk spin-orbit torque within the ferromagnetic layer itself, which produces a spurious symmetric and antisymmetric ST-FMR signal that is thickness-dependent. The measurement machinery is three-terminal spin-torque ferromagnetic resonance: the bilayer and control single-layer devices are driven by a radio-frequency current, the mixed voltage is fit to symmetric and antisymmetric Lorentzians, and the dampinglike efficiency is extracted from the inverse intercept of $1/\xi_{\mathrm{FMR}}$ versus $1/t_{\mathrm{FM}}$. The load-bearing step is subtracting the single-layer FM's symmetric and antisymmetric components from the bilayer's components before computing the Ta-only efficiency; without this subtraction, the Ni self-torque dominates the apparent Ta/Ni torque and flips its sign.

What would settle it

A decisive check is to measure the Ta/Ni dampinglike torque using harmonic Hall voltage on the same films with the Ni self-torque separately characterized, or to grow Ta/Ni with a series of Ta thicknesses at fixed Ni thickness and perform the same single-layer subtraction; if the corrected efficiency depends on Ni thickness or Ta thickness in a way not matching the Ta spin Hall conductivity, the subtraction premise is falsified.

Watch

Extended reading notes

Core claim

The central claim is that the dampinglike spin-orbit torque produced by a 5 nm Ta layer has essentially the same negative efficiency, about $-0.028$ after correction, for Ni, Ni$_{81}$Fe$_{19}$, Fe, Fe$_{60}$Co$_{20}$B$_{20}$, and Fe$_{50}$Pt$_{50}$, independent of ferromagnet type and thickness, and therefore originates solely from the negative spin Hall effect of Ta. The apparent positive efficiency of Ta/Ni bilayers in the Ni thickness range above about 2 nm is identified as an artifact: the Ni layer itself generates a strong self-induced ST-FMR signal of the same sign and comparable magnitude, and this signal grows with thickness. After subtracting the symmetric and antisymmetric responses of control single-layer ferromagnets from the bilayer responses, the Ta-contributed efficiency becomes uniformly negative and matches the non-Ni ferromagnets. Control experiments with a Si$_3$N$_4$ thermal sink and rf-power dependence exclude anomalous Nernst and spin-pumping explanations. The paper concludes that the orbital Hall effect of Ta, despite predicted conductivities 20 to 50 times larger than its spin Hall conductivity, makes no detectable contribution to the interfacial torque in Ta/ferromagnet systems.

Load-bearing premise

The subtraction step assumes that the spin-torque signal produced by a bare ferromagnet layer grown on just 1 nm of tantalum is identical, in size and phase, to the ferromagnet's own signal when it sits on the 5 nm tantalum layer being studied.

Editorial extensions

If this is right

The positive Ta/Ni dampinglike-torque efficiencies cited as evidence for orbital-current torque disappear once the self-induced ST-FMR signal of the Ni layer is subtracted, so those Ni-based claims need re-examination with single-layer controls.
The dampinglike torque from Ta remains negative and nearly constant across FM types and thicknesses, consistent with Ta's spin Hall effect being the sole source of the interfacial torque.
Extracted spin Hall conductivities of Ta from ST-FMR do not need an orbital-current correction of the kind suggested by the 20 to 50 times larger predicted orbital Hall conductivity.
ST-FMR thickness-series determinations of interfacial torques are unreliable for ferromagnets with strong bulk self-torque unless the ferromagnet's own signal is measured and subtracted, even for FM layers thicker than 10 nm.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

A natural extension is to look for the orbital Hall effect of Ta directly in bulk transport, e.g., by detecting the inverse orbital Hall effect in a Cu or nonmagnetic detector layer, which would separate whether an orbital current exists in Ta from whether it exerts a torque on the adjacent ferromagnet.
The same single-layer subtraction logic could be applied to orbital-torque claims in other light metals such as Cr, Ti, and Zr, whose reported torque signs may also be contaminated by self-induced FM signals.
Because the control samples use a 1 nm Ta adhesion layer, a stricter control would eliminate Ta entirely, e.g., by growing the FM on a nonmetallic seed; if that changes the extracted residual, the subtraction is not exact.
Varying the Ta thickness while holding the Ni thickness fixed would provide a quantitative test: the corrected Ta efficiency should track the Ta spin Hall conductivity and stay independent of Ni thickness if the central claim holds.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

The paper makes a genuinely new claim: the positive dampinglike torque efficiency in Ta/Ni, previously read as evidence for orbital current torque, is actually a self-induced ST-FMR signal from the Ni layer. That is a specific, testable idea, and the authors go after it with a systematic dataset—five ferromagnets, a thickness series, harmonic Hall measurements, a Si3N4 thermal sink, and a power-dependence check. If the central claim holds, it removes one of the most cited experimental pillars of the orbital-torque literature. Credit where it is due: the experiment is well designed, the controls are thoughtful, and the writing is clear about what the raw ST-FMR analysis can and cannot show.

