| Metadata | Details |
|---|
| Publication Date | 2025-09-29 |
| Journal | Nature Communications |
| Authors | Yuxin Li, Zhe Ding, Chen Wang, Haoyu Sun, Zhousheng Chen |
| Institutions | University of Science and Technology of China, Hefei National Center for Physical Sciences at Nanoscale |
| Citations | 1 |
| Analysis | Full AI Review Included |
- Core Achievement: Successfully utilized Nitrogen-Vacancy (NV) center quantum decoherence imaging to probe dynamic critical spin fluctuations in the two-dimensional van der Waals magnet Fe3GeTe2 (FGT).
- Critical Fluctuation Signature: Experimental data revealed that critical fluctuations generate a random magnetic field, causing a sharp, pronounced peak in the NV center decoherence rate (Γ) precisely at the critical temperature (Tc).
- Noise Spectral Crossover: A key finding was the explicit observation of a crossover in the magnetic noise spectrum: it exhibits 1/f noise characteristics near Tc, transitioning sharply to white noise behavior away from the critical regime.
- Theoretical Validation: A theoretical framework based on scaling theory was developed, showing excellent quantitative agreement with the measured temperature-dependent decoherence rates (Γ), linking Γ to the noise spectral density S(ω).
- Scaling Exponent Determination: The methodology enables the precise determination of critical exponents (e.g., v=1 and z=2.17 for the 2D Ising model fit), which are crucial for characterizing long-range correlations and universality classes.
- Methodological Advance: By adjusting the NV-FGT separation distance (d) using hBN spacers, the magnetic noise coupling was reduced, allowing reliable measurement of the NV coherence time (T2) even through the intense fluctuation regime at Tc.
| Parameter | Value | Unit | Context |
|---|
| Material System | Fe3GeTe2 (FGT) | N/A | Two-dimensional van der Waals ferromagnet. |
| Bulk Curie Temperature (Tc) | Approx 210 | K | Reference value for bulk FGT. |
| Measured Tc Range | 160 to 190 | K | Varies depending on FGT thickness (10 nm to 90 nm). |
| NV Center Intrinsic T2 | Approx 4.5 | µs | Coherence time in bare diamond chips. |
| Minimum Measured T2 (near Tc) | Approx 3.2 | µs | Characteristic T2 in FGT-covered regions (175 K). |
| Decoherence Rate Peak (Γ) | Hundreds | kHz | Maximum rate observed at Tc (Fig. 4b). |
| Critical Crossover Frequency (ω0) | Approx 0.4 | MHz | Frequency marking the transition between white noise and 1/f noise near Tc. |
| FGT Thickness Range | 10 to 90 | nm | Range of samples investigated. |
| NV-FGT Separation Distance (d) | 60 to 310 | nm | Varied using hBN spacers to control magnetic coupling. |
| Magnetic Fluctuation Decay Scaling | d-2.5 | N/A | Preliminary estimation of magnetic fluctuation decay with distance. |
| Dynamic Critical Exponent (z) | 2.17 | N/A | Used in theoretical fit (consistent with 2D Ising model). |
| Correlation Length Exponent (v) | 1 | N/A | Used in theoretical fit (consistent with 2D Ising model). |
| Laser Wavelength | 532 | nm | Used for NV center initialization and readout. |
- Sample and Sensor Integration: Exfoliated FGT flakes were transferred onto a [100] oriented diamond chip containing an ensemble layer of NV centers.
- Distance Engineering: Hexagonal boron nitride (hBN) flakes were inserted between the FGT and the diamond surface to precisely increase the separation distance (d) from 60 nm up to 310 nm. This reduced the magnetic noise coupling, enabling T2 measurements across the critical point.
- Quantum State Control: NV centers were initialized and read out using a 532 nm laser. Quantum state manipulation (π/2 and π pulses) was achieved using microwave (MW) radiation delivered via an Ω-shaped waveguide antenna.
- Static Magnetometry: Continuous Wave (CW) optically detected magnetic resonance (ODMR) spectroscopy was used to map the static magnetic field distribution and characterize spontaneous magnetization (T < Tc).
- Dynamic Fluctuation Sensing: The Hahn-echo pulse sequence (π/2 - τ - π - τ - π/2) was employed. This dynamic decoupling protocol acts as a narrow-band spectral filter, selectively coupling the NV coherence to magnetic noise components resonant with the pulse sequence periodicity (1/τ).
- Decoherence Analysis: The measured contrast C(t) was fitted using the stretched exponential function C(t) = C0 exp(-(t/T2)α) to extract the coherence time (T2) and stretch exponent (α). The decoherence rate was defined as Γ = 1/T2 - 1/T2,0.
- Noise Spectrum Modeling: A phenomenological noise spectral density S(ω) = A/(ωμ + ω0μ) was proposed, where ω0 is the crossover frequency related to the relaxation time (ω0 ~ |(T-Tc)/T|zν). This model was used to fit the temperature-dependent Γ data, confirming the 1/f noise (ω > ω0) to white noise (ω < ω0) transition.
- Quantum Sensing and Metrology: Utilizing NV center wide-field microscopy for high-resolution, non-invasive mapping of magnetic fields and fluctuations in complex condensed matter systems.
- 2D Materials Characterization: Providing a robust, microscopic methodology for characterizing phase transition dynamics, critical exponents, and correlation lengths in low-dimensional magnets, crucial for materials discovery.
- Spintronics and Memory Devices: Gaining fundamental insights into the noise mechanisms (1/f noise) near magnetic phase transitions, which is essential for designing stable and efficient spintronic devices and magnetic memory.
- Advanced Noise Spectroscopy: Employing quantum sensors as frequency-selective probes to reconstruct the spectral density S(ω) of magnetic noise, enabling the study of critical dynamics that are inaccessible by traditional relaxometry methods.
- Fundamental Physics Instrumentation: Establishing a universal framework for characterizing criticality through dynamic quantities, applicable to diverse systems including ferroelectrics, superconductors, and quantum magnets.
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.
- 1983 - Magnetic Phase Transitions [Crossref]
- 2007 - A Modern Approach to Critical Phenomena [Crossref]