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arxiv: 2604.25002 · v1 · submitted 2026-04-27 · ❄️ cond-mat.mes-hall

Probing Spin Dynamics Across Magnetic Phase Transitions in CrCl3 Nanoflakes Using Nitrogen-Vacancy Microscopy

Ben Hammons , Jitender Kumar , Sehrish Iqbal , Prem Karki , Karishma Prasad , Tianlin Li , Aram Pirali , Ayodimeji E. Aregbesola
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Rupak Timalsina Xia Hong Jian Wang Kapildeb Ambal Ilja Fescenko Abdelghani Laraoui
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Pith reviewed 2026-05-08 01:38 UTC · model grok-4.3

classification ❄️ cond-mat.mes-hall
keywords CrCl3nitrogen-vacancy centersspin dynamicsmagnetic phase transitionsvan der Waals magnetsmagnonicsquantum sensing
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The pith

Nitrogen-vacancy centers detect a two-order-of-magnitude rise in gigahertz magnetic noise from CrCl3 exactly in its ferromagnetic phase.

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

The work tracks spin dynamics in CrCl3 nanoflakes by monitoring optically detected magnetic resonance, Rabi oscillations, and the spin relaxation rate of shallow NV centers in diamond while the sample is cooled across its magnetic transitions. In the ferromagnetic window the NV signals show collapsed oscillations, reduced contrast, and a relaxation rate that jumps by roughly one hundred times, pointing to intense sample-generated noise at gigahertz frequencies. Broadband ferromagnetic resonance on bulk crystals confirms resonances in the 4-15 GHz band with a 24 mT linewidth. A simple model that adds independent noise contributions from the antiferromagnetic, ferromagnetic, and paramagnetic regimes reproduces the measured temperature curve for the relaxation rate. The results matter because CrCl3 is viewed as a tunable platform for 2D magnonics and hybrid quantum-magnon circuits, where knowing the phase with strongest noise helps avoid or exploit that regime.

Core claim

In CrCl3 nanoflakes the ferromagnetic phase produces the strongest magnetic noise in the gigahertz range, shown by a collapse of NV Rabi oscillations, lowered resonance contrast, and a two-order-of-magnitude increase in the relaxation rate G1 = 1/T1; a phenomenological model that superposes fluctuation channels from the antiferromagnetic, ferromagnetic, and paramagnetic states accounts for the temperature dependence of G1, and broadband FMR independently confirms resonances between 4 and 15 GHz.

What carries the argument

NV-center relaxation rate G1 = 1/T1 together with a three-channel phenomenological model that adds magnetic-noise spectra from antiferromagnetic, ferromagnetic, and paramagnetic fluctuation channels.

Load-bearing premise

The large observed changes in NV contrast, Rabi collapse, and T1 are produced by magnetic fluctuations from the CrCl3 nanoflakes rather than by temperature-dependent instrumental effects or non-magnetic noise sources.

What would settle it

If the same NV relaxation enhancement and Rabi collapse appear in a control run with a non-magnetic flake held at identical temperature and field, or if the enhancement vanishes when the external field is swept to suppress ferromagnetic order while temperature is held fixed.

Figures

Figures reproduced from arXiv: 2604.25002 by Abdelghani Laraoui, Aram Pirali, Ayodimeji E. Aregbesola, Ben Hammons, Ilja Fescenko, Jian Wang, Jitender Kumar, Kapildeb Ambal, Karishma Prasad, Prem Karki, Rupak Timalsina, Sehrish Iqbal, Tianlin Li, Xia Hong.

Figure 3
Figure 3. Figure 3: (a) Rabi oscillations (fluorescence vs time) of NVs underneath the CrCl3 flake recorded at the three different magnetic state temperature regimes: PM (21 K, 26 K), FM (14 K, 18 K), and AFM (6.3 K). (b) Fast Fourier Transformation (FFT) of the Rabi oscillations in (a) and Figure S3.3.1 recorded at temperatures in the range 6.3 – 32 K. Rabi amplitude (c) and decay (TR) versus temperature measured at H of 1.4… view at source ↗
Figure 4
Figure 4. Figure 4: (a) Measured (scattered) and fitted (solid lines) T1 (fluorescence vs time) curves at three temperatures (4.3, 15, and 30 K) on and off the 60-nm CrCl3 flake. (b) 1 vs temperature at H of 1.45 and 13.95 mT on the flake. The off-flake 1 curve is zoomed in the inset for better presentation, where no effect is observed. (c) Temperature-dependent FMR (intensity vs applied magnetic field H) spectra of CrCl3 m… view at source ↗
read the original abstract

