Quantitative nanoscale vortex imaging using a cryogenic quantum magnetometer

At a Glance

Metadata	Details
Publication Date	2016-04-29
Journal	Nature Nanotechnology
Authors	Lucas Thiel, Dominik Rohner, Marc Ganzhorn, Patrick Appel, Elke Neu
Institutions	University of Tübingen, University of Basel
Citations	195
Analysis	Full AI Review Included

Technical Documentation & Analysis: Cryogenic NV Magnetometry

Research Paper Analyzed: L. Thiel et al., “Quantitative nanoscale vortex-imaging using a cryogenic quantum magnetometer” (arXiv:1511.02873v1, 2015).

Executive Summary

This research successfully demonstrates the first operation of a Nitrogen-Vacancy (NV) based scanning probe magnetometer under cryogenic conditions (4.2 K), achieving quantitative nanoscale magnetic imaging critical for condensed matter physics.

Cryogenic Operation: Achieved stable, high-resolution NV magnetometry at 4.2 K, overcoming a major limitation for studying complex electronic systems like superconductors.
Nanoscale Resolution: Utilized single NV centers embedded in all-diamond nanopillars, achieving an ultimate NV-to-sample standoff distance of approximately 10 nm.
Quantitative Results: Enabled non-invasive, quantitative mapping of magnetic stray fields from individual Pearl vortices in YBa₂Cu₃O_7-δ (YBCO).
Material Determination: Directly determined the local bulk London penetration depth (λ_L = 251 ± 14 nm), a notoriously difficult quantity to measure.
Sensor Design: The sensor relies on high-purity Single Crystal Diamond (SCD) material, requiring precise NV implantation (10 ± 8 nm depth) and ultra-smooth surface finishing (sample Ra = 1.5 nm).
Future Potential: Establishes NV magnetometry as a powerful tool for exploring pseudogap phases, inhomogeneous superconductivity, and strongly correlated electron systems.

Technical Specifications

Parameter	Value	Unit	Context
Base Operating Temperature	4.2	K	Liquid ⁴He bath cryostat
NV-to-Sample Standoff Distance (z_NV)	~10	nm	Ultimate imaging resolution
NV Implantation Energy	6	keV	Corresponds to implantation depth
NV Implantation Depth	10 ± 8	nm	Required for close proximity sensing
Typical Magnetic Field Sensitivity	11.9	µT/√Hz	Direct ESR measurement
Enhanced Magnetic Field Sensitivity	nT/√Hz	range	Coherent spin manipulation
YBCO Film Thickness (Sample A)	100	nm	Superconducting layer
Capping Layer Thickness (Sample A)	60	nm	Gold (Au) protective/contact layer
Determined London Penetration Depth (λ_L)	251 ± 14	nm	Quantitative result from Pearl-vortex fit
Sample Surface Roughness (Ra)	1.5	nm	Measured via AFM (Sample A)
NV Excitation Wavelength	532	nm	Solid-state laser (Green)
Maximum Applied Magnetic Field	0.5	T	3D vector magnet capability

Key Methodologies

The experiment combined advanced material fabrication with a specialized cryogenic scanning probe setup.

Sample Fabrication (YBCO):
- Epitaxial c-axis oriented YBa₂Cu₃O_7-δ (YBCO) thin films were grown on SrTiO₃ (STO) single crystal (001) substrates via Pulsed Laser Deposition (PLD).
- Samples were capped either with in-situ electron-beam-evaporated Gold (Au, 60 nm) or epitaxially grown STO (20 nm) for protection.
NV Sensor Fabrication:
- Single NV centers were created in an all-diamond scanning probe (nanopillar on a cantilever).
- NV centers were implanted at 6 keV, targeting a shallow depth (10 ± 8 nm) for optimal proximity to the sample surface.
Cryogenic Setup:
- A combined confocal and Atomic Force Microscope (AFM) was operated in a ⁴He bath cryostat at 4.2 K.
- The system included a 3D vector magnet (0.5 T) and a cryogenic objective (0.82 NA) for optical access.
NV Magnetometry:
- NV spins were optically excited using a 532 nm laser.
- Microwave (MW) signals were delivered via a 25 µm gold wire positioned across the sample to drive Electron Spin Resonance (ESR).
- Magnetic field mapping (B_NV) was achieved by measuring the Zeeman-splitting in the optically detected ESR at each pixel.
Quantitative Analysis:
- High-resolution line-scans of the vortex stray field were performed.
- Data was fitted to Pearl’s analytic model for thin-film superconductors, allowing for the independent determination of the London penetration depth (λ_L) and the NV-to-vortex standoff distance (h_NV).

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced MPCVD diamond materials and customization services required to replicate and extend this groundbreaking cryogenic NV magnetometry research.

Applicable Materials

To achieve the high sensitivity and nanoscale resolution demonstrated, the NV sensor requires ultra-low strain, high-purity SCD diamond.

Research Requirement	6CCVD Material Solution	Key Benefit
High-Purity Substrate	Optical Grade Single Crystal Diamond (SCD)	Ultra-low nitrogen content and strain essential for long NV coherence times (T₂) and high magnetic sensitivity.
Shallow NV Implantation	SCD Wafers (Custom Thickness)	SCD material supplied with precise orientation control, optimized for subsequent low-energy ion implantation (6 keV) and reactive ion etching (RIE) to form nanopillars.
Surface Quality	SCD Polishing Service	Achieves surface roughness Ra < 1 nm, critical for maintaining the required ~10 nm NV-to-sample standoff distance in AFM contact mode.

Customization Potential

The complexity of the NV scanning probe and the YBCO sample structure necessitates highly customized material solutions, which are a core strength of 6CCVD.

Custom Dimensions and Geometry: 6CCVD provides custom laser cutting services to produce SCD plates and wafers in the precise dimensions required for integration into specialized AFM cantilevers and cryogenic microscope heads. We supply plates/wafers up to 125 mm (PCD) and substrates up to 10 mm thick.
Integrated Metalization: The research utilized a 60 nm Au capping layer on Sample A and a gold wire for MW delivery. 6CCVD offers internal metalization capabilities (Au, Pt, Pd, Ti, W, Cu) on both SCD and PCD substrates, allowing researchers to receive pre-patterned diamond components for integrated microwave delivery or sample protection layers.
Thickness Control: We offer precise thickness control for SCD (0.1 µm - 500 µm) and PCD (0.1 µm - 500 µm), ensuring the starting material meets the exact specifications for subsequent nanofabrication steps (e.g., RIE for nanopillar formation).
Boron-Doped Diamond (BDD): For future extensions of this work requiring integrated conductive elements or electrochemical sensing, 6CCVD supplies Boron-Doped Diamond (BDD) films with tunable conductivity.

Engineering Support

The successful implementation of cryogenic NV magnetometry relies heavily on material quality and precise integration. 6CCVD’s in-house PhD team specializes in diamond material science and can assist researchers with:

Material Selection: Guidance on selecting the optimal SCD grade (purity, orientation, and strain) to maximize NV center performance (T₂ and T₁).
Post-Processing Optimization: Consultation on surface preparation and polishing techniques necessary to achieve the ultra-smooth surfaces (Ra < 1 nm) required for high-resolution scanning probe applications.
Cryogenic Integration: Support for material specifications suitable for high-vacuum and ultra-low temperature environments.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

Tech Support

Original Source

DOI: https://doi.org/10.1038/nnano.2016.63