{"id":"0d500ba9-3131-4ceb-a6b2-1e600f1843ea","arxiv_id":"2411.13414","paper_version":1,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":2,"one_line_summary":"At millikelvin temperatures, the dissipation rate of propagating spin waves in YIG/GGG increases with wavenumber by up to 55%, an effect attributed to dipolar coupling to the partially magnetized GGG substrate.","lead":"This paper measures how much energy propagating spin waves lose in a yttrium iron garnet film on a gadolinium gallium garnet substrate at temperatures down to 26 millikelvin. It finds that losses grow with the spin-wave wavenumber at cryogenic temperatures, which the authors explain by magnetic coupling to the partially magnetized substrate, and it sketches implications for building quantum devices from magnons.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The causal attribution to GGG is not quantitatively established at the headline temperature: the model is admitted to overestimate the k-dependent rise below 1 K, so the 26 mK increase is not explained by the invoked mechanism.","rationale":"The reader's weakest assumption correctly identifies that the causal attribution to GGG rests on a model that fails below 1 K. I agree with this assessment and find it to be the most load-bearing concern because the paper's central claim is precisely that the k-dependent dissipation is caused by GGG dipolar coupling. The model's quantitative agreement at 4–1 K provides some support, and the micromagnetic simulations at 2 K are a useful independent check. However, the manuscript explicitly acknowledges that below 1 K the model overestimates the k-dependence and predicts a temperature trend (continued increase down to 26 mK) that contradicts the experimental plateau below 500 mK. Since the headline number (55% at 26 mK) is quoted at a temperature where the model is not valid, the causal statement in the abstract is stronger than what the evidence supports. This does not invalidate the experimental observation, which appears robust and is supported by the difference-spectra analysis in Fig. 4(a), but it means the paper should be conditional on either performing a GGG-free control experiment or refining the GGG model with measured low-temperature parameters. The reader's CONDITIONAL verdict is appropriate, so no adjustment is needed.","tokens_in":14267,"tokens_out":14251,"duration_ms":137432,"concrete_test":"Measure the same PSWS protocol at 26 mK on a YIG film of similar thickness on GGG but with a diamagnetic spacer layer (e.g., Y3Sc2.5Al2.5O12) that decouples YIG from GGG paramagnetism. If the wavenumber-dependent rise in Γk disappears or is drastically reduced, the GGG-dipolar mechanism is confirmed; if it persists, the k-dependent loss has a different origin. Alternatively, measure the low-temperature GGG paramagnetic resonance linewidth at 26 mK (e.g., via EPR) and replace the fixed μ0ΔH=400 mT in Eq. (2b) with the measured value; if the model then quantitatively reproduces the experimental Γk(k), the failure is resolved.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's central causal claim—that the wavenumber-dependent increase in Γk at millikelvin temperatures is caused by dynamic dipolar coupling to the partially magnetized GGG substrate—is not quantitatively established at the temperature where it is stated. The semi-analytical model (Eq. 4 with the GGG permeability tensor, Eq. 2b) reproduces the Γk(k) curve only for 4 K to 1 K, using a temperature-independent inhomogeneous linewidth μ0ΔH≈400 mT and a Brillouin-paramagnet M_GGG. Below 1 K, the authors state in Section III.B that 'the theoretical predictions deviate significantly from the experiment and overestimate the change of the dissipation rate Γk with wavenumber.' Specifically, the model predicts a continued increase of Γk between 500 mK and 26 mK as M_GGG grows, whereas the experiment shows a plateau below 500 mK in both transmission amplitude and Γk. Thus the observed 55% increase at 26 mK is not explained by the mechanism invoked. The agreement at 4–1 K is suggestive, but the failure of the model's temperature dependence below 1 K means the attribution to GGG is an extrapolation. The possibility remains that the low-temperature k-dependence arises from a different source—e.g., edge modes or inhomogeneous stray-field broadening—or that the actual GGG magnetic state at mK temperatures produces a weaker coupling than the Brillouin model assumes. This is an explicit limitation in the manuscript, and it directly undermines the abstract's causal claim.