Critical fluctuations and noise spectra in two-dimensional Fe3GeTe2 magnets

At a Glance

Metadata	Details
Publication Date	2025-09-29
Journal	Nature Communications
Authors	Yuxin Li, Zhe Ding, Chen Wang, Haoyu Sun, Zhousheng Chen
Institutions	Hefei National Center for Physical Sciences at Nanoscale, University of Science and Technology of China
Citations	1
Analysis	Full AI Review Included

Technical Documentation & Analysis: Critical Fluctuations in 2D Magnets using NV Centers

This document analyzes the research paper “Critical fluctuations and noise spectra in two-dimensional Fe${3}$GeTe${2}$ magnets” to provide technical specifications and align the material requirements with 6CCVD’s advanced MPCVD diamond capabilities, focusing on quantum sensing applications.

Executive Summary

This research successfully leverages Nitrogen-Vacancy (NV) centers in diamond to probe critical spin fluctuations in the van der Waals magnet Fe${3}$GeTe${2}$ (FGT), establishing a new methodology for characterizing phase transitions.

Quantum Sensing Achievement: Utilized NV center quantum decoherence imaging (Hahn-echo sequence) to map magnetic noise spectra generated by critical spin fluctuations near the Curie temperature ($T_{c}$).
Noise Crossover Signature: Explicitly observed a fundamental noise crossover: the magnetic noise spectral density transitions from $1/f$ noise characteristics near $T_{c}$ to white noise behavior away from the critical regime.
Decoherence Peak: The NV center decoherence rate ($\Gamma$) exhibits a pronounced peak precisely at $T_{c}$, confirming the strongest magnetic fluctuations occur at the critical point.
Scaling Theory Validation: Developed a theoretical framework that quantitatively links the decoherence rate to the noise spectra, successfully fitting experimental data using critical exponents ($\nu=1, z=2.17$) derived from the 2D Ising model.
Interface Engineering: Demonstrated the necessity of precise control over the NV-sample separation distance ($d$) (achieved using hBN spacers) to mitigate noise coupling and enable reliable $T_{2}$ measurements across the phase transition.
Paradigm Shift: Establishes a robust, microscopic methodology for studying phase transition dynamics and determining critical exponents across diverse quantum materials.

Technical Specifications

The following hard data points were extracted from the experimental results and theoretical modeling:

Parameter	Value	Unit	Context
Critical Temperature (T_c) Range	175 - 210	K	Dependent on FGT flake thickness (10 nm to 90 nm)
Intrinsic NV Coherence Time (T_2,0)	4.5	µs	Measured in FGT-free regions (bare diamond)
Minimum Measured T₂ (Near T_c)	3.2	µs	Characteristic T₂ in FGT-covered regions
NV-FGT Separation Distance (d)	60, 245, 310	nm	Adjusted using hBN flakes to control noise coupling
NV Zero-Field Splitting ($\Delta_{0}$)	2.87	GHz	Resonant frequency for relaxation measurements
Magnetic Noise Frequency Range	MHz	N/A	Detected by the Hahn-echo pulse sequence
Critical Slowing Down Time (1/$\omega_{0}$)	2.5	µs	Relaxation time of critical fluctuations near (T-T_c)/T $\approx$ 0.05
Dynamic Critical Exponent (z)	2.17	N/A	Used in the theoretical scaling model
Correlation Length Exponent ($\nu$)	1	N/A	Used in the theoretical scaling model

Key Methodologies

The experiment relied on advanced quantum sensing techniques and precise material integration:

Material Stacking and Transfer: Exfoliated FGT flakes were transferred onto a [100] oriented diamond chip containing a single-layer ensemble of Nitrogen-Vacancy (NV) centers.
Interface Engineering: Hexagonal Boron Nitride (hBN) flakes were strategically inserted above and below the FGT to prevent oxidation and, critically, to adjust the NV-FGT separation distance ($d$) from tens to hundreds of nanometers.
Quantum State Manipulation: Microwave (MW) radiation was delivered via an antenna/waveguide to manipulate the NV quantum state. An in-plane magnetic bias field ($B_{0}$) was applied to lift the degeneracy of the NV centers.
Decoherence Measurement: The Hahn-echo pulse sequence ($\pi/2 - \tau - \pi - \tau - \pi/2$) was employed for wide-field coherence detection. This sequence acts as a narrow-band spectral filter, selectively coupling to magnetic noise components whose frequencies match the pulse sequence periodicity.
Wide-Field Imaging: A 532 nm laser was used for initialization and readout, with photoluminescence recorded by a CMOS camera to generate spatial maps of $T_{2}$ coherence time and CW spectral characteristics.
Noise Analysis: Decoherence curves $C(t)$ were fitted using a stretched exponential function $C(t) = C_{0} \exp(-(t/T_{2})^{\alpha})$ to extract the coherence time ($T_{2}$) and the stretch exponent ($\alpha$), which reflects the properties of the magnetic noise.

