Imaging non-collinear antiferromagnetic textures via single spin relaxometry

At a Glance

Metadata	Details
Publication Date	2021-02-03
Journal	Nature Communications
Authors	Aurore Finco, Angela Haykal, Rana Tanos, Florentin Fabre, S. Chouaieb
Institutions	Centre National de la Recherche Scientifique, Université Paris-Saclay
Citations	92
Analysis	Full AI Review Included

Technical Documentation & Analysis: Nanoscale Antiferromagnetic Imaging via NV Relaxometry

Reference Paper: Finco et al., “Imaging non-collinear antiferromagnetic textures via single spin relaxometry,” Nature Communications (2021).

Executive Summary

This research successfully demonstrates a novel, all-optical method for imaging non-collinear antiferromagnetic (AFM) spin textures—including domain walls, spin spirals, and skyrmions—at the nanoscale using a single Nitrogen-Vacancy (NV) defect in diamond.

Core Achievement: Nanoscale imaging of AFM textures (zero net magnetization) by sensing localized magnetic noise generated by thermal magnons.
Sensing Mechanism: Single NV spin relaxometry ($T_{1}$ measurement) under continuous laser illumination, where magnetic noise increases the spin relaxation rate ($\Gamma_{1}$), causing a measurable reduction (quenching) in the photoluminescence (PL) signal.
Key Finding: Magnetic noise is significantly stronger at domain walls ($T_{1}$ drops from $120$ µs in the uniform domain to $22$ µs at the wall), confirming the presence of thermally-excited gapless magnon modes.
Material Requirement: The technique relies fundamentally on the exceptional spin coherence properties of the NV center, necessitating ultra-high purity, low-strain Single Crystal Diamond (SCD).
6CCVD Value Proposition: 6CCVD supplies the requisite Optical Grade SCD wafers and plates, featuring industry-leading purity and surface polishing (Ra < 1 nm), essential for maximizing NV sensor sensitivity and minimizing surface-induced noise.
Application Potential: This method opens new avenues for studying localized spin wave modes, magnon transport, and magnetic order in next-generation spintronic devices.

Technical Specifications

Parameter	Value	Unit	Context
NV ESR Frequency ($f_{0}$)	2.87	GHz	Zero magnetic field transition frequency.
NV Flying Distance ($d_{NV}$)	79 ± 5	nm	Distance between NV sensor and SAF surface.
$T_{1}$ (Retracted/Bulk)	860 ± 300	µs	Baseline spin relaxation time (limited by tip impurities).
$T_{1}$ (Uniform SAF Domain)	120 ± 10	µs	Relaxation time above the uniform magnetic domain.
$T_{1}$ (SAF Domain Wall)	22 ± 2	µs	Relaxation time above the domain wall (maximum noise).
Maximum Static Stray Field ($B_{NV}$)	~500	µT	Measured above the domain wall (too weak for PL quenching).
Optical Saturation Power ($P_{sat}$)	450	µW	Saturation power of the NV defect optical transition.
Spin Spiral Period ($\lambda$)	~250	nm	Measured period in the SAF with vanishing anisotropy.
AFM Skyrmion Width (FWHM)	76 ± 29	nm	Average width of isolated PL quenching spots.
SCD Polishing Requirement	Ra < 1	nm	Required for minimizing surface noise and maximizing sensor proximity.

Key Methodologies

The experiment utilized a scanning-NV magnetometer operating under ambient conditions, integrated with a confocal optical microscope.

Sample Preparation: Synthetic Antiferromagnets (SAFs) were fabricated using sputtered [Pt/Co/Ru]×2 multilayer stacks, with Co thickness ($t_{Co}$) tuned to control effective perpendicular magnetic anisotropy (e.g., $t_{Co} = 1.41$ nm for domain walls, $t_{Co} = 1.47$ nm for spin spirals).
Sensor Setup: A commercial diamond tip hosting a single NV defect (Qnami Quantilever MX) was scanned above the SAF surface using an Atomic Force Microscope (AFM) setup.
All-Optical Relaxometry: The NV defect was illuminated continuously with a green laser. Magnetic noise was detected as a reduction (quenching) of the NV photoluminescence (PL) signal.
$T_{1}$ Measurement Sequence: The longitudinal spin relaxation time ($T_{1}$) was measured using a standard sequence:
- Initialization: Laser pulse to polarize the NV into the $m_{s}=0$ sublevel.
- Relaxation: Dark delay ($\tau$) allowing spin relaxation.
- Readout: Second laser pulse to read the final population via spin-dependent PL signal.
Imaging Contrast Optimization: The optical excitation power ($P$) was adjusted relative to the saturation power ($P_{sat}$) to optimize the PL quenching contrast, demonstrating that the imaging mechanism is noise-driven ($\Gamma_{1}$ increase) rather than static field-driven.
Micromagnetic Simulation: Simulations were performed to model the dispersion relation of spin waves and the resulting power spectral density (PSD) of magnetic noise, confirming that the NV sensor detects thermally-excited gapless magnon modes localized at the domain walls.

6CCVD Solutions & Capabilities

The success of nanoscale quantum sensing relies entirely on the quality and purity of the diamond material used to host the NV centers. 6CCVD is an expert supplier of MPCVD diamond engineered specifically for quantum and spintronic applications.

Applicable Materials

To replicate or extend this research, the highest quality diamond is required to ensure long spin coherence times ($T_{1}$ and $T_{2}$) and minimal background noise.

6CCVD Material Solution	Specification & Relevance
Optical Grade Single Crystal Diamond (SCD)	Ultra-high purity, low nitrogen content (< 1 ppb), and low strain. Essential for maximizing the $T_{1}$ time of the NV defect, thereby enhancing magnetic noise sensitivity.
High-Purity SCD Substrates	Available in thicknesses from 0.1 µm up to 500 µm, suitable for both bulk NV implantation/growth and thin-film integration onto spintronic stacks.
Polycrystalline Diamond (PCD) Plates	While SCD is preferred for single NV sensing, high-quality PCD (up to 125mm) is available for large-area applications requiring ensemble NV sensing or thermal management in integrated devices.

Customization Potential

6CCVD’s in-house engineering capabilities directly address the complex integration needs of advanced spintronics research:

Precision Polishing: The paper notes that surface impurities reduce the baseline $T_{1}$. 6CCVD guarantees Ra < 1 nm polishing on SCD, minimizing surface defects and maximizing sensor proximity ($d_{NV} \approx 79$ nm) for optimal signal-to-noise ratio.
Custom Dimensions and Geometry: We provide custom laser cutting and shaping services for diamond plates and wafers, allowing researchers to integrate the NV sensor chip directly into complex experimental setups or fabricate custom scanning tips.
Advanced Metalization: For integrating diamond sensors onto SAF or other spintronic stacks, 6CCVD offers internal metalization capabilities, including deposition of Au, Pt, Ti, and W, enabling custom electrical contacts or bonding layers.

Engineering Support

The detection of magnetic noise via thermal magnons is a highly specialized field. 6CCVD’s in-house PhD team offers authoritative professional support:

Material Selection for Magnonics: Our experts can assist researchers in selecting the optimal SCD grade and surface termination necessary to minimize parasitic noise sources and maximize the sensitivity of the NV sensor for similar Magnon Transport and Antiferromagnetic Imaging projects.
Interface Optimization: We consult on surface preparation techniques crucial for achieving the ultra-low flying distances ($d_{NV} \approx 80$ nm) required to detect high-wave-vector ($k$) magnons, as demonstrated in this study.

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

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41467-021-20995-x