CrCl3, a layered van der Waals (vdW) magnet, exhibits in-plane magnetic anisotropy and enhanced interlayer coupling upon stacking, making it an ideal platform to host exotic nanoscale magnetic phenomena such as magnon hydrodynamics and meron-like topological spin defects. When interfaced with other vdW materials, its antiferromagnetic-to-ferromagnetic and ferromagnetic-to-paramagnetic phase transitions and magnetic anisotropy can be tuned by voltage, strain, and layer stacking. Understanding the spin dynamics of CrCl3 at its magnetic phase transitions is crucial to its applications in magnonics. Here, we investigate the spin dynamics of CrCl3 nanoflakes using cryogenic diamond quantum sensing microscopy, based on measuring optically detected magnetic resonance, Rabi oscillations, and spin-lattice relaxation time (T1) of shallow nitrogen vacancy (NV) centers in diamond. In the ferromagnetic regime, we observe a pronounced reduction in the NV spin resonance contrast, a collapse of the Rabi oscillations, and a strong enhancement by two orders of magnitude of the relaxation rate, G1 = 1/T1. These observations indicate intensified spin fluctuations in the gigahertz range. Broadband ferromagnetic resonance spectroscopy on CrCl3 microcrystals reveals resonance frequencies in the 4-15 GHz range together with a linewidth of ~24 mT, further supporting the NV measurements. A phenomenological model of magnetic-noise-induced NV relaxation reproduces the temperature dependence of G1 by combining antiferromagnetic, ferromagnetic, and paramagnetic fluctuation channels, indicating that magnetic noise is strongest in the ferromagnetic regime and evolves markedly across the phase diagram. These results are crucial for using CrCl3 in 2D magnonics and hybrid quantum-magnon systems.

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

3 major / 2 minor

Summary. The manuscript uses cryogenic NV-center microscopy on CrCl3 nanoflakes to measure ODMR contrast, Rabi oscillations, and T1 relaxation across magnetic phase transitions. It reports a pronounced reduction in NV contrast, collapse of Rabi oscillations, and ~100-fold enhancement of the relaxation rate G1=1/T1 specifically in the ferromagnetic regime, interpreted as evidence for intensified GHz-range spin fluctuations from the sample. Broadband FMR on microcrystals shows resonances in the 4-15 GHz range with ~24 mT linewidth. A phenomenological model superposes antiferromagnetic, ferromagnetic, and paramagnetic fluctuation channels with adjustable relative amplitudes to reproduce the measured G1(T) dependence, concluding that magnetic noise is strongest in the FM regime.

Significance. If the magnetic origin of the NV signal changes is established, the work offers direct experimental access to spin fluctuations across phase transitions in a vdW magnet, with clear relevance to magnonics and hybrid quantum-magnon devices. The reported magnitude of the T1 change (two orders) and the multi-technique combination of NV sensing with FMR constitute genuine strengths. The phenomenological model provides a useful framework for interpreting the temperature dependence, though its fitting freedom limits its predictive independence.

major comments (3)
  1. The central attribution of the contrast reduction, Rabi collapse, and G1 enhancement (reported in the ferromagnetic regime) to CrCl3 magnetic fluctuations lacks control experiments. No bare-diamond or non-magnetic-flake datasets are described to exclude temperature-dependent instrumental effects, laser-induced heating, or surface adsorbates, which directly undermines the weakest assumption in the interpretation.
  2. The phenomenological model reproduces G1(T) by superposing AF/FM/PM spectral densities whose relative amplitudes are adjusted to fit the data. This introduces fitting freedom that reduces the independence of the model prediction from the measured curve, weakening the claim that magnetic noise is strongest in the FM regime.
  3. The supporting FMR spectra (4-15 GHz resonances, ~24 mT linewidth) were acquired on microcrystals rather than the exfoliated nanoflakes used for NV sensing. It is unclear whether the GHz noise spectrum is representative of the measured devices, which is load-bearing for linking FMR to the NV results.
minor comments (2)
  1. The abstract provides no quantitative error bars, statistical details, or mention of controls for the reported changes in contrast, Rabi, and G1.
  2. Notation for G1 = 1/T1 and the fluctuation channels could be defined more explicitly in the main text to improve clarity for readers unfamiliar with NV relaxation modeling.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their thorough review and insightful comments on our manuscript. We address each of the major comments point by point below, providing clarifications and indicating revisions to the manuscript where appropriate.

read point-by-point responses

  1. Referee: The central attribution of the contrast reduction, Rabi collapse, and G1 enhancement (reported in the ferromagnetic regime) to CrCl3 magnetic fluctuations lacks control experiments. No bare-diamond or non-magnetic-flake datasets are described to exclude temperature-dependent instrumental effects, laser-induced heating, or surface adsorbates, which directly undermines the weakest assumption in the interpretation.