6CCVD Solutions & Capabilities

This research highlights the critical need for high-quality, precisely engineered diamond substrates for advanced quantum magnetometry. 6CCVD is uniquely positioned to supply the foundational materials required to replicate, scale, and extend this work.

Applicable Materials

To replicate or extend this research, the highest quality diamond is required to ensure long intrinsic coherence times ($T_{2,0}$) and minimal strain.

Material Requirement	6CCVD Material Recommendation	Rationale
High-Purity Substrate	Optical Grade Single Crystal Diamond (SCD)	Essential for minimizing intrinsic decoherence ($T_{2,0}$) and maximizing the signal-to-noise ratio in quantum sensing experiments.
NV Layer Integration	Custom SCD Wafers with Controlled Nitrogen Doping	We offer MPCVD growth tailored for creating shallow, high-density ensemble NV layers, crucial for maximizing magnetic field sensitivity near the surface.
Interface Layer	High-Purity Polycrystalline Diamond (PCD)	For applications requiring larger area coverage (up to 125 mm) or specific thermal management, our PCD offers excellent thermal conductivity and customizable thickness (0.1 µm - 500 µm).
Substrate Thickness	SCD Substrates up to 10 mm	Provides robust mechanical support and superior heat sinking for cryogenic setups (T < 210 K) used in this research.

Customization Potential

The experiment required precise integration of the FGT sample and the microwave delivery system. 6CCVD’s in-house engineering capabilities directly address these needs:

Custom Dimensions: The research utilized small diamond chips. 6CCVD offers custom laser cutting and shaping services to produce diamond plates and wafers in any required geometry, up to 125 mm (PCD).
Interface Metalization: The experiment requires efficient delivery of microwave (MW) radiation via an antenna/waveguide. 6CCVD provides internal metalization services including deposition of Au, Pt, Pd, Ti, W, and Cu, enabling the fabrication of integrated microwave structures directly onto the diamond sensor chip.
Surface Preparation: Achieving optimal coupling requires an ultra-smooth interface. 6CCVD guarantees superior polishing, achieving surface roughness Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD wafers, minimizing surface noise and strain.

Engineering Support

6CCVD’s in-house PhD team specializes in MPCVD growth and material optimization for quantum technologies. We can assist researchers in similar Quantum Magnetometry and Phase Transition projects by:

Optimizing Nitrogen Concentration: Tailoring the nitrogen concentration during MPCVD growth to achieve the desired density and depth of ensemble NV centers.
Strain Management: Providing low-strain SCD materials critical for maintaining long coherence times in cryogenic environments.
Custom Layer Stacks: Consulting on the optimal diamond thickness and surface termination for integrating 2D materials like Fe${3}$GeTe${2}$ and hBN.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Critical fluctuations play a crucial role in determining spin orders in low-dimensional magnetic materials. However, experimentally linking these fluctuations to scaling theory-and thereby uncovering insights into spin interaction models-remains a challenge. Here, we utilize a nitrogen-vacancy center-based quantum decoherence imaging technique to probe critical fluctuations in the van der Waals magnet Fe<sub>3</sub>GeTe<sub>2</sub>. Our data reveal that critical fluctuations produce a random magnetic field, with noise spectra undergoing significant changes near the critical temperature. To explain this phenomenon, we developed a theoretical framework showing that the spectral density exhibits 1/f noise characteristics near the critical temperature, transitioning to white noise behavior away from this regime. By experimentally adjusting the sample-to-diamond distance, we identified the crossover temperature between these two noise types. These findings offer an approach to studying phase transition dynamics through critical fluctuations, enabling precise determination of critical exponents associated with long-range correlations. This methodology holds promise for advancing our understanding of critical phenomena across diverse physical systems.