Imaging non-collinear antiferromagnetic textures via single spin relaxometry
At a Glance
Section titled âAt a Glanceâ| 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, Laboratoire Albert Fert |
| Citations | 92 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research introduces a breakthrough in nanoscale imaging of antiferromagnetic (AFM) materials, traditionally challenging due to their zero net magnetization.
- Core Value Proposition: Achieves nanoscale, all-optical imaging of non-collinear AFM spin textures (domain walls, spirals, skyrmions) by sensing local magnetic noise, rather than static stray fields.
- Methodology: Utilizes a scanning quantum sensor based on a single Nitrogen-Vacancy (NV) defect in diamond, operating via longitudinal spin relaxometry (T1 measurement).
- Contrast Mechanism: Magnetic noise generated by thermally-activated magnons locally increases the NV spin relaxation rate (T1 drops significantly, e.g., from 120 ”s to 22 ”s), resulting in a measurable quenching of the NV photoluminescence (PL) signal.
- Material System: Demonstrated successfully on Synthetic Antiferromagnets (SAFs) composed of [Pt/Co/Ru]x2 multilayers, where magnetic parameters are highly tunable.
- Resolution and Features: Successfully imaged spin spirals with periods of approximately 250 nm and antiferromagnetic skyrmions with an average width of 76 nm.
- Significance: Opens new opportunities for studying the physics of localized spin wave modes (magnonics) and magnetic order in low-moment materials under ambient conditions.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| NV Sensor Flying Distance (dNV) | 79 ± 5 | nm | Distance above SAF surface during scanning. |
| NV ESR Frequency (f0) | 2.87 | GHz | Electron Spin Resonance frequency of the NV defect (zero field). |
| T1 Relaxation Time (Retracted) | 860 ± 300 | ”s | Background T1 attributed to paramagnetic impurities on the tip surface. |
| T1 Relaxation Time (Uniform Domain) | 120 ± 10 | ”s | T1 above a uniform SAF domain. |
| T1 Relaxation Time (Domain Wall) | 22 ± 2 | ”s | Minimum T1 observed, indicating maximum magnetic noise. |
| Maximum Static Stray Field (BNV) | ~500 | ”T | Measured above domain wall (insufficient to cause static PL quenching). |
| SAF Structure (General) | [Pt/Co/Ru]x2 | Multilayer | Stack with broken inversion symmetry for DMI. |
| Co Layer Thickness (tCo) | 1.41 | nm | Sample used for domain wall imaging (perpendicular anisotropy). |
| Co Layer Thickness (tCo) | 1.47 | nm | Sample used for spin spiral imaging (vanishing anisotropy). |
| Spin Spiral Period (λ) | ~250 | nm | Measured period in the vanishing anisotropy SAF. |
| Skyrmion Width (FWHM) | 76 ± 29 | nm | Average full width at half maximum of isolated antiferromagnetic skyrmions. |
| DMI Constant (Inferred) | 1 | mJ/m2 | Estimated from spin spiral period (assuming A = 20 pJ/m). |
| Saturation Power (Psat) | 450 | ”W | Saturation power of the NV defect optical transition. |
Key Methodologies
Section titled âKey MethodologiesâThe imaging procedure relies on nanoscale, all-optical relaxometry using a scanning NV magnetometer.
- Sample Preparation: Synthetic Antiferromagnets (SAFs) were fabricated via sputtering, consisting of a Ta/Pt seed layer, two identical Co layers coupled antiferromagnetically via a Ru/Pt spacer, and a Pt capping layer.
- Magnetic Texture Control:
- Domain walls were stabilized in SAFs with significant effective perpendicular magnetic anisotropy (tCo = 1.41 nm).
- Spin spirals were stabilized by adjusting tCo (1.47 nm) to achieve vanishing effective magnetic anisotropy.
- Antiferromagnetic skyrmions were stabilized by adding a uniformly magnetized bias layer coupled ferromagnetically to the SAF.
- Sensor Setup: A commercial diamond tip hosting a single NV defect (Quantilever MX) was integrated into an Atomic Force Microscope (AFM) and scanned at a fixed flying distance (dNV ~ 80 nm).
- Relaxometry Measurement (T1):
- The NV defect was initialized into the ms=0 sublevel using a green laser pulse.
- The spin was allowed to relax in the dark for a time Ï.
- A second laser pulse read out the final ms=0 population by recording the spin-dependent PL signal.
- The longitudinal spin relaxation time (T1) was extracted by fitting the decay curve.
- All-Optical Imaging Mode: The PL signal was recorded under continuous green laser illumination. PL quenching (reduction) directly maps the local magnetic noise intensity, as noise increases the spin relaxation rate (Î1) and degrades NV polarization.
- Imaging Optimization: The imaging contrast was maximized by operating at low optical excitation power (P), where the PL signal is most sensitive to changes in T1 (e.g., P/Psat = 0.05).
- Noise Modeling: Micromagnetic simulations were used to calculate the Power Spectral Density (PSD) of the magnetic noise, confirming that the strongest noise intensity occurs at domain walls due to thermally-excited, gapless spin wave modes.
Commercial Applications
Section titled âCommercial ApplicationsâThis NV relaxometry technique provides a crucial tool for characterizing materials central to emerging spintronic and magnonic technologies.
- Next-Generation Spintronics:
- Characterization of high-speed, robust antiferromagnetic materials for memory and logic devices.
- Imaging and optimization of AFM domain walls for high-density racetrack memory applications.
- Magnonics and Spin Wave Technologies:
- Direct mapping of localized spin wave modes (magnons) in complex textures (domain walls, skyrmions).
- Studying magnon transport and scattering mechanisms at the nanoscale, which is inaccessible by conventional methods.
- Quantum Sensing and Metrology:
- Advancing the development of high-sensitivity, table-top magnetic noise sensors capable of operating under ambient conditions.
- Extending NV relaxometry to study magnetic order and disorder in other low-moment systems, such as two-dimensional (2D) van der Waals magnets.
- Fundamental Materials Research:
- Investigating dynamic phenomena in magnetic materials, including current-induced dynamics and thermal fluctuations.