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper reports propagating spin-wave spectroscopy (PSWS) measurements of a 7.78 µm YIG film on a GGG substrate at temperatures from 293 K down to 26 mK. The authors convert the frequency-domain transmission spectra into wavenumber space using a semi-analytical dispersion relation for a dipolar-coupled YIG/GGG bilayer (Eq. 4), and then extract the wavenumber-dependent dissipation rate Γk using Eq. 8. They observe a clear increase of Γk with wavenumber at all cryogenic temperatures, reaching about 55% between the FMR point and k ≈ 450 rad/cm at 26 mK. They attribute this k-dependence to dynamic dipolar coupling between YIG and the partially magnetized GGG substrate, supported by the semi-analytical model and micromagnetic simulations at 4 K–1 K. They also observe a temperature-dependent decrease in transmission at 4 K, attributed to rare-earth ion relaxation, and a plateau below 500 mK. The model reproduces the measured Γk(k) for 4 K–1 K after adding a k-independent offset Γ0, but the authors explicitly state that below 1 K the model overestimates the k-dependence and deviates significantly from experiment.","tokens_in":14541,"tokens_out":5344,"duration_ms":59818,"significance":"The experimental observation of a wavenumber-dependent dissipation rate in YIG/GGG at millikelvin temperatures is both novel and directly relevant to quantum magnonics, where propagating magnons at millikelvin temperatures are envisioned. The extraction method (Eq. 8) is transparent, the assumption that J² is k-independent and temperature-independent is clearly stated, and the model curves for 4 K–1 K are convincing. The paper also includes a substantial micromagnetic modelling effort, including a careful treatment of the static stray field of the thick GGG substrate. The main limitation is that the central causal claim—that the millikelvin k-dependence is caused by dipolar coupling to the partially magnetized GGG—is not quantitatively established at the headline temperature, as the model fails below 1 K. If the causal claim is appropriately qualified, the paper remains a valuable contribution; as written, the abstract and conclusions overstate the support for the mechanism.","major_comments":[{"comment":"The central claim that the wavenumber-dependent dissipation rate at millikelvin temperatures is \"caused by dipolar coupling to the partially magnetized GGG substrate\" is not supported by the quantitative comparison at the temperatures where the claim is strongest. The authors state in Section III.B that below 1 K \"the theoretical predictions deviate significantly from the experiment and overestimate the change of the dissipation rate Γk with wavenumber.\" Since the 26 mK data are the headline result, the causal attribution is an extrapolation from the 4 K–1 K agreement. The possibility remains that the low-temperature k-dependence arises from another mechanism (e.g., edge modes, stray-field inhomogeneity) or from a weaker temperature-dependent GGG coupling than the Brillouin-paramagnet model assumes. The abstract and conclusions should be reframed to present the GGG coupling as a plausible mechanism supported at intermediate temperatures, not as an established cause at millikelvin temperatures, or the model should be extended to account for GGG’s low-temperature magnetic phase transitions.","section":"Abstract and Section III.B, Fig. 4(c)"},{"comment":"The failure of the model below 1 K is attributed to the complex magnetic phase transitions of GGG, but the model itself uses a single temperature-independent inhomogeneous linewidth μ₀ΔH ≈ 400 mT and a Brillouin-paramagnet magnetization for GGG. These assumptions are known to break down in the sub-Kelvin range. Because the causal attribution rests on this model, the quantitative prediction at 26 mK is unreliable. The paper should either (i) present a sensitivity analysis showing how the predicted Γk(k) changes when plausible temperature dependencies of μ₀ΔH and M_GGG