ac Susceptometry of 2D van der Waals Magnets Enabled by the Coherent Control of Quantum Sensors
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-09-28 |
| Journal | PRX Quantum |
| Authors | XinâYue Zhang, YuâXuan Wang, Thomas A. Tartaglia, Siyuan Ding, Mason Gray |
| Institutions | Boston College |
| Citations | 21 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Breakthrough Technique: A quantum-enabled AC susceptometry method was developed using coherently controlled Nitrogen-Vacancy (NV) centers in diamond, enabling ultra-sensitive measurement of dynamic magnetism in two-dimensional (2D) van der Waals (vdW) magnets.
- Ultra-High Sensitivity: The technique achieved a remarkable AC magnetic field resolution of ~40 nT, exceeding prior DC magnetometry sensitivity on 2D magnetic monolayers by two orders of magnitude.
- Material Characterization: The platform was applied to few-layer Chromium Bromide (CrBr3), providing quantitative DC hysteresis and AC susceptibility data across varying temperature, field, and frequency.
- Domain Wall Dynamics: Measurements illuminated the formation, mobility, and consolidation of magnetic domain walls, showing that mobility is enhanced in ultrathin CrBr3 and remains robust for excitation frequencies exceeding hundreds of kilohertz.
- Phase Transition Quantification: The ferromagnetic phase transition was quantified, yielding a Curie temperature (Tc) of 30.5 ± 0.5 K and a critical exponent (γ) of 1.1 ± 0.3.
- Versatile Platform: This multi-modal approach establishes a generic, nanoscale platform for characterizing both static and sub-gigahertz dynamic magnetic phenomena in a wide range of spintronic materials.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| AC Field Resolution | ~40 | nT | Achieved via coherent control (Dynamical Decoupling). |
| NV Center Depth | ~60 | nm | Near-surface ensemble NV layer in diamond. |
| NV Center Transition Slope | -28 | MHz/mT | Used for the ms=0 â ms=-1 transition. |
| CrBr3 Flake Thickness (Flake A) | 10 | layers (7.4 nm) | Ultrathin flake investigated. |
| Saturation Moment Density (Simulated) | 148 | ”B/nm2 | Assumed for 10-layer CrBr3 (3.0 ”B per Cr ion). |
| Curie Temperature (Tc) (Flake A) | 30.5 ± 0.5 | K | Determined via field-cooling (FC) initial susceptibility. |
| Critical Exponent (γ) (Flake A) | 1.1 ± 0.3 | Dimensionless | Scaling exponent for XAC divergence near Tc. |
| Maximum XAC (Flake A, FW) | 20 | emu/(mol Oe) | Local measurement during field-warming (FW). |
| AC Excitation Frequency (fAC) Range | 119 to 714 | kHz | Tested range using XY8-N sequences. |
| Domain Wall Relaxation Frequency (wc/2Ï) | ~2 to ~1 | MHz | Decreases monotonically as temperature drops from 22 K to 10 K. |
| Static Bias Field (HDC) Alignment | 54.7 | ° | Angle relative to the surface normal (z-axis). |
Key Methodologies
Section titled âKey MethodologiesâThe experiment combined nanoscale sample preparation with advanced quantum control sequences for magnetic sensing.
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Sample Preparation and Integration:
- Exfoliation: Ultrathin CrBr3 flakes (6 to 18 layers) were mechanically exfoliated onto a diamond substrate containing a shallow ensemble of NV centers.
- Environment: All sample handling and measurements were conducted in a cryogenic system within an argon-filled glovebox to prevent degradation.
- Field Application: An insulated wire coil adjacent to the diamond delivered the radio frequency (RF) AC excitation field (HAC, ~100 kHz) and gigahertz microwave pulses for NV spin manipulation.
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DC Magnetometry (ODMR):
- Measurement: Optically-Detected Magnetic Resonance (ODMR) was used to measure the static stray field (BSC) from the CrBr3 flake, which is proportional to the DC magnetization (M).
- Hysteresis Mapping: By sweeping the static bias field (HDC), magnetic hysteresis loops were mapped, revealing domain nucleation, pinning (Barkhausen jumps), and domain consolidation behavior.
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Quantum-Enabled AC Susceptometry:
- Sensing Protocol: Dynamical Decoupling (DD) sequences, specifically XY8-N, were applied to the NV center spin superposition state. This sequence acts as a lock-in amplifier, making the NV center sensitive only to AC fields synchronized to the pulse repetition rate.
- Frequency Matching: The Ï-pulse spacing (Ï) in the DD sequence was set such that Ï = 1/(2fAC), matching the NV precession to the coil excitation frequency (fAC).
- Phase Control: The phase delay (ÎŽ) between the coil field HAC and the train of Ï-pulses was tuned (ÎŽ = 0 or ÎŽ = Ï/2) to selectively measure the real (in-phase) or imaginary (out-of-phase) components of the AC susceptibility (XAC).
- Artifact Correction: A baseline subtraction procedure was implemented using simultaneous DC magnetization data to remove systematic artifacts caused by the flakeâs DC stray field misaligning the NV center axis relative to HDC.
Commercial Applications
Section titled âCommercial Applicationsâ- Quantum Sensing and Metrology:
- Nanoscale Dynamic Probes: Enabling the development of next-generation quantum sensors for high-frequency (sub-gigahertz) magnetic characterization at the single-particle or few-layer limit.
- Coherence Engineering: Utilizing DD sequences to actively protect sensor coherence (T2), crucial for high-precision quantum metrology applications.
- Spintronics and Data Storage:
- 2D Magnet Optimization: Providing critical dynamic data (domain wall mobility, damping, relaxation times) necessary for optimizing 2D vdW magnets (CrBr3, CrI3) for integration into spintronic devices.
- Domain-Based Memory: Informing the design and performance limits of high-speed, low-power domain-based memory and logic devices (e.g., racetrack memory) by characterizing pinning landscapes.
- Advanced Materials Research:
- Complex Magnetic Systems: Offering a generic, quantitative tool for investigating dynamic phenomena in materials lacking magneto-optical coupling, such as antiferromagnets, quantum spin liquids, and single-molecule magnets.
- Critical Phenomena: High-precision measurement of critical exponents and phase transitions in ultra-thin films, essential for fundamental condensed matter physics.
View Original Abstract
Precision magnetometry is fundamental to the development of novel magnetic\nmaterials and devices. Recently, the nitrogen-vacancy (NV) center in diamond\nhas emerged as a promising probe for static magnetism in 2D van der Waals\nmaterials, capable of quantitative imaging with nanoscale spatial resolution.\nHowever, the dynamic character of magnetism, crucial for understanding the\nmagnetic phase transition and achieving technological applications, has rarely\nbeen experimentally accessible in single 2D crystals. Here, we coherently\ncontrol the NV centerâs spin precession to achieve ultra-sensitive,\nquantitative ac susceptometry of a 2D ferromagnet. Combining dc hysteresis with\nac susceptibility measurements varying temperature, field, and frequency, we\nilluminate the formation, mobility, and consolidation of magnetic domain walls\nin few-layer CrBr3. We show that domain wall mobility is enhanced in ultrathin\nCrBr3, with minimal decrease for excitation frequencies exceeding hundreds of\nkilohertz, and is influenced by the domain morphology and local pinning of the\nflake. Our technique extends NV magnetometry to the multi-functional ac and dc\nmagnetic characterization of wide-ranging spintronic materials at the\nnanoscale.\n