The soft spot is the subtraction procedure. The paper subtracts the ST-FMR spectrum of a single FM layer (grown on only 1 nm Ta) from the bilayer spectrum (5 nm Ta) to isolate the Ta contribution. That subtraction assumes the self-induced signal in the control matches the self-induced component in the bilayer. The paper does not report normalizing to the FM current density. Because 5 nm Ta shunts current away from the FM, the FM current fraction in the bilayer is visibly lower than in the control. If the control signal is larger than the self-induced component in the bilayer, subtracting it unnormalized over-subtracts, and a large positive self-torque could artificially shift the residual Ta torque negative. The sign reversal from +0.041 to −0.028 is exactly the direction this would produce. The harmonic Hall correction appears to have the same issue. This is not a minor detail; it is the load-bearing joint of the paper's core claim. The authors may well have normalized in the Supplementary Note, but the main text does not say so, and the data are not public.

Other concerns are minor: the control uses 1 nm Ta rather than the bilayer's 5 nm Ta, which changes the interface environment, and the raw data are not deposited. But the paper is honest in scope, and the framework—separating bulk self-torque from interfacial torque—is a useful contribution even if the quantitative conclusion is not yet nailed down.

Who this is for: spintronics experimenters who work on ST-FMR and orbital torque. It deserves a serious referee, because the claim is important and the experimental effort is substantial. But I would not cite it as evidence for the absence of orbital torque until the current-density normalization is either confirmed in the supplementary material or demonstrated in a follow-up. My own verdict is conditional, and I would push the editor to demand the normalization detail before acceptance.

Referee Report

3 major / 4 minor

Summary. The paper reports ST-FMR and harmonic-Hall measurements on Ta(5 nm)/FM bilayers with several ferromagnets (Ni, Ni81Fe19, Fe, Fe60Co20B20, Fe50Pt50) and on control FM single layers grown on a 1-nm Ta adhesion layer. The authors find that, without correction, Ta/Ni yields a positive dampinglike torque efficiency of about +0.041, whereas the other Ta/FM systems yield negative values around -0.03. They attribute the positive Ni value to a thickness-dependent self-induced ST-FMR signal of the Ni layer itself. After subtracting the self-induced S and A signals measured on the single-layer controls, they obtain a negative Ta-contributed efficiency of about -0.028 for Ta/Ni, similar to the other FMs, and conclude that the orbital Hall effect of Ta makes no detectable contribution to the interfacial torque, so that previously reported positive orbital torques in Ta/Ni are artifacts.

Significance. If the central claim holds, the paper would resolve a prominent controversy by showing that the positive dampinglike torques reported for Ta/Ni bilayers in the orbital-torque literature arise from an overlooked self-induced bulk torque in the Ni layer, not from orbital-to-spin conversion. The paper's strength is its multi-pronged experimental approach: ST-FMR across five FM types and a range of thicknesses, a Si3N4 thermal-sink test for anomalous Nernst contributions, a power-dependence check, and independent harmonic-Hall measurements. The conclusion is falsifiable and quantitatively testable. However, the load-bearing subtraction step that separates the Ta contribution from the self-induced FM contribution rests on assumptions about the equivalence of the control and bilayer samples that are not yet validated in the manuscript.

major comments (3)