are used, or (ii) explicitly state that the model is only valid for T ≥ 1 K and that the sub-Kelvin mechanism remains an open question. As written, the model agreement at 4 K–1 K is suggestive but does not establish the sub-Kelvin mechanism.","section":"Section II.B, Eq. 2b and Section III.B"},{"comment":"The extraction of the wavenumber-dependent Γk relies on the group velocity v_g(k) from the same semi-analytical model (Eq. 4) that is being used to explain the mechanism. If the model dispersion is inaccurate at low temperatures, part of the apparent k-dependence of Γk could be an artefact of the conversion. The micromagnetic simulations validate the dispersion up to about 2000 rad/cm, but the comparison is shown for a limited temperature set (the dotted data in Fig. 4(c) are for 2 K only). Please provide a quantitative sensitivity analysis: e.g., propagate the uncertainty in the fitted effective anisotropy field H_a and in M_YIG through Eq. 8 to show that the reported 55% k-dependence at 26 mK is robust, or provide an independent check of v_g(k) at millikelvin temperatures (e.g., phase-resolved measurements).","section":"Section III.B, Eq. 8 and Fig. 3(b)–(d)"},{"comment":"The paper reports fitted values of the k-independent offset Γ0 only for 4 K, 2 K, and 1 K (9, 5, and 1 MHz, respectively). For the sub-Kelvin temperatures where the model overestimates the data, no Γ0 is quoted. It should be stated explicitly whether any choice of Γ0 could bring the model into agreement below 1 K. If not, that is a strong indication that the model is inapplicable in that regime, and the corresponding curves in Fig. 4(c) should not be presented without a clear caveat.","section":"Section III.B, fitted Γ0 values"}],"minor_comments":[{"comment":"The phrase \"We demonstrate an increase in the dissipation rate with wavenumber at cryogenic temperatures, caused by dipolar coupling\" conflates the direct observation (the increase) with the interpretation (the cause). Consider separating these in the abstract to avoid overclaiming.","section":"Abstract and Introduction"},{"comment":"The word \"shored\" in \"a shored AMI superconducting vector magnet\" should likely be \"short\" or \"short-circuited\"; please correct the typo.","section":"Section II.A, experimental setup"},{"comment":"The sentence \"it could proof beneficial\" should be \"it could prove beneficial.\" Also, the phrase \"we attribute the recorded k-dependency\" should be softened in view of the major comment above.","section":"Section IV, Conclusions"},{"comment":"The secular equation Eq. (4) is stated to be solved numerically, but the definitions of F_YIG and E_GGG in Eqs. (5c) and (5d) appear to be unused in the main text; please clarify the role of these terms (or whether the secular equation should contain both plus and minus combinations of D and E terms).","section":"Section II.B, Eq. 4"},{"comment":"Reference [54] is given as \"R. Corporation, RT/duroid 6010.2lm laminates datasheet\"; please format the publisher properly (Rogers Corporation) and include a URL or document number.","section":"Reference [54]"}],"recommendation":"major_revision","confidential_remarks":"The paper contains an honest and explicit statement of the model's failure below 1 K, which is commendable. However, the abstract and conclusions do not consistently respect that limitation. The referee report recommends major revision rather than rejection because the experimental dataset and the 4 K–1 K modelling are of high quality and the sub-Kelvin discrepancy could be addressed by reframing the causal claim and adding robustness checks. The journal may also wish to consider whether the acknowledgement mentioning a personal wedding is appropriate, though this does not affect the scientific assessment."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The one thing to know: this is a solid experimental paper with a clear limitation that the authors openly acknowledge. They measure the dissipation rate of propagating magnetostatic surface waves in a 7.78 µm YIG film on GGG from 4 K down to 26 mK, and find that Γk increases with wavenumber at cryogenic temperatures, up to about 55% between k = 0 and 450 rad/cm at 26 mK. That is new. Prior FMR work saw GGG-induced damping, but nobody had extracted the wavenumber dependence for propagating magnons at these temperatures. The extraction method is transparent: they convert S21 from frequency to wavenumber using a layered-susceptibility dispersion, then get Γk from Eq. 8. The k-dependent shape is a parameter-free output of the model given literature parameters, not a fit.