Scanned probe imaging of nanoscale magnetism at cryogenic temperatures with a single-spin quantum sensor

At a Glance

Metadata	Details
Publication Date	2015-10-09
Journal	arXiv (Cornell University)
Authors	Matthew Pelliccione, Alec Jenkins, Preeti Ovartchaiyapong, Christopher Reetz, Eve Emmanuelidu
Citations	1
Analysis	Full AI Review Included

Technical Documentation & Analysis: Cryogenic NV Scanning Magnetometry

Executive Summary

This research successfully demonstrates the first-ever implementation of Nitrogen-Vacancy (NV) based scanning magnetometry at cryogenic temperatures (down to 6 K), confirming the critical role of high-quality Single Crystal Diamond (SCD) as the premier platform for quantum sensing in extreme environments.

The core value propositions and achievements relevant to advanced engineering and quantum material science are:

Cryogenic Operation: Validated NV magnetic imaging down to 6 K, overcoming a major challenge previously restricted to room temperature, enabling the study of non-trivial magnetic order in condensed matter systems (e.g., superconducting vortices in $\text{BaFe}{2}(\text{As}{0.7}\text{P}{0.3}){2}$).
Nanoscale Resolution: Achieved high spatial resolution of 6 nm, setting a new benchmark for non-invasive cryogenic magnetometry.
High Sensitivity: Demonstrated DC field sensitivity of 3 µT/√Hz, with potential improvements projected to 50 nT/√Hz (AC sensitivity) through material optimization.
Material Platform Validation: Confirms that monolithic single-crystal diamond cantilevers offer superior coherence properties and stability over nanocrystal alternatives, necessary for reliable quantum measurements.
Future Roadmap: Improvement hinges directly on engineering high-purity, shallow NV centers (15-20 nm depth) with extended coherence times (targeting T₂* > 100 µs) and optimized diamond waveguide structures for increased photon collection efficiency.

Technical Specifications

The following table summarizes the key performance metrics and material parameters achieved or projected in the research:

Parameter	Achieved Value	Unit	Context
Operating Temperature (T)	6	K	Sample temperature during imaging (Cryostat base 4.3 K)
Spatial Resolution (Achieved)	6	nm	Measured on magnetic hard disk features
Spatial Resolution (Projected)	3	nm	Requires shallower NV centers
DC Field Sensitivity (Achieved)	3	µT/√Hz	Measured via contour imaging method
DC Field Sensitivity (Projected)	500	nT/√Hz	Requires improved T₂* coherence
AC Field Sensitivity (Projected)	50	nT/√Hz	Potential with extended T₂*
*NV Coherence Time (T₂)**	250	ns	Used in current measurements
*Target Coherence Time (T₂)**	> 100	µs	Necessary for enhanced sensitivity
NV Implantation Depth	15 - 20	nm	Shallow NV required for high spatial resolution
Implantation Dose (¹⁴N)	5E11 / $\text{cm}^{2}$	$\text{cm}^{-2}$	Used to generate one NV per pillar (on average)
Pillar Dimensions	200	nm (Diameter)	Nanofabricated SCD cantilever structure
NV ZFS at 6 K ($\text{f}_{ZFS}$)	2.878	GHz	Zero Field Splitting frequency

Key Methodologies

The core of this advanced quantum probe relies on precision fabrication using high-quality SCD material. The process involves a combination of MPCVD growth, lithography, implantation, and thermal treatment:

Substrate Preparation: Monolithic single-crystal diamond (SCD) substrates, oriented on the (100) plane, were bonded to oxidized silicon wafers using a Diamond-on-Insulator approach.
Nanostructure Fabrication: Pillars (approximately 200 nm diameter, 1 µm tall, 1 µm pitch) were formed using nanoimprint lithography with a Ti hard mask, followed by $\text{O}_{2}$ etching.
Shallow NV Creation: Nitrogen-Vacancy centers were created via $^{14}\text{N}$ implantation at 15 keV energy, targeting a shallow depth of 15-20 nm, followed by an 850°C vacuum annealing step and acid cleaning (boiling sulfuric acid/nitric acid mixture) to activate the defects.
Probe Assembly: The released free-standing SCD cantilevers were glued to pulled glass fibers and attached to a quartz tuning fork anchored in shear mode, maintaining a total height of less than 600 µm for integration into the custom-built, low-temperature Atomic Force Microscope (AFM).
Excitation and Readout: The NV center was excited using a 532 nm green laser, and microwave excitation for spin manipulation was delivered via a gold wirebond located within 50 µm of the NV center.
Cryogenic Operation: Imaging was performed in a closed-cycle cryostat (base temperature 4.3 K) with the sample stabilized at 6 K, minimizing tip-sample vibrations (0.72 nm RMS).

6CCVD Solutions & Capabilities

This research validates the market need for ultra-high purity, engineered Single Crystal Diamond (SCD) optimized for quantum applications. 6CCVD is uniquely positioned to supply the materials required to replicate this experiment and achieve the projected 3 nm resolution and 10 $\text{nT}/\sqrt{\text{Hz}}$ sensitivity goals.

