Long-Time-Scale Magnetization Ordering Induced by an Adsorbed Chiral Monolayer on Ferromagnets
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-02-16 |
| Journal | ACS Nano |
| Authors | Idan Meirzada, Nir Sukenik, Galya Haim, Shira Yochelis, L. T. Baczewski |
| Institutions | Institute of Physics, Polish Academy of Sciences |
| Citations | 44 |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Long-Timescale Magnetization Ordering Induced by an Adsorbed Chiral Monolayer on Ferromagnets
Section titled âTechnical Documentation & Analysis: Long-Timescale Magnetization Ordering Induced by an Adsorbed Chiral Monolayer on FerromagnetsâThis document analyzes the application of Nitrogen-Vacancy (NV) centers in diamond for high-precision quantum magnetometry, specifically focusing on the Chiral Induced Spin Selectivity (CISS) effect. The findings validate the need for ultra-high-purity, highly polished Single Crystal Diamond (SCD) substrates, a core offering of 6CCVD.
Executive Summary
Section titled âExecutive SummaryâThis research utilizes NV wide-field microscopy in diamond to provide quantitative, vectorial measurements of magnetization reorientation induced by chiral molecules (Alpha-Helix L polyalanine, AHPA) adsorbed onto a ferromagnetic (FM) thin film.
- Core Achievement: Direct experimental evidence confirming the persistent, long-timescale nature of the Chiral Induced Spin Selectivity (CISS) effect, resolving a long-standing open question in spintronics.
- Measurement Technique: High-sensitivity quantum magnetometry using a dense, quasi 2D layer of Nitrogen-Vacancy (NV) centers in diamond.
- Key Correlation: The magnetization tilt angle of the Cobalt (Co) layer was found to directly follow the adsorption tilt angle of the AHPA molecular monolayer over periods of hours to days.
- Material Requirement: The experiment relies on high-quality SCD diamond with precise NV layer placement and extremely low surface roughness (Ra < 1nm) to maintain the critical ~10 ”m standoff distance.
- Spintronics Impact: The results offer crucial insights for the implementation of CISS in next-generation spintronic and magnetic memory devices, requiring robust, persistent spin polarization.
- Observed Mechanism: Adsorption of molecules polarizing only 10% of the Co electrons is sufficient to tilt the Co layer easy axis, stabilized by a large spin exchange energy (> 250 meV).
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data points were extracted from the experimental setup and results:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| NV Zero Field Splitting | 2.87 | GHz | Between ms = 0 and ms = ±1 spin states |
| NV Gyromagnetic Ratio ($\gamma$) | 2.8 | MHz/Gauss | Used for Zeeman splitting calculation |
| FM Layer Thickness (Co) | 1.6 - 1.8 | nm | Grown by Molecular Beam Epitaxy (MBE) |
| FM Stack Composition | Au/Co/Au/Pt | Ă | 50/X/200/50 Ă layers (X = 16-18 Ă ) |
| Coercive Field (Hc) | ~150 | G | Magnetization easy axis is out of plane |
| NV Standoff Distance | ~10 | ”m | Distance between NV layer and FM sample |
| Initial Magnetization Tilt ($\theta$) | 40 ± 10 | degrees | Relative to sample normal (simulated) |
| Azimuthal Angle ($\phi$) | 90 ± 3 | degrees | Average azimuthal angle throughout adsorption area |
| Magnetization Decay Time | Hours to Days | Time | Demonstrating persistent CISS effect |
| AHPA Molecule Length | 5.4 | nm | Calculated length of Alpha-Helix L polyalanine |
| Adsorption Pattern Area | 1x1 | ”m2 | Checkerboard pattern size (6x6 array) |
| Magnetic Field Sensitivity | picoTesla / $\sqrt{\text{Hz}}$ | N/A | Demonstrated capability of NV centers |
Key Methodologies
Section titled âKey MethodologiesâThe experiment combined advanced thin-film growth, molecular self-assembly, and quantum sensing techniques:
- FM Substrate Fabrication: Epitaxial thin film samples (Au/Co/Au/Pt stack) were grown using Molecular Beam Epitaxy (MBE). The Au capping layers prevent oxidation and enhance perpendicular anisotropy.
- Molecular Patterning: A checkerboard pattern (6x6 array of 1x1 ”m2 squares) was fabricated in PMMA using E-beam lithography on the FM substrate.
- Self-Assembled Monolayer (SAM) Deposition: Alpha-Helix L polyalanine (AHPA) molecules were adsorbed selectively onto the patterned areas by dipping the sample in a 1 mM Ethanol solution for 3 hours.
- NV Sensor Preparation: A diamond substrate containing a dense, quasi 2D layer of NV centers was prepared.
