Magnetic domains and domain wall pinning in atomically thin CrBr3 revealed by nanoscale imaging
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-03-31 |
| Journal | Nature Communications |
| Authors | Qiâchao Sun, Tiancheng Song, Eric Anderson, Andreas Brunner, Johannes Förster |
| Institutions | Max Planck Institute for Solid State Research, Max Planck Institute for Intelligent Systems |
| Citations | 107 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThe research details the successful application of cryogenic scanning Nitrogen-Vacancy (NV) center magnetometry to quantitatively analyze magnetic domains and coercivity mechanisms in atomically thin Chromium Bromide (CrBr3).
- Quantitative Nanoscale Probing: The study provides the first real-space imaging of magnetic domains and their evolution in atomically thin CrBr3, overcoming limitations of conventional micro-scale techniques.
- High Sensitivity and Resolution: The setup utilizes a pulsed Optically Detected Magnetic Resonance (ODMR) scheme, achieving a high magnetic field sensitivity of ~0.3 ”THz-1 and a spatial resolution of ~80 nm.
- Thermal Management: The pulsed measurement significantly reduces microwave heating, limiting the sample temperature increase to only a few hundred milli-Kelvin (mK) above the 4.2 K base temperature.
- Magnetization Quantification: The saturation magnetization (Msat) of a CrBr3 bilayer was quantitatively determined to be approximately 26 Bohr magnetons per square nanometer (”B/nm2).
- Coercivity Mechanism Identified: Domain wall pinning at specific defect sites is unambiguously proven to be the dominant coercivity mechanism governing magnetic reversal in the CrBr3 bilayer.
- Defect Mapping: The high spatial resolution allows for the precise location of defects that pin domain walls and nucleate reverse domains, crucial for understanding material imperfections.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Base Measurement Temperature | < 5 | K | Liquid helium cryostat operation. |
| CrBr3 Sample Thickness | ~2 | nm | Bilayer sample, confirmed by AFM step height. |
| NV Center Sensitivity | ~0.3 | ”THz-1 | Optimal magnetic field sensitivity achieved via pulsed ODMR. |
| Spatial Resolution | ~80 | nm | Limited by the distance (h) between the NV center and the sample. |
| Saturation Magnetization (Bilayer) | ~26 | ”B/nm2 | Experimentally determined Msat for CrBr3. |
| Theoretical Msat (Bilayer) | ~32 | ”B/nm2 | Based on 3 ”B saturation moment per Cr3+ ion. |
| NV Axis Angle | ~54.7 | ° | Angle relative to the vertical direction (out-of-plane). |
| External Field for Saturation (Bsat) | 11 | mT | Field used to fully polarize the sample. |
| Microwave Heating Effect | Few hundred | mK | Temperature increase observed during pulsed ODMR. |
| Microwave Pulse (pi-pulse) Duration | ~80 | ns | Used for spin manipulation. |
| Laser Pulse Duration | 600 | ns | Used for optical initialization/readout. |
| Sample Encapsulation | hBN | N/A | Used on both sides of CrBr3 for protection. |
Key Methodologies
Section titled âKey Methodologiesâ-
Cryogenic Scanning NV Magnetometry Setup:
- A single NV center implanted in a diamond pillar probe is attached to an Atomic Force Microscope (AFM) tuning fork, operating in frequency modulation mode (oscillation amplitude ~1.5 nm).
- The microscope head is suspended in an insertion tube filled with helium buffer gas, dipped into a liquid helium cryostat (T < 5 K) equipped with vector superconducting coils for applying external magnetic fields (Bext).
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Sample Preparation and Integration:
- CrBr3 flakes were exfoliated and encapsulated in hexagonal boron nitride (hBN) within a pure nitrogen glovebox (H2O/O2 concentration <0.1 p.p.m.).
- The hBN/CrBr3/hBN stack was transferred into the gap of a pre-patterned coplanar waveguide (CPW) deposited on a SiO2/Si substrate, enabling efficient microwave delivery.
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Pulsed ODMR Measurement Scheme:
- The stray magnetic field (Bs) is mapped by measuring the electronic spin resonance spectrum using a pulsed ODMR sequence (Fig. 1d).
- Short laser pulses (600 ns) and Ï-pulse microwaves (~80 ns) are used. Microwave pulses are applied 600 ns after the laser is switched off to prevent disturbance from laser-induced excitations.
- Three pairs of sinusoidal signals are generated to simultaneously excite the three hyperfine split transitions of the 14N nuclear spin, optimizing sensitivity.
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Magnetization Reconstruction:
- The stray magnetic field (Bs) is measured along the NV axis (54.7° relative to vertical).
- The magnetization (Mz) image is reconstructed from Bs using a reverse-propagation protocol, assuming out-of-plane magnetization (justified by Bext being much lower than the in-plane critical field).
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Domain Evolution and Coercivity Study:
- Samples were thermally demagnetized (heated to 45 K, cooled under zero field) before measurement cycles.
- Magnetization images were taken successively while increasing the external magnetic field (2 mT to 6 mT) to observe domain wall motion and pinning effects.
- Hysteresis loops were extracted by cycling the external field and calculating the average magnetization ratio (M/Msat) based on pixel counts of positive and negative domains.
Commercial Applications
Section titled âCommercial Applicationsâ- Spintronic Device Development: Provides critical quantitative data (Msat, coercivity mechanisms) necessary for engineering next-generation 2D vdW magnet-based spintronic devices, such as magnetic tunnel junctions and memory elements.
- Quantum Sensing and Metrology: Advances the state-of-the-art in cryogenic NV center magnetometry, offering a high-sensitivity, low-thermal-load platform for probing complex magnetic textures in quantum materials.
- Materials Characterization (2D Magnets): Essential tool for non-invasive, nanoscale characterization of novel 2D magnetic materials and vdW heterostructures, particularly for imaging topological spin textures (e.g., skyrmions) and studying spin dynamics.
- Defect and Interface Engineering: Enables precise localization of defects and grain boundaries that govern magnetic reversal processes, allowing for targeted material synthesis and defect control to optimize magnetic performance.
- Cryogenic Instrumentation: The demonstrated pulsed ODMR technique is valuable for any cryogenic experiment requiring high magnetic sensitivity while strictly minimizing thermal dissipation.