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6CCVD Research

Scanning nitrogen-vacancy magnetometry down to 350 mK

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2022-05-30
JournalApplied Physics Letters
AuthorsPatrick Scheidegger, S. Diesch, Marius L. Palm, Christian L. Degen
InstitutionsETH Zurich
Citations25
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”
  • Record Low Temperature: Demonstrated robust scanning Nitrogen-Vacancy (NV) magnetometry in a dry dilution refrigerator, achieving an operational base temperature of 350 mK.
  • High Sensitivity: Achieved a magnetic field sensitivity of 3 µT/sqrt(Hz) (best case) by employing pulsed Optically Detected Magnetic Resonance (ODMR).
  • NV Center Stability: NV spin and optical properties (Photoluminescence contrast, C) remained stable and constant (within 0.5%) across the entire experimental range (0.35 K to 3 K).
  • Heating Mitigation: Pulsed ODMR combined with efficient Co-Planar Waveguide (CPW) microwave delivery successfully minimized laser- and MW-induced local heating, allowing the sample temperature (Tsample) to closely match the stage temperature (TRuO2) under low-power conditions.
  • Vortex Imaging: Successfully imaged superconducting vortices in 50-nm-thin aluminum micro-discs (Tc ≈ 1.25 K), confirming the feasibility of non-invasive magnetic imaging in the sub-Kelvin regime.
  • Technical Challenges: The primary limitations identified were experimental heat load, thermalization of the probe, and spurious mechanical vibrations (estimated at tens of nanometers) originating from the dry pulse-tube cooling circuit.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Minimum Stage Temperature (TRuO2)350mKOperational base temperature for scanning.
Mixing Chamber Base Temperature~25mKBase temperature without experimental heat load.
Best Magnetic Sensitivity3µT/sqrt(Hz)Achieved via line scans at 1 mT bias field.
Pixel-by-Pixel Sensitivity14µT/sqrt(Hz)Measured at 6 mT bias field (1-second integration).
NV Center Stability Range0.35 - 3KTemperature range showing stable spin/optical properties.
PL Contrast Variation< 0.5%Variation across the 0.35 K to 1.4 K range.
Aluminum Film Thickness (Discs)50nmSuperconducting test structures.
Aluminum CPW Thickness150nmMicrowave Co-Planar Waveguide structure.
Aluminum Critical Temperature (Tc)~1.25KSuperconducting transition temperature for 50 nm film.
NV-to-Sample Distanceca. 110nmWorking distance for contact mode scanning.
NV Implantation Energy7keVLow-energy ion implantation for shallow NV centers.
Estimated Vibration AmplitudeTens ofnmDue to rigid suspension in dry pulse-tube cryostat.
MW Input Power (High Setting)300µWNominal input power (time-averaged, accounting for 5 dB attenuation).
Laser Power (High Setting)140µWMeasured at room temperature in front of the objective (time-averaged).

Key Methodologies

Section titled “Key Methodologies”
  1. Cryogenic Integration: The experiment utilized a combined Atomic Force/Confocal Microscope (AFM/CFM) rigidly suspended within the cold insert of a CF-CS110 dry dilution refrigerator. A rigid suspension was chosen over spring-suspension to maintain high beam-pointing stability despite pulse-tube vibrations.
  2. Thermal Monitoring and Control: The sample holder included a resistive heater and a calibrated ruthenium oxide (RuO2) thermometer (TRuO2). Local heating (Tsample) was indirectly characterized by monitoring the peak magnetic field (ΔBmax) near the aluminum critical temperature (Tc).
  3. NV Probe Fabrication: Scanning tips were monolithic blocks of single-crystal diamond ({100} surface orientation). NV centers were created using 7 keV low-energy ion implantation followed by dry etching steps.
  4. Microwave (MW) Delivery: MW signals were guided through semi-rigid coaxial lines (including superconducting NbTi-NbTi sections) to an impedance-matched, tapered Co-Planar Waveguide (CPW) lithographically patterned from 150 nm thick aluminum on a sapphire substrate.
  5. Pulsed ODMR Detection: Pulsed optically detected magnetic resonance was employed to manipulate the NV spin. This scheme reduces the duty cycle of laser and MW irradiation, minimizing heating and decreasing the spin resonance linewidth.
  6. Sample Preparation: Test structures consisted of 50-nm-thick aluminum micro-discs (1-5 µm diameter) patterned within the gaps of the CPW on a sapphire substrate via e-beam evaporation (0.4 nm/s rate).
  7. Magnetic Field Mapping: Scanning was performed in contact mode. The local stray field (ΔB) was derived from the frequency shift of the NV resonance, which is proportional to the magnetic field component along the NV symmetry axis (~55° angle relative to the out-of-plane direction).

Commercial Applications

Section titled “Commercial Applications”
  • Quantum Materials and 2D Physics: Enabling non-invasive magnetic field imaging in novel quantum materials (e.g., twisted bilayer graphene, oxide interfaces) where critical phenomena occur at temperatures below 1 K.
  • Cryogenic Electronics and Transport: Spatial mapping of nanoscale electronic transport, persistent ring currents, edge currents, and hydrodynamic electron flow in mesoscopic conductors at ultra-low temperatures.
  • Superconductivity Research: Detailed characterization of Type II superconductors, including the visualization of vortex lattices, vortex pinning mechanisms, and the determination of local critical temperatures (Tc) and critical fields (Bc2).
  • Advanced Scanning Probe Microscopy (SPM): Development and commercialization of next-generation, high-resolution, cryogenic AFM/CFM systems incorporating NV magnetometry for multi-parameter material analysis in dry cryostats.
  • Quantum Sensing Technology: Advancing the technical readiness level of NV-based quantum sensors for operation in complex, high-vibration, low-temperature environments, paving the way for improved sensitivity (e.g., through AC magnetometry or gradiometry).
View Original Abstract

We report on the implementation of a scanning nitrogen-vacancy (NV) magnetometer in a dry dilution refrigerator. Using pulsed optically detected magnetic resonance combined with efficient microwave delivery through a co-planar waveguide, we reach a base temperature of 350 mK, limited by experimental heat load and thermalization of the probe. We demonstrate scanning NV magnetometry by imaging superconducting vortices in a 50-nm-thin aluminum microstructure. The sensitivity of our measurements is approximately 3 μT per square root Hz. Our work demonstrates the feasibility for performing noninvasive magnetic field imaging with scanning NV centers at sub-Kelvin temperatures.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2022 - Nanoscale magnetic field imaging for 2D materials [Crossref]
  2. 2008 - Scanning magnetic field microscope with a diamond single-spin sensor [Crossref]
  3. 2012 - A robust scanning diamond sensor for nanoscale imaging with single nitrogen-vacancy centres [Crossref]
  4. 2017 - Real-space imaging of non-collinear antiferromagnetic order with a single-spin magnetometer [Crossref]
  5. 2019 - Nanomagnetism of magnetoelectric granular thin-film antiferromagnets [Crossref]
  6. 2021 - Coexistence of Bloch and Neél walls in a collinear antiferromagnet [Crossref]
  7. 2021 - Characterization of room-temperature in-plane magnetization in thin flakes of CrTe2 with a single-spin magnetometer [Crossref]
  8. 2017 - Nanoscale imaging of current density with a single-spin magnetometer [Crossref]
  9. 2017 - Quantum imaging of current flow in graphene [Crossref]
  10. 2020 - Imaging viscous flow of the Dirac fluid in graphene [Crossref]

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