[Methods and Fig. 3a-b] The central subtraction of the single-layer FM self-induced S and A signals from the bilayer signals requires that the two measurements be normalized to the same radio-frequency current density in the ferromagnet. The control samples have only 1 nm Ta, while the bilayers have 5 nm Ta, so the FM current fraction differs between the two stacks; with the reported Ta resistivity of 200 µΩ cm and Ni thicknesses of 2-4 nm, the Ni current density in the bilayer is roughly 10-20% lower than in the control. The manuscript does not state that the control S and A amplitudes were scaled to the bilayer FM current density before subtraction. Without such normalization, subtracting the large positive Ni self-torque from the bilayer would over-subtract and could artificially shift the residual Ta contribution negative. The same issue applies to the harmonic-Hall correction described in Supplementary Note 1. This is the decisive step for the paper's main claim and must be documented and justified.
[Fig. 3 and Supplementary Note 2] The subtraction assumes that the self-induced ST-FMR signal measured in a single FM layer grown on a 1-nm Ta adhesion layer is identical in amplitude and phase to the self-induced signal inside the Ta(5 nm)/FM bilayer. However, the 5-nm Ta underlayer can change the texture, strain, interface electronic structure, and possibly the magnetic damping of the FM compared with the 1-nm-Ta control. The manuscript offers no independent validation of this equivalence, such as a control with 5 nm Ta on the back side of the substrate or a comparison of magnetic properties, damping, or FMR linewidth between the control and bilayer samples. If the self-induced signal differs between the two stacks, the residual S and A attributed to Ta are not established as the true spin Hall torque.
[Eq. (2) and Fig. 3a] Equation (2) defines ξ_FMR using the heavy-metal thickness d_HM, but for the single-layer control samples there is no heavy-metal layer, and the text does not specify what thickness is inserted when computing the apparent self-induced ξ_FMR for those controls. Since the correction procedure depends on converting S and A into efficiency units before subtraction, the exact convention used for the control samples must be stated explicitly; otherwise the subtracted quantities are ambiguous.

minor comments (4)

[Introduction] The phrase 'triggers bloomed interest' in the opening sentence is ungrammatical and should be revised.
[Fig. 2a] The figure caption should state how many devices and how many field sweeps contribute to each standard deviation, and whether the error bars include the uncertainty from the linear fits in Fig. 1d and Fig. 3b.
[Additional information] The placeholder 'available at xxx' for the Supplementary Information should be replaced with a working link or reference before publication.
[Abstract and Discussion] The word 'unambiguous' in the abstract overstates the strength of the evidence given that the key subtraction relies on assumptions that are not yet validated; a more measured claim would better match the presented data.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: the Ta torque sign follows from direct single-layer baseline subtraction and independent controls, not from fitted inputs or self-citations.

full rationale

The paper's central claim is that the positive Ta/Ni ST-FMR efficiency is an artifact of a self-induced FM signal, and that after subtracting that artifact the Ta contribution is negative and consistent with the spin Hall effect. The derivation chain is empirical: bilayer and control single-layer spectra are separately fitted to Eq. (1); the single-layer S and A amplitudes are subtracted from the bilayer amplitudes; and the residual is reinterpreted through Eqs. (2) and (3). The control is an independently measured sample, not a parameter fitted to force the conclusion, so the negative sign of the residual is an experimental outcome rather than an input. The claim is further cross-checked by harmonic Hall measurements and by a thermal-sink experiment. The paper does cite prior work by the same group on bulk spin-orbit torques in single layers (refs. 45, 62, 63), but that citation is corroborative rather than load-bearing: the presence of the Ni self-induced signal is directly demonstrated in Fig. 3a and by independent harmonic Hall data. Concerns that the 1-nm-Ta control may not perfectly match the 5-nm-Ta bilayer in current density or self-torque are control-matching and correctness issues, not circularity: no equation defines the conclusion in terms of itself, and no fitted parameter is renamed as a prediction.

Assumptions & free parameters 1 free parameters · 5 assumptions · 0 invented entities

No new particles, forces, or conserved quantities are introduced. The central measurement is an efficiency fitted from ST-FMR data; the main assumptions are about the validity of the ST-FMR model, the additivity of self-induced and interfacial signals, and the equivalence of control and bilayer environments.

free parameters (1)

Dampinglike torque efficiency xi_DL^j from linear fits = Ta/Ni: +0.041 before correction, -0.028 after correction; other FMs: -0.026 to -0.033
These efficiencies are obtained from the inverse intercept of linear fits of 1/xi_FMR versus 1/t_FM. They are the measured quantities used to conclude that the torque is negative and independent of FM type.