\n\nWhat the paper does well: the layered-model calculation with a complex permeability for GGG is a legitimate way to capture dynamic dipolar coupling, and the micromagnetic simulations support the dispersion. The comparison with experiment at 4, 2, and 1 K is genuinely good, and the model fit uses only a k-independent offset Γ0. The authors also correctly describe the plateau below 500 mK and the complex phase transitions of GGG.\n\nWhere it gets soft: the causal attribution at the headline temperature is not quantitatively established. Below 1 K the model overestimates the k-dependent rise. The authors state this in Section III.B and say the model does not capture GGG's complex phase transitions. So the 26 mK 55% increase is observed, but the mechanism—dynamic dipolar coupling to the partially magnetized GGG—is an extrapolation from the 4–1 K agreement. That is an explicit and honest limitation, but it means the abstract's 'caused by dipolar coupling' is stronger than the support. Also, there are no error bars on the k-dependent Γk curves, the antenna efficiency J2 is assumed temperature-independent without direct verification, and no data or code are shared. These are addressable and do not overturn the observation. The measurement itself is not in question.\n\nWho this is for: people designing YIG-based quantum magnonic devices at millikelvin temperatures. The practical takeaway—that substrate-induced k-dependent losses matter and may be smaller for short-wavelength exchange magnons—is plausible and worth knowing, even if the mechanism at the lowest temperatures needs more work.\n\nRecommendation: send this to peer review. It deserves a serious referee. The central observation is real, the method is reusable, and the authors are straightforward about where the model fails. A referee can push for error bars and a more cautious causal wording in the abstract and conclusions.","headline":"A transparent and honest measurement of k-dependent dissipation in YIG/GGG at millikelvin temperatures, but the causal mechanism at the headline temperature is an extrapolation the authors themselves flag.","tokens_in":15160,"tokens_out":1919,"would_cite":true,"duration_ms":19331,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"This paper shows that at millikelvin temperatures the dissipation rate of propagating spin waves in a YIG-on-GGG film grows with wavenumber, by up to 55% at 26 mK, because of dynamic dipolar coupling to the partially magnetized GGG…","keywords":["yttrium iron garnet","gadolinium gallium garnet","millikelvin spin-wave spectroscopy","magnetostatic surface spin waves","wavenumber-dependent damping","dipolar coupling","quantum magnonics","cryogenic magnon transport"],"falsifier":"Remeasure the same sample after removing or isolating the GGG substrate (for example by etching it away or inserting a nonmagnetic spacer): if $\\Gamma_k$ still rises by roughly 55% between $k \\approx 0$ and $k \\approx 450$ rad/cm at 26 mK, the dipolar-coupling mechanism is not the cause.","tokens_in":14045,"feed_emoji":"🧲","tokens_out":8199,"duration_ms":82520,"temperature":0.7,"pith_summary":"The paper asks where magnetic losses come from for propagating magnons in the workhorse magnonic material, a yttrium iron garnet (YIG) film grown on gadolinium gallium garnet (GGG), at the millikelvin temperatures needed for quantum magnonics. Using propagating spin-wave spectroscopy from 293 K down to 26 mK, it extracts the wavenumber-dependent dissipation rate $\\Gamma_k$ for magnetostatic surface spin waves in a 7.78 µm YIG film. The central claim is that $\\Gamma_k$ increases with wavenumber at cryogenic temperatures, by up to 55% between the uniform mode ($k=0$) and the largest probed wavenumber ($k \\approx 450$ rad/cm) at 26 mK, and that this increase is caused by dynamic dipolar coupling to the partially magnetized GGG substrate. A consequence is that the substrate, normally treated as inert, is an intrinsic loss