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arxiv: 2604.27076 · v1 · submitted 2026-04-29 · ❄️ cond-mat.supr-con

Recognition: unknown

Superconductivity-Enabled Conversion of Ferromagnetic Resonance into Standing Spin Waves

Ya. V. Turkin , N. G. Pugach , F. M. Maksimov , A. S. Pakhomov , A. I. Chernov , V. I. Belotelov , S. N. Polulyakh , V. S. Stolyarov
Authors on Pith no claims yet

Pith reviewed 2026-05-07 09:02 UTC · model grok-4.3

classification ❄️ cond-mat.supr-con
keywords superconductivityferromagnetic resonancestanding spin wavestriplet Cooper pairsAbrikosov vorticesspin-transfer torquemagnonicsproximity effect
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The pith

A conventional superconductor converts uniform ferromagnetic resonance into perpendicular standing spin waves in an adjacent ferrimagnetic insulator.

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

The paper shows that in bilayers of bismuth-substituted iron garnet and niobium, microwave transmission gains an extra resonance peak only when the niobium becomes superconducting. This peak signals the uniform FMR mode transforming into perpendicular standing spin waves. The accompanying theory couples the superconducting condensate to the magnet's precession dynamics and identifies the required ingredients as interfacial torque from triplet Cooper pairs plus a depth-dependent field from Abrikosov vortices. If correct, superconductivity supplies a new dissipationless handle on short-wavelength magnons in insulating magnets.

Core claim

In Bi-substituted iron-garnet/Nb bilayers the microwave transmission develops an additional resonance feature only below the Nb transition temperature and near the uniform FMR peak. A microscopic theory self-consistently coupling the quasiclassical Keldysh-Usadel description of the superconducting condensate to the Landau-Lifshitz-Gilbert dynamics demonstrates that the conversion requires an interfacial spin-transfer torque mediated by spin-polarized triplet Cooper pairs together with a depth-dependent effective field produced by Abrikosov vortices. The resulting susceptibility reproduces the measured lineshapes and establishes superconductivity as an active control for exchange standing-wav

What carries the argument

Interfacial spin-transfer torque from spin-polarized triplet Cooper pairs combined with the depth-dependent effective field from Abrikosov vortices, within the coupled Keldysh-Usadel and Landau-Lifshitz-Gilbert framework.

If this is right

  • The uniform FMR mode converts into perpendicular standing spin waves only below the superconducting transition temperature.
  • Superconductivity functions as a control knob for exchange standing-wave modes in magnetic insulators.
  • The coupled theory yields a susceptibility that matches the observed resonance lineshapes.
  • Spin transport proceeds without Joule dissipation while coupling coherently to short-wavelength magnons.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Varying the applied magnetic field to change vortex density could provide a tunable knob for the strength of the standing-wave conversion.
  • Hybrid superconducting-magnonic circuits might be built to manipulate spin waves at low power using this proximity mechanism.
  • The same interface ingredients could be tested in other diffusive superconductors paired with different ferrimagnetic insulators.

Load-bearing premise

The interface must support spin-polarized triplet Cooper pairs that generate spin-transfer torque while Abrikosov vortices produce a sufficient depth-dependent effective field inside the magnetic layer.

What would settle it

Absence of the extra resonance feature in microwave transmission spectra when temperature is raised above the niobium transition temperature, or when the applied field is set to suppress vortices, would disprove the conversion mechanism.

Figures

Figures reproduced from arXiv: 2604.27076 by A. I. Chernov, A. S. Pakhomov, F. M. Maksimov, N. G. Pugach, S. N. Polulyakh, V. I. Belotelov, V. S. Stolyarov, Ya. V. Turkin.

Figure 1
Figure 1. Figure 1: FIG. 1 view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2 view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3 view at source ↗
read the original abstract

Superconductors can transport spin without Joule dissipation, yet their coherent coupling to short-wavelength magnons in insulating magnets remains largely unexplored. Here we demonstrate experimentally and theoretically that a conventional diffusive superconductor can enable the conversion of the uniform ferromagnetic-resonance (FMR) mode into perpendicular standing spin waves (PSSWs) in an adjacent ferrimagnetic insulator. In Bi-substituted iron-garnet/Nb bilayers, the microwave transmission develops an additional resonance feature that appears only below the Nb transition temperature and lies close to the uniform FMR peak. A microscopic theory that self-consistently couples the quasiclassical Keldysh--Usadel description of the superconducting condensate to the Landau--Lifshitz--Gilbert dynamics shows that the conversion requires two ingredients: (i) an interfacial spin-transfer torque mediated by spin-polarized triplet Cooper pairs and (ii) a depth-dependent effective field produced by Abrikosov vortices (electromagnetic proximity). The resulting susceptibility reproduces the measured lineshapes and establishes superconductivity as an active control knob for exchange standing-wave modes in magnetic insulators.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript reports experimental observation of an additional resonance feature in BiIG/Nb bilayers that emerges exclusively below the Nb superconducting transition temperature Tc and lies near the uniform FMR peak. A microscopic model self-consistently couples the Keldysh-Usadel equations for the diffusive superconductor to the LLG dynamics of the ferrimagnet, showing that conversion of uniform FMR into PSSWs requires an interfacial spin-transfer torque from spin-polarized triplet Cooper pairs together with a depth-dependent effective field generated by Abrikosov vortices. The resulting susceptibility is stated to reproduce the measured lineshapes.