assumptions (5)

domain assumption The ST-FMR mixing voltage model (Eq. 1) and the torque efficiency formulas (Eqs. 2-3) are valid for these bilayers.
The analysis adopts standard ST-FMR theory from refs. 5 and 46 without re-deriving it. If the mixing voltage decomposition or the transparency model is inadequate, the extracted efficiencies are biased.
domain assumption The self-induced ST-FMR signal of a bare FM layer is identical in the single-layer control and inside the Ta/FM bilayer, so subtracting the single-layer spectrum isolates the Ta contribution.
This is the central correction assumption, introduced around Fig. 3 and Supplementary Note 2. It is not independently demonstrated and is the main structural risk to the conclusion.
domain assumption A 1 nm Ta adhesion layer in the control samples introduces negligible spin and orbital current compared with the 5 nm Ta layer.
The Methods state this property of the control samples, but no measurement is shown to verify that the 1 nm Ta layer contributes zero torque. If it contributes a non-negligible torque, the subtraction is incomplete.
domain assumption The bulk spin-orbit coupling of the Fe alloys follows a linear composition dependence.
Used in Fig. 2b to estimate SOC values for Ni81Fe19, CoFeB, and FePt. The paper notes this is an estimate, and the qualitative conclusion is said to survive under weaker assumptions.
domain assumption The spin Hall conductivity of Ta is negative, from prior experimental and theoretical work.
The paper treats the negative sign of Ta's spin Hall effect as established background from refs. 1, 41, and 42. The conclusion that the measured negative torque matches the spin Hall effect depends on this prior result.

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Pith. "Pith review of Absence of orbital current torque in Ta/ferromagnet bilayers." pith.science (2026). https://pith.science/paper/MWAYMYXU

@misc{pith2026250110260,
  author       = {Pith},
  title        = {Pith review of: Absence of orbital current torque in Ta/ferromagnet bilayers},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/MWAYMYXU}},
  note         = {Machine review of arXiv:2501.10260}
}

read the original abstract

It has become a heated debate as to whether the orbital Hall effect of a material could generate a non-local orbital current and a non-zero spin-orbit torque on an adjacent magnetic layer. Here, we report unambiguous evidence that, regardless of the ferromagnets (FMs) (e.g., Ni, Ni81Fe19, Fe, Fe60Co20B20, and FePt), the spin-orbit torque generated by an adjacent Ta, which is predicted to have a 50 times greater positive orbital Hall conductivity than the negative spin Hall conductivity, has essentially the same, negative efficiency, in agreement with the spin Hall effect of Ta being the only source of the interfacial torque. We identify that the constant, positive estimate of the torque of the Ta/FM samples from spin-torque ferromagnetic resonance (ST-FMR) analysis in a specific FM thickness range (>2 nm for Ni), that was heavily cited in the literature to signify an orbital current torque but strongly disagrees with the fairly long relaxation length in other orbital current torque claims, results from the overlook of a significant thick-dependent self-induced ST-FMR signal of the FM. These results indicate the absence of orbital current torque in Ta/ferromagnet systems, regardless of the type and the layer thickness of the ferromagnets.

Figures

Figures reproduced from arXiv: 2501.10260 by the authors.

**Figure 2.** c shows the magnetic anisotropy energy density (Ks) of the FM interfaces as the indicator of the interfacial SOC57-59 that is distinct from the bulk SOC and sensitive to the short-range ordering and the spin-orbit proximity effect at the interface. Here, the values of Ks for the Ta/FM samples are determined from the slope of the linear fit of 𝑀eff vs 1/tFM following the relation of 𝑀eff = Ms - 2Ks/MstFM (Supplementa… view at source ↗

**Figure 3.** Self-induced ST-FMR signal. (a) Self-induced STFMR spectra for Ni 4, Ni81Fe19 3.8, Fe 3.3, Fe60Co20B20 3, and Fe50Pt50 3 single layers (φ = 45o ), with the three solid curves plotting the best fit of the data to Eq. (1) (in orange), the symmetric (in red), and antisymmetric (in blue) components. (b) Inverse thickness dependence of 1/ξFMR contributed from the Ta layers of the Ta/Ni, Ta/Ni81Fe19, Ta/Fe, Ta/Fe60Co20B2… view at source ↗

**Figure 4.** Origin of the positive estimate of the SOT [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗

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Pith tools

Reviewed August 10, 2026 · model on record in the stance chip above.

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