channel that must be accounted for in designs for detecting individual propagating magnons.","feed_headline":"At 26 mK, spin-wave losses climb up to 55% with wavenumber","feed_subtitle":"Magnetic coupling to the substrate, not the film alone, drives the extra dissipation at cryogenic temperatures.","key_machinery":"The carrier of the argument is the semi-analytical magnetostatic-wave model of a layered ferrite/paramagnet structure, with complex frequency $\\tilde{\\omega}_k = \\omega_k + i \\Gamma_k$ and permeability tensors for YIG and GGG. The dispersion and damping follow from the secular equation $D_{\\mathrm{YIG}}D_{\\mathrm{GGG}} + E_{\\mathrm{YIG}}E_{\\mathrm{GGG}} = 0$, where GGG is modeled as a paramagnet with a phenomenological inhomogeneous linewidth $\\mu_0\\Delta H \\approx 400$ mT. The k-dependence of the extra loss comes from the magnetostatic Green function $G_{k,xx} = -G_{k,zz} = \\frac{|k_x|}{2} e^{-|k_x||z-z'|}$, which makes the YIG-GGG dipolar coupling vanish at $k=0$, rise linearly at small $k$, and decay exponentially at large $k$. This same function explains both the measured increase of $\\Gamma_k$ with wavenumber and the predicted decrease for $k > 400$ rad/cm.","core_discovery":"On its own terms, the paper demonstrates a temperature- and wavenumber-dependent loss mechanism in YIG-GGG heterostructures: the propagating-wave dissipation rate $\\Gamma_k$ grows with $k$ at low temperatures even as the total damping falls below 4 K, and the growth saturates below 500 mK. The quantitative extraction uses the measured transmission $S_{21}$, the calculated group velocity, and the assumption that antenna efficiency is temperature independent, giving $\\Gamma_k$ directly from $S_{21} = J^2 \\exp(-\\Gamma_k L / v_g)$. The rise in $\\Gamma_k$ is reproduced by a dipolar-coupled two-layer model in which GGG carries a much larger internal damping than YIG; like coupled oscillators, the higher-quality YIG mode loses energy into the lower-quality GGG mode, and because the magnetostatic coupling grows linearly at small $k$ before decaying exponentially, the added loss is k-dependent. The paper reports that rare-earth ion relaxation adds a k-independent offset (largest at 4 K), while the substrate coupling produces the k-dependent part, and it states that the model overestimates the k-dependence below 1 K because it does not include GGG's complex magnetic phase transitions.","pith_inferences":["If the dipolar-coupling mechanism is the true cause, a clean test is to deposit a thin nonmagnetic spacer between YIG and GGG (or transfer the film): the k-dependent rise should vanish while the k-independent rare-earth loss survives.","The same coupled-oscillator reasoning should generalize: any low-damping magnetic film on a lossier magnetic or paramagnetic substrate should show a similar k-dependent loss, so this is a design constraint beyond YIG-GGG.","At the lowest temperatures the model's mismatch means the quantitative prediction should be checked by measuring GGG's own dynamic response at 26 mK rather than inferred from YIG spectra.","The model's predicted turnover near $k > 400$ rad/cm implies an optimum wavenumber for low-loss propagation; narrower antennas that excite shorter magnons would test this directly."],"forward_implications":["GGG-induced dissipation is additive to YIG's intrinsic damping: $\\Gamma_k = \\Gamma_0 + \\Gamma^{\\mathrm{(dip)}}(k)$, so a measured $\\Gamma_k(k)$ curve can be separated into a k-independent rare-earth relaxation part and a k-dependent substrate part.","At wavenumbers above roughly $k_x d_{\\mathrm{YIG}} \\gtrsim 3$ (above ~400 rad/cm here), the dipolar coupling falls exponentially, so the model predicts lower dissipation for short-wavelength exchange-dominated magnons.","Below 500 mK the dissipative state is frozen: neither transmission nor $\\Gamma_k$ changes with temperature, matching the plateau in GGG's magnetic response.","Quantum-magnonic experiments using propagating, rather than standing, spin waves in YIG-on-GGG must treat the substrate as a loss channel that grows with wavenumber at low temperature."],"supporting_citations":[{"why":"Supplies the magnetostatic-wave secular equation for layered magnetic structures on which the dispersion and damping extraction are