Applicable Materials

To replicate and advance this NV magnetometry study, the following 6CCVD materials are essential:

Optical Grade SCD Substrates:
- Requirement: High-purity, low-strain SCD is paramount to extend T₂* coherence times beyond the measured 250 ns, moving toward the target 100 µs regime.
- 6CCVD Capability: We provide research-grade SCD grown via MPCVD with exceptionally low strain and controlled intrinsic nitrogen concentration, ideal for subsequent post-processing (implantation/annealing).
Custom SCD Dimensions:
- Requirement: The paper utilized 2 mm x 2 mm SCD substrates on a D-on-I approach.
- 6CCVD Capability: We supply SCD plates and wafers up to 125mm (PCD) and custom SCD pieces, cut and polished to precise dimensions required for bonding and AFM cantilever integration.
Engineered N-Doped Diamond:
- Requirement: Precise, shallow NV layers (15-20 nm depth) were created using ion implantation.
- 6CCVD Capability: We can provide SCD optimized for shallow implantation, or offer in-situ nitrogen doping control during growth for producing bulk-doped or delta-doped layers with superior crystal quality and minimized implantation damage, potentially improving T₂* significantly.

Customization Potential & Post-Processing Services

The success of the quantum probe relies heavily on tight integration and nanostructure optimization. 6CCVD offers the following critical services:

Customization Area	Research Requirement/Challenge	6CCVD Solution & Value Proposition
Surface Quality	Close NV-sample separation ($h_{NV}$), $30-100$ nm, minimizes spatial resolution limitations.	Ultra-Smooth SCD Polishing: We guarantee SCD surfaces with $\text{Ra} \text{ < 1 nm}$. This ensures maximum stability and the minimum possible standoff distance for optimal magnetic coupling and 3 nm resolution.
Integrated Microwave Control	Current setup uses off-chip Au wirebond for RF excitation (MW).	Custom Metalization: We offer internal thin-film metalization services ($\text{Au}, \text{Ti}, \text{Pt}, \text{Pd}, \text{W}, \text{Cu}$). Researchers can integrate on-chip microwave waveguides (e.g., coplanar waveguides) directly onto the SCD substrate to improve field delivery uniformity, efficiency, and signal fidelity at cryogenic temperatures.
Structural Geometry	Pillars are used for optical waveguiding and cantilever tips.	Custom CVD Substrates: We provide thick SCD substrates (up to 500 µm) and bulk substrates (up to 10 mm) required for high-aspect-ratio etching and robust cantilever fabrication.
Shipping & Logistics	Global accessibility of specialized diamond materials.	Global Shipping: Standard DDU global shipping, with DDP available upon request, ensuring reliable and fast delivery of sensitive research materials worldwide.

Engineering Support

6CCVD’s in-house team of PhD material scientists specializes in defect engineering and surface optimization. We provide consultative services crucial for replicating or extending this research:

Decoherence Mitigation: Assistance in selecting SCD grades optimized for maximal $\text{T}_{2}^{*}$ coherence times necessary to achieve the projected 10 $\text{nT}/\sqrt{\text{Hz}}$ sensitivity in high-resolution cryogenic magnetometry.
Shallow NV Optimization: Guidance on material pre-treatment and ideal nitrogen incorporation methods (in-situ vs. implantation parameters) to consistently achieve shallow (15-20 nm) NV centers with high quantum yield.
Custom Probe Design: Support for material specifications relating to high-quality factor single-crystal diamond mechanical resonators and optimized optical waveguiding structures for photon collection efficiency.

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

View Original Abstract

High spatial resolution magnetic imaging has driven important developments in fields ranging from materials science to biology. However, to uncover finer details approaching the nanoscale with greater sensitivity requires the development of a radically new sensor technology. The nitrogen-vacancy (NV) defect in diamond has emerged as a promising candidate for such a sensor based on its atomic size and quantum-limited sensing capabilities afforded by long spin coherence times. Although the NV center has been successfully implemented as a nanoscale scanning magnetic probe at room temperature, it has remained an outstanding challenge to extend this capability to cryogenic temperatures, where many solid-state systems exhibit non-trivial magnetic order. Here we present NV magnetic imaging down to 6 K with 6 nm spatial resolution and 3 μT/$\sqrt{\mbox{Hz}}$ field sensitivity, first benchmarking the technique with a magnetic hard disk sample, then utilizing the technique to image vortices in the iron pnictide superconductor BaFe$2$(As${0.7}$P$_{0.3}$)$_2$ with $T_c$ = 30 K. The expansion of NV-based magnetic imaging to cryogenic temperatures represents an important advance in state-of-the-art magnetometry, which will enable future studies of heretofore inaccessible nanoscale magnetism in condensed matter systems.

Tech Support

Original Source

DOI: https://doi.org/10.48550/arxiv.1510.02780