Nanoscale Vector Magnetic Sensing with Current‐Driven Stochastic Nanomagnet
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2024-01-09 |
| Journal | Advanced Electronic Materials |
| Authors | Shuai Zhang, Shihao Li, Zhe Guo, Yan Xu, Ruofan Li |
| Institutions | Hubei University, Wuhan National Laboratory for Optoelectronics |
| Citations | 5 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research demonstrates a novel, all-electric approach for nanoscale 3D vector magnetic field sensing, utilizing the stochastic switching behavior of a single nanomagnet driven by Spin-Orbit Torque (SOT).
- Core Value Proposition: Achieves nanoscale vector magnetic sensing (200 x 200 nm2) using a simple, all-electric device, eliminating the complex optical and microwave setups required by traditional Nitrogen-Vacancy (NV) center magnetometers.
- Sensing Mechanism: The device monitors the transition probability (Pup) of the nanomagnet relaxing from a SOT-excited metastable state (Mz = 0) back to a settled state (Mz = ±1). This probability is linearly modulated by the external magnetic field (Zeeman splitting).
- 3D Vector Reconstruction: By applying driving currents along the x- and y-directions (Jd,x and Jd,y) and performing addition/subtraction operations on the resulting Pup values, the contributions of the Hx, Hy, and Hz field components are successfully isolated.
- High Sensitivity: The highest measured sensitivity is 3.43% Oe-1 for the out-of-plane (Hz) component (N=500 pulse events).
- Scalability and Resolution: The device is CMOS-compatible and highly scalable. Theoretical projections suggest the minimum detectable field can be improved to less than 1 µT by increasing the number of pulse events (N ~ 106).
- Integration Potential: The technology opens avenues for integrated sensing, memory, and probabilistic computing functions within a single nanospintronic device.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Nanomagnet Size | 200 x 200 | nm2 | Spatial resolution limit |
| Film Stack (Bottom up) | Ta/CoFeB/MgO/Ta | 10/1/2/2 nm | Thicknesses of layers |
| Driving Current Density (Jd) | 12 | MA cm-2 | SOT excitation current |
| Driving Pulse Duration | 0.2 | s | Time to excite to metastable state |
| Reading Current Density (Jr) | 0.5 | MA cm-2 | Used for Anomalous Hall Effect (AHE) measurement |
| Hz Sensitivity (S) | 3.43 | % Oe-1 | Out-of-plane field response |
| Hx Sensitivity (S) | 1.02 | % Oe-1 | In-plane field response |
| Hy Sensitivity (S) | 1.09 | % Oe-1 | In-plane field response |
| Hz Linearity Error (N=500) | 4.7 | % | Deviation within the linear range |
| Hz Minimum Detectable Field (δHmin, N=500) | 0.7 | Oe | Calculated from APmax/S |
| Projected Field Resolution (N~106) | < 1 | µT | Theoretical limit based on 1/sqrt(N) scaling |
| Projected Noise Floor (Hz) | 65 | nT Hz-1/2 | Assuming 2 ns cycle time for real applications |
| Hz Linear Range | -8 to +8 | Oe | Range for accurate linear detection |
Key Methodologies
Section titled “Key Methodologies”-
Sample Preparation (Film Deposition):
- A film stack of Ta (10 nm)/CoFeB (1 nm)/MgO (2 nm)/Ta (2 nm) was deposited onto a thermally oxidized Si substrate using magnetron sputtering at room temperature.
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Device Fabrication (Patterning):
- Hall Bar Definition: Electron Beam Lithography (EBL) and Argon-Ion Milling (AIM) were used to pattern the film into Hall bar structures (1 µm width) for current application and voltage detection.
- Nanomagnet Definition: A 10 nm thick hard mask (Ti) was defined via EBL and electron beam evaporation at the center of the Hall bar. AIM was then used to etch the stack outside the dot region, resulting in a 200 x 200 nm2 nanomagnet structure.
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Electrical Measurement Setup:
- A d.c. current source (Keithley model 6221) was used for current application, and a nanovoltmeter (Keithley model 2182A) was used to measure the Hall voltage (Anomalous Hall Resistance, RAHE).
- External magnetic fields (Hx, Hy, Hz) were generated using a Helmholtz coil driven by a power supply.
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Stochastic Sensing Protocol (Pup Measurement):
- Excitation: A high driving current pulse (Jd = 12 MA cm-2, 0.2 s duration) was applied along the x- or y-direction (Jd,x or Jd,y) to drive the nanomagnet to the metastable state (Mz = 0) via SOT.
- Relaxation: The driving current was turned off, allowing the magnetization to relax stochastically to the stable states (Mz = +1 or Mz = -1).
- Reading: A small reading current (Jr = 0.5 MA cm-2) was applied to measure the RAHE and determine the final relaxed state (up or down).
- Probability Calculation: The process was repeated N=500 times for each field point to statistically determine the up-state probability (Pup = Nup/N).
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3D Field Reconstruction:
- Pup was measured under both positive and negative driving currents (+Jd and -Jd).
- Hz Isolation: P(Hz) was calculated using the addition operation: P(Hz) = [Pup(+Jd) + Pup(-Jd)] / 2.
- Hx/Hy Isolation: P(Hx,y) was calculated using the subtraction operation: P(Hx,y) = [Pup(+Jd) - Pup(-Jd)] / 2 + 0.5.
Commercial Applications
Section titled “Commercial Applications”This technology is highly relevant to fields requiring compact, high-resolution magnetic sensing and integrated spintronic functionality:
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Spintronics and Memory:
- Integration of sensing and memory functions within Magnetic Tunnel Junction (MTJ) arrays.
- Development of scalable, all-electric vector magnetometers compatible with CMOS fabrication processes.
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Probabilistic and Neuromorphic Computing:
- Utilization of the inherent stochastic switching behavior for building probabilistic bits (p-bits) and true random number generators (TRNGs).
- Integrated sensing and computing functions in advanced nanospintronic circuits.
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High-Resolution Magnetic Imaging:
- Nanoscale mapping of magnetic domains and fields in advanced materials research.
- Development of compact sensor arrays for simultaneous magnetic imaging over wide areas (e.g., using 3D stacking of MTJs).
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Medical and Biological Diagnostics:
- Detection of extremely weak magnetic fields generated by biological processes (e.g., magnetocardiography or magnetoencephalography) with high spatial localization.
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Fundamental Physics Research:
- Probing local magnetic phenomena and material properties at the nanometer scale where traditional sensors are limited by size or complexity.
View Original Abstract
Abstract Detection of vector magnetic fields at nanoscale dimensions is critical in applications ranging from basic material science and fundamental physics to information storage and medical diagnostics. So far, nanoscale vector magnetic field sensing is achieved solely by exploiting a single nitrogen‐vacancy (NV) center in a diamond, by evaluating the Zeeman splitting of NV spin qubits by using the technique of an optically‐detected magnetic resonance. This protocol requires a complex optical setup and expensive detection systems to detect the photoluminescence light, which may limit miniaturization and scalability. Here, a simple approach with all‐electric operation to sensing a vector magnetic field at 200 × 200 nm 2 dimensions is experimentally demonstrated, by monitoring a stochastic nanomagnet’s transition probability from a metastable state, excited by a driving current due to spin‐orbit torque, to a settled state.