built.","marker":"[46]"},{"why":"Provides the temperature-dependent GGG stray-field and magnetization model used to correct the applied fields and interpret the FMR shifts.","marker":"[34]"},{"why":"Is the source of the ~400 mT phenomenological inhomogeneous linewidth used for GGG's permeability tensor.","marker":"[47]"},{"why":"Demonstrates propagating spin-wave spectroscopy in YIG at millikelvin temperatures and the GGG stray-field procedure extended here.","marker":"[30]"},{"why":"Reports millikelvin magnon damping in YIG films and identifies GGG as an additional damping source.","marker":"[31]"},{"why":"Gives the magnetostatic Green function whose k-dependent coupling is the mechanism for the added dissipation.","marker":"[55]"},{"why":"Shows that isolating YIG from GGG with a diamagnetic spacer reduces damping, supporting the substrate-loss attribution.","marker":"[33]"},{"why":"Supplies the finite-element magnetostatic solver used to compute the static GGG stray field.","marker":"[48]"},{"why":"Provides the GPU micromagnetic solver used for the dispersion and lifetime simulation.","marker":"[49]"}],"fun_headline_variants":["Substrate coupling drives wavenumber-dependent magnon losses at millikelvin","YIG-GGG losses grow with wavenumber below 1 K due to GGG damping","Millikelvin spin-wave losses rise with k via dipolar coupling to GGG","Extra magnon dissipation at mK traced to substrate, not film","Wavenumber-dependent losses in YIG-GGG pinned to GGG magnetism"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The entire explanation rests on treating the GGG substrate as a simple paramagnet with a single, temperature-independent internal loss parameter, and the paper itself reports that this model overestimates the wavenumber dependence below 1 K because it omits GGG's complex low-temperature phase behavior.","fun_headline_variants_meta":{"raw":{"variants":["Substrate coupling drives wavenumber-dependent magnon losses at millikelvin","YIG-GGG losses grow with wavenumber below 1 K due to GGG damping","Millikelvin spin-wave losses rise with k via dipolar coupling to GGG","Extra magnon dissipation at mK traced to substrate, not film","Wavenumber-dependent losses in YIG-GGG pinned to GGG magnetism"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000258,"raw_usage":{"total_tokens":1635,"prompt_tokens":1051,"completion_tokens":584,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":667,"completion_tokens_details":{"reasoning_tokens":480}},"tokens_in":667,"tokens_out":584,"duration_ms":6449,"temperature":1.0,"reasoning_tokens":480,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T16:26:16.631555+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Remeasure the same sample after removing or isolating the GGG substrate (for example by etching it away or inserting a nonmagnetic spacer): if $\\Gamma_k$ still rises by roughly 55% between $k \\approx 0$ and $k \\approx 450$ rad/cm at 26 mK, the dipolar-coupling mechanism is not the cause.","supporting_citations":[{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Supplies the magnetostatic-wave secular equation for layered magnetic structures on which the dispersion and damping extraction are built."},{"cited_title":"Barak, M","cited_arxiv_id":null,"evidence_quote":"Is the source of the ~400 mT phenomenological inhomogeneous linewidth used for GGG's permeability tensor."},{"cited_title":"Knauer, K","cited_arxiv_id":null,"evidence_quote":"Demonstrates propagating spin-wave spectroscopy in YIG at millikelvin temperatures and the GGG stray-field procedure extended here."},{"cited_title":"Kalinikos and A","cited_arxiv_id":null,"evidence_quote":"Gives the magnetostatic Green function whose k-dependent coupling is the mechanism for the added dissipation."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Shows that isolating YIG from GGG with a diamagnetic spacer reduces damping, supporting the substrate-loss attribution."},{"cited_title":"Bruckner, C","cited_arxiv_id":null,"evidence_quote":"Supplies the finite-element magnetostatic solver used to compute the static GGG stray field."},{"cited_title":"Bruckner, S","cited_arxiv_id":null,"evidence_quote":"Provides the GPU micromagnetic solver used for the dispersion and lifetime simulation."}],"review_version":1}