- NV Wide-Field Microscopy: The FM sample was placed on the diamond, with the adsorbed molecules facing the NV layer at a standoff distance of ~10 ”m. A custom-built system used a 532 nm green laser for initialization and microwave (MW) fields (via an Ω-shaped antenna) for spin manipulation.
- Vectorial Magnetization Measurement: CW, 16-point Electron Spin Resonance (ESR) measurements were performed per pixel to extract 2D vectorial magnetic images (Bx, By, Bz).
- Correlation Measurement: Atomic Force Microscopy (AFM) was used in parallel to measure the molecular monolayer height and tilt angle over time, correlating the physical structure change with the magnetic reorientation dynamics.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & Capabilitiesâ6CCVD is the ideal partner for researchers replicating or extending this work, providing the critical SCD material necessary for high-fidelity NV magnetometry and spintronic integration.
Applicable Materials for NV Magnetometry
Section titled âApplicable Materials for NV MagnetometryâThe success of this experiment hinges on the quality and preparation of the diamond sensor. 6CCVD provides the necessary foundation:
- Single Crystal Diamond (SCD): We offer high-purity, low-strain SCD substrates, essential for maximizing NV coherence time (T2) and achieving the highest magnetic sensitivity (picoTesla / $\sqrt{\text{Hz}}$).
- Custom NV Integration: While the paper used a quasi 2D NV layer, 6CCVD can supply SCD substrates optimized for near-surface NV creation (via implantation or in-situ growth) to achieve the required dense NV ensemble for wide-field imaging.
- Polishing Excellence: The critical ~10 ”m standoff distance requires exceptional surface quality. 6CCVD guarantees Ra < 1nm polishing on SCD, minimizing surface scattering and ensuring uniform coupling between the FM sample and the NV sensor layer.
Customization Potential & Integration Services
Section titled âCustomization Potential & Integration ServicesâThe complexity of the FM stack (Au/Co/Pt) and the need for precise integration with the diamond sensor are directly addressed by 6CCVDâs in-house capabilities:
| Research Requirement | 6CCVD Capability | Technical Advantage |
|---|---|---|
| Substrate Dimensions | Plates/wafers up to 125mm (PCD) and large SCD plates. | Supports wide-field imaging and scale-up of experimental setups. |
| Thin Film Integration | SCD thickness control from 0.1”m to 500”m. | Allows for precise optical and thermal management of the NV sensor. |
| Custom Metalization | In-house deposition of Au, Pt, Ti, W, Cu, Pd. | We can replicate or modify the complex Au/Co/Pt stack directly onto the diamond or supply pre-metalized substrates for bonding. |
| Patterning & Shaping | High-precision laser cutting and shaping services. | Enables custom geometries for integrated waveguides (like the Ω-shaped MW antenna used) or specific sample mounting requirements. |
Engineering Support
Section titled âEngineering SupportâThe correlation between molecular tilt angle and magnetization direction is a powerful finding for CISS applications. 6CCVDâs in-house PhD team specializes in solid-state quantum systems and material interfaces. We can assist researchers in optimizing diamond material selection (e.g., nitrogen concentration, surface termination) for similar Spintronic and Quantum Magnetometry projects, ensuring the highest performance for NV-based sensing platforms.
Call to Action: For custom specifications, high-purity SCD substrates, or material consultation on integrating diamond quantum sensors with magnetic thin films, visit 6ccvd.com or contact our engineering team directly. We ship globally (DDU default, DDP available).
View Original Abstract
When an electron passes through a chiral molecule, there is a high probability for correlation between the momentum and spin of the charge, thus leading to a spin polarized current. This phenomenon is known as the chiral-induced spin selectivity (CISS) effect. One of the most surprising experimental results recently demonstrated is that magnetization reversal in a ferromagnet with perpendicular anisotropy can be realized solely by chemisorbing a chiral molecular monolayer without applying any current or external magnetic field. This result raises the currently open question of whether this effect is due to the bonding event, held by the ferromagnet, or a long-time-scale effect stabilized by exchange interactions. In this work we have performed vectorial magnetic field measurements of the magnetization reorientation of a ferromagnetic layer exhibiting perpendicular anisotropy due to CISS using nitrogen-vacancy centers in diamond and followed the time dynamics of this effect. In parallel, we have measured the molecular monolayer tilt angle in order to find a correlation between the time dependence of the magnetization reorientation and the change of the tilt angle of the molecular monolayer. We have identified that changes in the magnetization direction correspond to changes of the molecular monolayer tilt angle, providing evidence for a long-time-scale characteristic of the induced magnetization reorientation. This suggests that the CISS effect has an effect over long time scales which we attribute to exchange interactions. These results offer significant insights into the fundamental processes underlying the CISS effect, contributing to the implementation of CISS in state-of-the-art applications such as spintronic and magnetic memory devices.