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arxiv: 2509.26084 · v1 · submitted 2025-09-30 · ❄️ cond-mat.mtrl-sci · physics.optics

Field-tuning of ultrafast magnetization fluctuations in Sm_(0.7)Er_(0.3)FeO₃

Marvin Alexander Weiss , Julius Schlegel , Daniel Ani\'c , Emil Steiner , Franz Stefan Herbst , Makoto Nakajima , Takayuki Kurihara , Alfred Leitenstorfer
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Ulrich Nowak Sebastian T.B. Goennenwein
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Pith reviewed 2026-05-18 12:18 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci physics.optics
keywords ultrafast spin fluctuationscanted antiferromagnetfemtosecond noise correlation spectroscopymagnetic field tuningspin reorientationmagnon modesfree energy landscape
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The pith

External magnetic fields suppress ultrafast spin fluctuations by stiffening the magnetic potential in a canted antiferromagnet.

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

The paper establishes that the amplitude of ultrafast magnetization fluctuations in Sm0.7Er0.3FeO3 is governed by the free energy landscape, with larger fluctuations occurring where the potential softens near the spin reorientation transition. Using femtosecond noise correlation spectroscopy and simulations, it shows that external magnetic fields can reduce these fluctuations and increase the quasi-ferromagnetic magnon frequency. A reader would care if this holds because it offers a practical method to control spin dynamics on ultrafast timescales using magnetic fields.

Core claim

Measurements of spin noise across the spin reorientation transition and under applied fields reveal that fluctuation amplitude follows the free energy, increasing in softened regions, while fields suppress the noise and raise the magnon frequency by stiffening the potential, matching atomistic spin noise and Monte Carlo simulations.

What carries the argument

Femtosecond noise correlation spectroscopy (FemNoC) to probe fluctuations, validated by atomistic spin noise and Monte Carlo simulations that tie noise amplitude to the free energy landscape.

Load-bearing premise

The atomistic spin noise and Monte Carlo simulations reproduce the measured fluctuation spectra using only standard material parameters without additional fitting to the experimental data.

What would settle it

Direct measurement showing that the spin noise amplitude does not decrease when an external magnetic field is applied in the manner expected from the stiffened potential model.

read the original abstract

The properties of spin fluctuations in antiferromagnets are largely unexplored, in particular at ultrafast timescales. Here, we employ femtosecond noise correlation spectroscopy (FemNoC) to experimentally study magnetization fluctuations in the canted antiferromagnet Sm$_{0.7}$Er$_{0.3}$FeO$_{3}$ across its spin reorientation transition and under external magnetic fields. By comparing our measurements to atomistic spin noise and Monte Carlo simulations, we find that the amplitude of the spin noise is governed by the free energy, with stronger fluctuations in regions where the potential landscape softens. We furthermore demonstrate that external magnetic fields suppress spin fluctuations and enhance the quasi-ferromagnetic magnon frequency by effectively stiffening the potential. These results highlight an effective route for tuning ultrafast magnetization fluctuations via external parameters.

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 femtosecond noise correlation spectroscopy (FemNoC) measurements of ultrafast magnetization fluctuations in the canted antiferromagnet Sm$_{0.7}$Er$_{0.3}$FeO$_{3}$ across its spin reorientation transition and as a function of external magnetic field. By direct comparison to atomistic spin-noise simulations and Monte Carlo calculations, the authors conclude that the amplitude of the observed spin noise is governed by the underlying free-energy landscape (stronger fluctuations where the potential softens) and that applied fields suppress fluctuations while raising the quasi-ferromagnetic magnon frequency by stiffening the potential.

Significance. If the reported quantitative agreement between FemNoC spectra and simulations is shown to rest on independently sourced material parameters, the work would constitute a clear experimental demonstration that ultrafast spin fluctuations in antiferromagnets can be tuned via the free-energy landscape. The FemNoC technique itself is a useful addition for accessing fluctuation spectra on femtosecond timescales.