    Authors: We agree that control experiments are essential to firmly establish the magnetic origin of the observed NV signal changes. While the sharp correlation of the G1 enhancement with the ferromagnetic phase transition temperature provides strong evidence against purely instrumental effects, we will include in the revised manuscript additional datasets from bare diamond substrates and non-magnetic flakes (such as hBN) measured under identical cryogenic conditions. These controls will demonstrate that the two-order-of-magnitude increase in relaxation rate is specific to the CrCl3 nanoflakes. revision: yes

  2. Referee: The phenomenological model reproduces G1(T) by superposing AF/FM/PM spectral densities whose relative amplitudes are adjusted to fit the data. This introduces fitting freedom that reduces the independence of the model prediction from the measured curve, weakening the claim that magnetic noise is strongest in the FM regime.

    Authors: The model is presented as phenomenological in the manuscript, intended to provide a framework for understanding the temperature dependence rather than an ab initio prediction. The relative amplitudes are fitted, but the key conclusion—that the ferromagnetic channel dominates in the FM regime—arises because only with a strong FM contribution can the model capture the pronounced peak in G1 at the relevant temperatures. In the revision, we will add an analysis showing the sensitivity to parameters and demonstrate that suppressing the FM channel leads to poor fits, thereby reinforcing the interpretation without overclaiming predictive power. revision: partial

  3. Referee: The supporting FMR spectra (4-15 GHz resonances, ~24 mT linewidth) were acquired on microcrystals rather than the exfoliated nanoflakes used for NV sensing. It is unclear whether the GHz noise spectrum is representative of the measured devices, which is load-bearing for linking FMR to the NV results.

    Authors: We recognize that FMR on the exact nanoflakes would be ideal but is challenging due to the limited volume and weak signal from thin flakes. The microcrystals are from the same source material and display consistent magnetic properties. The resonance frequencies and linewidth are in agreement with prior studies on CrCl3 thin films and flakes. In the revised manuscript, we will expand the discussion to include references to literature showing that spin dynamics in CrCl3 are robust across sample thicknesses, and note that the FMR linewidth provides a conservative estimate for the noise spectrum relevant to the NV centers. revision: partial

Circularity Check

1 steps flagged

Phenomenological model of G1(T) reproduces data via adjustable superposition of fluctuation channels

specific steps
  1. fitted input called prediction [Abstract (and corresponding Results/Discussion section on phenomenological model)]
    "A phenomenological model of magnetic-noise-induced NV relaxation reproduces the temperature dependence of G1 by combining antiferromagnetic, ferromagnetic, and paramagnetic fluctuation channels, indicating that magnetic noise is strongest in the ferromagnetic regime."

    The model superposes three spectral-density channels whose relative amplitudes are adjusted to fit the measured G1(T) data points; the claimed reproduction therefore follows by construction from the fitting procedure rather than from an independent first-principles calculation or external constraint.

full rationale

The paper's key interpretive step is a phenomenological model that combines AF/FM/PM magnetic fluctuation channels to match the measured temperature dependence of the NV relaxation rate G1. This reproduction is achieved by fitting the relative amplitudes of the channels to the experimental curve rather than deriving them independently. The raw observations (contrast loss, Rabi collapse, G1 enhancement) remain direct measurements and are not themselves circular. No self-definitional equations, load-bearing self-citations, uniqueness theorems, or ansatz smuggling are present. The FMR spectra provide qualitative support but are acquired on microcrystals, not the nanoflakes; this is a limitation of evidence strength, not circularity. Overall circularity is moderate and confined to the model's fitting procedure.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central interpretation assumes NV relaxation is dominated by sample magnetic noise; the phenomenological model requires adjustable amplitudes for each fluctuation channel. No invented entities are introduced.

free parameters (1)
  • relative amplitudes of AF/FM/PM fluctuation channels
    Adjusted to reproduce the measured temperature dependence of G1; exact values not stated in abstract.
axioms (1)
  • domain assumption NV-center T1 relaxation rate is dominated by magnetic noise from the CrCl3 sample in the GHz frequency window.
    Invoked to attribute the two-order-of-magnitude enhancement directly to intensified spin fluctuations rather than other mechanisms.

pith-pipeline@v0.9.0 · 5671 in / 1459 out tokens · 72082 ms · 2026-05-08T01:38:35.093834+00:00 · methodology

discussion (0)

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