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 | Hebrew University of Jerusalem, Polish Academy of Sciences |
| Citations | 44 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research resolves a fundamental question regarding the Chiral Induced Spin Selectivity (CISS) effect by demonstrating its long-timescale persistence and dynamic correlation with molecular structure.
- Persistent Magnetization Ordering: The magnetization reorientation induced in a ferromagnet (FM) by an adsorbed chiral monolayer (Alpha-Helix L polyalanine, AHPA) is not immediate but is a persistent, steady-state effect lasting hours to days.
- Structural Correlation: The direction of the FM magnetization vector directly follows the time-dependent change in the molecular monolayer tilt angle, moving slowly toward the in-plane direction over time.
- Quantitative Vectorial Measurement: Nitrogen-Vacancy (NV) center wide-field microscopy was employed to perform quantitative, vectorial measurements of the stray magnetic field, providing precise data on magnetization angle and magnitude.
- High Coupling Efficiency: Simulations indicate that polarization of only 10% of the Cobalt (Co) electrons is sufficient to induce the measured magnetic field magnitude, suggesting strong coupling between the molecules and the substrate.
- Stabilization Mechanism: The persistent effect is attributed to strong coupling between the magnetic substrate and the molecular magnetic dipole, stabilized by a large spin exchange energy estimated to be greater than 250 meV.
- Methodological Advance: The combination of NV magnetometry and Atomic Force Microscopy (AFM) provides a powerful modality for distinguishing between transient (immediate) and persistent (long-timescale) mechanisms in spintronic phenomena.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Ferromagnet Thickness (Co) | 1.6 - 1.8 | nm | Grown by Molecular Beam Epitaxy (MBE) |
| Coercive Field (Hc) | ~ 150 | G | Magnetization easy axis is out-of-plane |
| Molecular Monolayer (AHPA) Length | 5.4 | nm | Calculated length of Alpha-Helix L polyalanine |
| Initial Magnetization Tilt Angle | 40 ± 10 | degrees | Relative to sample normal (measured hours after adsorption) |
| Initial Azimuthal Angle (Ï) | 90 ± 3 | degrees | Average azimuthal angle of magnetic dipoles |
| Magnetization Decay Timescale | Hours | Time | Time over which the magnetization tilt angle increases (moves in-plane) |
| Molecular Tilt Change Timescale | Days | Time | Time over which the molecular tilt angle increases (moves in-plane) |
| Required Co Electron Polarization | â„ 10 | % | Fraction of Co electrons polarized to match measured field magnitude |
| Exchange Interaction Energy | > 250 | meV | Energy stabilizing the persistent CISS effect (10 * KBT) |
| NV Center Zero Field Splitting | 2.87 | GHz | Ground state splitting between ms = 0 and ms = ±1 |
| NV-Sample Standoff Distance | ~ 10 | ”m | Distance for stray magnetic field measurement |
Key Methodologies
Section titled âKey MethodologiesâThe study utilized a multi-step fabrication and characterization process combining thin-film growth, lithography, molecular self-assembly, and advanced quantum sensing.
-
Ferromagnet Substrate Growth:
- Epitaxial thin film stacks (Pt/Au/Co/Au) were grown using Molecular Beam Epitaxy (MBE).
- Co thickness was controlled between 1.6 nm and 1.8 nm.
- Au capping layers (50 A top, 200 A bottom) ensured Co epitaxial growth, prevented oxidation, and enhanced perpendicular magnetic anisotropy.
-
Patterning and Adsorption:
- A checkerboard pattern consisting of 6x6 squares (1x1 ”m2 area each) was defined using E-beam lithography in PMMA resist.
- Alpha-Helix L polyalanine (AHPA) molecules were deposited as a Self-Assembled Monolayer (SAM).
- Deposition involved dipping the patterned sample into a 1 mM Ethanol solution of AHPA for 3 hours.
-
Resist Removal and Cleaning:
- PMMA was removed by rinsing with Acetone, leaving only covalently bonded molecules in the patterned areas.
- Final rinsing was performed using Ethanol, followed by drying in Nitrogen.
-
Vectorial Magnetization Measurement (NV Microscopy):
- A custom-built NV wide-field magnetic imaging microscope was used.
- The FM sample was placed on the diamond surface, resulting in a standoff distance of ~ 10 ”m.
- A green laser (532 nm) was used for NV initialization and readout.
- Helmholtz coils generated a bias magnetic field (~ 50 G) to split the Electron Spin Resonance (ESR) spectra of the four NV orientations.
- A CW, 16-point ESR measurement per pixel allowed extraction of the 2D vectorial magnetic image (Bx, By, Bz components).
-
Time Dynamics and Structural Correlation:
- Magnetization measurements were recorded at specific time points (e.g., t = 4, 8, and 12 hours) after adsorption.
- Atomic Force Microscopy (AFM) topography measurements were performed on similar samples to track the molecular monolayer height over time, providing a direct measure of the molecular tilt angle.
- Fourier Transform Infra-Red Spectroscopy (FTIR) was also used to confirm the molecular tilt angle.
Commercial Applications
Section titled âCommercial ApplicationsâThe findings regarding persistent, chemically induced spin ordering and the use of NV centers for quantitative nanoscale magnetometry have direct implications across several high-tech sectors.
| Industry | Application/Product Relevance |
|---|---|
| Spintronics | Development of chemically controlled spin filters and spin valves. The CISS effect allows for spin polarization without external fields or complex magnetic structures. |
| Magnetic Memory (MRAM) | Potential for ultra-low power magnetization switching. Using chiral molecules to reorient or flip magnetization eliminates the need for high currents or strong external magnetic fields typically required for switching. |
| Quantum Sensing & Metrology | NV-center magnetometry is validated as a powerful tool for quantitative, vectorial imaging of magnetic phenomena at the nanoscale, crucial for characterizing novel magnetic materials and interfaces. |
| Chiral Chemistry & Separation | Fundamental understanding of the CISS mechanism aids in designing highly efficient chiral separation techniques and spin-selective chemical reactions. |
| Advanced Materials Characterization | The NV microscopy technique can be applied to study magnetic exchange coupling and coherent spin dynamics in thin films and nanostructures down to the single-spin level. |
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.