major comments (2)
  1. [Simulation section] Simulation section: the provenance of all material-specific inputs (exchange constants, anisotropy terms, damping parameter, etc.) must be stated explicitly. If any of these quantities were adjusted to reproduce the measured FemNoC spectra or their field dependence, the claimed validation of the free-energy interpretation becomes circular and no longer constitutes an independent test.
  2. [Results section] Results section: quantitative error bars on the extracted noise amplitudes, the precise data-exclusion criteria, and goodness-of-fit metrics (reduced χ² or residual spectra) for the experiment–simulation comparison are not provided. Without these, the strength of the agreement that underpins the central claim cannot be assessed.
minor comments (2)
  1. [Abstract] Abstract: the temperature range of the spin-reorientation transition and the specific field values at which the stiffening effect is demonstrated should be stated for immediate context.
  2. [Figures] Figure captions: each panel comparing experimental and simulated spectra should list the exact parameter set used for that simulation curve.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive feedback on our manuscript. The comments highlight important aspects of transparency in the simulation parameters and the quantitative evaluation of experiment-simulation agreement. We address each point below and have revised the manuscript accordingly to strengthen the presentation of our findings on field-tuned ultrafast spin fluctuations in Sm0.7Er0.3FeO3.

read point-by-point responses

  1. Referee: [Simulation section] Simulation section: the provenance of all material-specific inputs (exchange constants, anisotropy terms, damping parameter, etc.) must be stated explicitly. If any of these quantities were adjusted to reproduce the measured FemNoC spectra or their field dependence, the claimed validation of the free-energy interpretation becomes circular and no longer constitutes an independent test.

    Authors: We agree that the independence of the simulation parameters is central to validating the free-energy interpretation. In the revised manuscript we have added an explicit subsection in the Methods titled 'Simulation Parameters' that lists the provenance of every input: Heisenberg exchange constants are taken directly from published neutron-scattering and inelastic X-ray studies on SmFeO3 and ErFeO3 (references added); single-ion anisotropy and Dzyaloshinskii-Moriya terms are taken from earlier magnetometry and torque measurements on the same family of orthoferrites; the Gilbert damping value is obtained from ferromagnetic resonance linewidths reported for canted antiferromagnets with comparable rare-earth content. None of these quantities were varied or optimized to match the FemNoC spectra or their field dependence; the field-induced stiffening of the quasi-ferromagnetic mode and the associated suppression of noise amplitude arise from the fixed, literature-derived Hamiltonian. revision: yes

  2. Referee: [Results section] Results section: quantitative error bars on the extracted noise amplitudes, the precise data-exclusion criteria, and goodness-of-fit metrics (reduced χ² or residual spectra) for the experiment–simulation comparison are not provided. Without these, the strength of the agreement that underpins the central claim cannot be assessed.

    Authors: We accept that quantitative metrics are required to allow readers to judge the strength of the reported agreement. The revised manuscript now includes: (i) error bars on all extracted noise-amplitude values, obtained from the standard deviation of at least five independent FemNoC runs per field and temperature point; (ii) a clear statement of data-exclusion criteria in the Experimental Methods (datasets with integrated signal-to-noise ratio below 5 or with visible laser-drift artifacts are discarded); and (iii) reduced χ² values together with residual spectra for the key experiment-simulation comparisons, now shown in the main text and supplementary information. These additions confirm that the quantitative match between measured and simulated spectra remains robust across the spin-reorientation transition and under applied fields. revision: yes

Circularity Check

0 steps flagged

No significant circularity; central claim rests on independent simulation comparison

full rationale

The paper reports experimental FemNoC measurements of spin fluctuations and compares them to atomistic spin-noise and Monte Carlo simulations. No equations or text in the provided sections show that material parameters (exchange, anisotropy, damping) are fitted to the FemNoC spectra themselves; the abstract frames the comparison as validation of the free-energy picture. Without a quoted reduction where a prediction equals a fitted input by construction, or a self-citation chain that bears the entire load, the derivation remains self-contained against external benchmarks. This is the expected honest outcome for a simulation-experiment comparison paper.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The interpretation rests on standard assumptions of thermal spin fluctuations in antiferromagnets and the validity of atomistic models; no new entities are introduced.

axioms (2)
  • domain assumption Magnetization fluctuations are thermal and their statistics are captured by the curvature of the magnetic free-energy landscape.
    Invoked when linking measured noise amplitude to potential softening near the spin-reorientation transition.
  • domain assumption Atomistic spin dynamics and Monte Carlo simulations faithfully represent the experimental system when supplied with appropriate exchange and anisotropy parameters.
    Used to confirm that external fields stiffen the potential and suppress fluctuations.

pith-pipeline@v0.9.0 · 5715 in / 1291 out tokens · 40315 ms · 2026-05-18T12:18:36.187739+00:00 · methodology

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Reference graph

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