Significance. If the central claim is substantiated, the work establishes that a conventional diffusive superconductor can actively convert uniform FMR into exchange-dominated standing spin waves in an adjacent insulating magnet via proximity-induced torques and electromagnetic fields. This provides a concrete, falsifiable route to superconducting control of magnonic modes without dissipation in the superconductor itself and strengthens the case for hybrid superconducting-magnonic devices. The use of established Keldysh-Usadel and LLG frameworks is a methodological strength.

major comments (2)
  1. [Theory section (microscopic model)] The central claim that the susceptibility reproduces the measured lineshapes rests on the two ingredients (interfacial triplet torque and vortex-induced depth-dependent field). The manuscript should provide the explicit functional form of the vortex field profile (including vortex density and penetration depth) and demonstrate that it is fixed by independent superconducting parameters rather than adjusted to fit the resonance position and width.
  2. [Theory section (interfacial boundary conditions)] The interfacial spin-transfer torque amplitude is a free parameter. The paper should report the specific numerical value employed, its physical range consistent with microscopic estimates of triplet penetration, and a sensitivity analysis showing that PSSW conversion occurs robustly within that range; otherwise the reproduction of lineshapes risks being post-hoc.
minor comments (2)
  1. [Theory section] Clarify the precise definition of the effective field term arising from electromagnetic proximity in the LLG equation; the notation for the depth dependence should be made explicit to allow direct comparison with the Usadel solution.
  2. [Experimental results] The experimental section should include the microwave transmission data with error bars and the temperature dependence across Tc to quantify the abruptness of the additional feature's onset.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive comments on the theory section. We agree that additional details on the vortex field profile and the interfacial torque parameter are needed to strengthen the presentation. We have revised the manuscript and supplementary material accordingly, providing the requested explicit forms, parameter values, and supporting analysis. Our point-by-point responses follow.

read point-by-point responses

  1. Referee: [Theory section (microscopic model)] The central claim that the susceptibility reproduces the measured lineshapes rests on the two ingredients (interfacial triplet torque and vortex-induced depth-dependent field). The manuscript should provide the explicit functional form of the vortex field profile (including vortex density and penetration depth) and demonstrate that it is fixed by independent superconducting parameters rather than adjusted to fit the resonance position and width.

    Authors: We agree that the explicit functional form and its independence from fitting are essential. In the revised manuscript we now state the vortex-induced field explicitly as B_v(z) = (Phi_0 n_v / 2 pi lambda) K_0(r / lambda) projected along the film depth, where n_v = mu_0 H_ext / Phi_0 is the vortex areal density fixed by the applied field and flux quantum, lambda = 90 nm is taken from independent critical-field measurements on the same Nb films, and the London screening length is not varied. The resulting depth-dependent effective field is inserted into the LLG equation without any adjustment to match the observed PSSW position or width; the lineshape agreement is obtained with these a-priori values. A new supplementary figure shows the profile for the experimental field range. revision: yes

  2. Referee: [Theory section (interfacial boundary conditions)] The interfacial spin-transfer torque amplitude is a free parameter. The paper should report the specific numerical value employed, its physical range consistent with microscopic estimates of triplet penetration, and a sensitivity analysis showing that PSSW conversion occurs robustly within that range; otherwise the reproduction of lineshapes risks being post-hoc.

    Authors: We have added the numerical value of the interfacial torque amplitude (gamma h_ST = 1.2 x 10^4 rad/s) together with its microscopic justification: this corresponds to a triplet spin polarization of order 8 % over a penetration depth of ~12 nm, consistent with Usadel-equation estimates for Nb/ferrimagnet interfaces. A sensitivity analysis has been included in the supplement; the PSSW feature remains visible and the lineshape qualitatively unchanged for torque amplitudes varied by ±40 % around the reported value. Only the precise amplitude of the additional peak scales linearly, while its position is set by the vortex field profile. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation self-contained

full rationale

The paper's central derivation couples the established Keldysh-Usadel framework for the diffusive superconductor to the LLG equation for the ferrimagnet via two standard physical mechanisms (triplet-mediated interfacial torque and vortex-induced depth-dependent field). These are implemented directly from prior literature without self-definition, parameter fitting that renames inputs as predictions, or load-bearing self-citations that reduce the result to tautology. The susceptibility calculation reproduces measured lineshapes as a consistency check rather than a circular fit, and the claim that superconductivity converts uniform FMR to PSSWs follows from the coupled dynamics without reducing to its own inputs by construction.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 2 invented entities

The central claim rests on two standard domain models plus two postulated interfacial mechanisms whose independent evidence is not supplied in the abstract. No explicit free parameters are named, but the reproduction of lineshapes implies at least one adjustable interfacial coupling strength.

free parameters (1)
  • interfacial spin-transfer torque amplitude
    Required to produce the observed conversion; value must be chosen to match the additional resonance feature.
axioms (2)
  • domain assumption Quasiclassical Keldysh-Usadel equations accurately describe the diffusive superconducting condensate.
    Standard framework invoked for the superconductor.
  • standard math Landau-Lifshitz-Gilbert dynamics govern the magnetization precession in the ferrimagnetic insulator.
    Established equation set for the magnetic layer.
invented entities (2)
  • spin-polarized triplet Cooper pairs at the interface no independent evidence
    purpose: Mediate the interfacial spin-transfer torque that converts uniform FMR into PSSWs
    Postulated mechanism; no independent falsifiable signature outside the resonance data is given.
  • depth-dependent effective field from Abrikosov vortices no independent evidence
    purpose: Provide the electromagnetic proximity effect that assists standing-wave formation
    Invoked as the second required ingredient; evidence is internal to the model fit.

pith-pipeline@v0.9.0 · 5537 in / 1618 out tokens · 49840 ms · 2026-05-07T09:02:36.462725+00:00 · methodology

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