Pulsed magnetic field gradient on a tip for nanoscale imaging of spins

At a Glance

Metadata	Details
Publication Date	2025-03-10
Journal	Communications Physics
Authors	Leora Schein-Lubomirsky, Yarden Mazor, Rainer Stöhr, Andrej Denisenko, Amit Finkler
Institutions	University of Stuttgart, Tel Aviv University
Analysis	Full AI Review Included

Executive Summary

This research introduces a novel scanning probe device featuring a pulsed, localized magnetic field gradient, significantly advancing Nanoscale Magnetic Resonance Imaging (nanoMRI) capabilities using Nitrogen-Vacancy (NV) centers in diamond.

Core Innovation: Developed a current-focusing metallic microwire on a quartz tip, providing a switchable magnetic field gradient localized at the nanoscale.
Gradient Performance: Achieved a maximum measured gradient of 0.95 µT nm^-1 at 1.54 mA, with theoretical potential for up to 3.3 µT nm^-1 (5-fold increase) using higher currents.
Temporal Control: Demonstrated rapid current switching with a measured rise/fall time of 600 ns, enabling advanced pulsed gradient sequences (e.g., Fourier imaging).
Resolution: Enables electron spin mapping with a projected 1 nm spatial resolution, overcoming limitations imposed by spectral linewidths (T₂* ~ 3 µs).
Rabi Power Modulation: The metallic tip geometry locally enhances the NV Rabi driving power by a factor of x3.5, providing an intrinsic spatial gradient source for selective spin manipulation.
Contrast Improvement: Operates at low magnetic fields (below 200 µT), avoiding detrimental off-axis fields that typically reduce NV contrast in systems relying on permanent magnets or ferromagnets.

Technical Specifications

Parameter	Value	Unit	Context
Maximum Measured Gradient	0.95	µT nm^-1	Measured at 1.54 mA current.
Maximum Simulated Gradient (Target)	2.78	µT nm^-1	Calculated for 2 mA current.
Potential Gradient (High Current)	3.3	µT nm^-1	Estimated 5-fold increase over measured value.
Spatial Resolution (Target)	1	nm	Required gradient for this resolution: 3.57 µT nm^-1.
Maximum Measured Field (DC)	259	µT	Field projected along the NV [111] axis.
Maximum Field (Simulation)	0.37	mT	At 300 nm stand-off distance.
Current Switching Time (Rise/Fall)	600	ns	Measured time for the current-controlled gradient.
Operating Current (Measurement)	1.54	mA	Used for gradient mapping experiments.
NV Dephasing Time (T₂*)	~3	µs	Limits the spectral resolution (Δν ~ 100 kHz).
Rabi Power Enhancement	x3.5	Factor	Modulation induced by the metallic tip geometry.
NV Implantation Energy	5	keV	For ¹⁵N⁺ implantation.
Annealing Temperature	950	°C	Vacuum annealing post-implantation.
Diamond Membrane Thickness	30	µm	CVD diamond substrate thickness.
Tip Apex Diameter (Design)	~1	µm	Optimized for gradient strength and heat dissipation.
Tip Deposition Thickness (Au)	120	nm	Final gold deposition layer at the apex.

Key Methodologies

NV Sensor Fabrication:
- Used e6 [100] electronic-grade CVD diamond, initially doped with a solid-state boron rod.
- NV centers created via ¹⁵N⁺ implantation at 5 keV, followed by vacuum annealing at 950 °C.
- Diamond thinned to a 30 µm membrane, and nanopillars were etched using electron beam lithography and Inductively Coupled Plasma (ICP).
Tip Fabrication (Current Focusing Device):
- Substrate: 75 mm quartz solid rod with grooves, pulled using a Sutter P-2000 to form tapers (~5 mm length).
- Initial Deposition: Thermal deposition of 2 nm Cr (adhesion) followed by 10 nm Au to form the long electrical leads.
- Apex Shaping: Focused Gallium Ion Beam (FIB) used to cut and shape the apex, achieving a 1 µm diameter for current focusing.
- Final Deposition: Second metal deposition applied 7 nm Cr and 120 nm Au to the apex and leads to ensure robust electrical contact.
Experimental Setup and Control:
- Optical Setup: Home-built confocal microscope using a 520 nm laser for NV initialization and readout.
- Positioning: Tip placed on a piezoelectric stage; lateral position controlled via stick-slip motion.
- Vertical Control: Atomic Force Microscope (AFM) feedback implemented using a quartz tuning fork (f₀ = 32.768 kHz) for precise vertical distance stabilization (fixed distance sensing).
- RF/Current Control: Microwave signal generated by R&S SGT100A and modulated by a Spectrum Instrumentation AWG, amplified by a Minicircuits ZHL-16W-43-S+ amplifier before injection into the co-planar waveguide.
Magnetic Field Characterization:
- Optically Detected Magnetic Resonance (ODMR) pulse sequence was used, interleaved with the current pulse (1.54 mA) through the tip.
- Reference measurements were alternated with and without current to cancel slow system drifts.
- Gradient calculation derived from the shift in the resonant dip center measured with and without the applied current.

Commercial Applications

The pulsed magnetic gradient tip technology is highly relevant for advanced research and commercial applications in quantum technology and materials science:

Quantum Computing and Sensing: Enables targeted, selective spin manipulation (using the Rabi power gradient) and high-fidelity readout protocols necessary for solid-state quantum processors based on NV centers.
Molecular Structure Analysis (NanoMRI): Provides the necessary spatial resolution (1 nm) and control for mapping individual electron spins in complex molecular structures, such as molecular rulers or biological macromolecules.
Advanced Materials Characterization: Useful for active and passive magnetic characterization of novel materials, including:
- Antiferromagnets and ferromagnets.
- Superconductors (especially when adapted for cryogenic operation).
- Exotic magnetic phases of matter.
High-Resolution Spectroscopy: The switchable gradient allows for unique pulse sequences (e.g., Fourier imaging, gradient pulse sequences) that enhance spectral separation and improve signal-to-noise ratio in nanoscale NMR/ESR spectroscopy.
Thermal Management Solutions: The demonstrated ability to increase Rabi driving power without increasing external MW power reduces thermal load on the sample, critical for sensitive measurements and cryogenic environments.

View Original Abstract

Abstract Nanoscale magnetic resonance imaging (nanoMRI) is crucial for advancing molecular-level structural analysis, yet existing techniques relying on permanent magnets face limitations in controllability and resolution. This study addresses the gap by introducing a switchable magnetic field gradient on a scanning tip, enabling localized, high-gradient magnetic fields at the nanoscale. Here, we demonstrate a device combining a metal microwire on a quartz tip with a nitrogen-vacancy (NV) center in diamond, achieving gradients up to 1 μT nm−1 at fields below 200 μT. This allows electron spin mapping with 1 nm resolution, overcoming challenges like emitter contrast and sample preparation rigidity. The current-controlled gradient, switchable in 600 ns, enhances precision and flexibility. Additionally, the metallic tip modifies Rabi power spatially, enabling selective spin manipulation with varying microwave effects. This innovation paves the way for advanced nanoMRI applications, including high-resolution imaging and targeted spin control in quantum sensing and molecular studies.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s42005-025-02019-y