Current Flow Mapping in Conducting Ferroelectric Domain Walls Using Scanning NV‐Magnetometry
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2025-06-26 |
| Journal | Advanced Electronic Materials |
| Authors | Conor J. McCluskey, James Dalzell, Amit Kumar, J. M. Gregg |
| Institutions | Queen’s University Belfast |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research utilizes Scanning Nitrogen Vacancy (NV) Center Magnetometry to directly map current flow in conducting ferroelectric Lithium Niobate (LNO) domain wall (DW) memristors, yielding critical insights into charge transport mechanisms.
- Direct Current Mapping: The Oersted magnetic field generated by current flow was mapped in situ using NV-magnetometry, allowing for the unique reconstruction of 2D current density vectors via Biot-Savart law inversion.
- Severe Current Channeling: The current density maps revealed that current flow is highly non-uniform and channeled. Only a strikingly small subset of the total domain wall network is strongly active in conduction, contradicting assumptions of uniform parallel conduction.
- Microstructure Correlation: Current channeling pathways were found to directly correlate with specific, highly dense regions of the domain wall microstructure near the electrode inlet.
- Carrier Density Correction: The finding of localized conduction forces a two-order-of-magnitude correction to the estimated 2D carrier density (n2D), raising the value from 3 x 105 cm-2 (initial estimate) to 1.9 x 107 cm-2 (corrected estimate).
- Transport Regime: The combination of high carrier mobility (3700 cm2V-1s-1) and extremely low corrected carrier density suggests that transport within the conducting domain walls is indicative of semiconducting behavior, rather than metallic 2DEG behavior.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Ferroelectric Material | Lithium Niobate (LNO) | N/A | Ion-sliced, z-cut thin film |
| Film Thickness | ≈500 | nm | Dielectric layer thickness |
| Electrode Material (Top/Bottom) | Pt / Au-Cr | N/A | Parallel plate capacitor structure |
| Switching Voltage (Vc) | ≈26 | V | Voltage required to inject conducting DWs |
| Conductance Change | ≈8 | orders of magnitude | Measured at 10 V, room temperature |
| Read Current (I) | ≈132 | µA | Measured at 10 V after switching |
| NV Mapping Current (INV) | ≈400 | µA | Constant current supplied during NV scan |
| Assumed DW Thickness (tDW) | 1 | nm | Used for area calculation |
| Initial DW Fractional Coverage (fDW) | ≈0.24 | N/A | Estimated from PFM amplitude map |
| Corrected Active DW Fraction (f’DW) | ≈0.22 | N/A | Based on NV current channeling map |
| Assumed Electron Mobility (µ) | 3700 | cm2V-1s-1 | Based on geometric magnetoresistance |
| Initial 2D Carrier Density (n2D) | 3 x 105 | cm-2 | Calculated assuming uniform conduction |
| Corrected 2D Carrier Density (n’2D) | 1.9 x 107 | cm-2 | Corrected based on current channeling |
| Theoretical Screening Density (nscreen) | 1.8 x 1014 | cm-2 | Expected from full polar discontinuity screening |
| Driving Electric Field (E) | 200 | kVcm-1 | Applied voltage (10 V) divided by film thickness (500 nm) |
Key Methodologies
Section titled “Key Methodologies”- Capacitor Fabrication and Poling: Square Pt electrodes (≈110 µm) were sputtered onto 500 nm thick ion-sliced z-cut LNO films. A triangular voltage pulse (up to 60 V) was applied via a tungsten probe to partially reverse polarization and inject 180° conducting domain walls, creating the memristor state.
- AFM Microstructural Characterization:
- PFM (Piezoresponse Force Microscopy): Used to map the complex, circular domain microstructure underneath the top electrode and estimate the initial fractional coverage of domain walls (fDW).
- cAFM (Conductive AFM): Performed after AFM micromachining removed the top electrode, confirming that regions of enhanced conductivity spatially correlate with DW locations.
- KPFM (Kelvin Probe Force Microscopy): Used in situ on a series-connected electrode structure to verify that the active capacitor electrode remained equipotential (1 V bias), suggesting a uniform driving field across the DW network.
- Scanning NV-Magnetometry: Measurements were performed in situ using a commercial scanning NV magnetometer (ProteusQ). A constant current (≈400 µA) was supplied to the capacitor structure. The magnetic field projection along the NV-axis was measured at each point using optical readout of the NV spin state.
- Current Density Reconstruction: The measured Oersted field map was inverted using the Biot-Savart law (implemented in MATLAB) to uniquely reconstruct the 2D current density vector field, revealing the localized current channeling.
- Finite Element Modeling (FEM): COMSOL Multiphysics was used to model current flow through the capacitor structure, comparing the results of a few localized conducting channels versus a distributed network, confirming that the experimental data aligns with the localized channeling scenario.
Commercial Applications
Section titled “Commercial Applications”- Neuromorphic and In-Memory Computing: The active control over conducting DW pathways and the resulting memristive behavior are foundational for developing high-density, low-power neuromorphic elements and artificial synapses.
- Reconfigurable Electronics: The ability to create, destroy, and move conducting pathways on demand allows for the development of reconfigurable logic gates, rectifiers, and transistors based on ferroelectric domain wall control.
- Non-Volatile Ferroelectric Memory: DW conductivity provides a mechanism for multi-level memory storage, offering high density and fast switching speeds compared to conventional charge-based memory.
- Advanced 2D Material Transport Studies: NV-magnetometry provides a crucial non-invasive tool for mapping current density in complex subsurface interfaces, essential for validating transport models in other high-mobility, low-carrier-density 2D systems.
- High-Mobility Semiconductor Interfaces: The observed high mobility (3700 cm2V-1s-1) in the DWs suggests potential for utilizing these ferroelectric interfaces as high-quality 2D semiconducting channels in specialized electronic devices.
View Original Abstract
Abstract The electrical conductivity of parallel plate capacitors, with ferroelectric lithium niobate as the dielectric layer, can be extensively and progressively modified by the controlled injection of conducting domain walls. Domain wall‐based memristor devices result. Microstructures, developed as a result of partial switching, are complex, and so simple models of equivalent circuits, based on the collective action of all conducting domain wall channels acting identically and in parallel, may not be appropriate. Here, the current density in ferroelectric domain wall memristors is directly mapped in situ by mapping Oersted fields, using nitrogen vacancy center microscopy. Current density maps are found to directly correlate with the domain microstructure, revealing that a strikingly small fraction of the total domain wall network is responsible for the majority of the current flow. This insight forces a two order of magnitude correction to the carrier densities, previously inferred from standard scanning probe or macroscopic electrical characterization.