Imaging Topological Spin Structures Using Light-Polarization and Magnetic Microscopy

At a Glance

Metadata	Details
Publication Date	2021-02-17
Journal	Physical Review Applied
Authors	Till Lenz, Georgios Chatzidrosos, Zhiyuan Wang, Lykourgos Bougas, Yannick Dumeige
Institutions	Centre National de la Recherche Scientifique, Helmholtz Institute Mainz
Citations	25
Analysis	Full AI Review Included

Technical Documentation & Analysis: Concurrent MOKE/NV-Based Magnetic Microscopy

This document analyzes the requirements and achievements detailed in the research paper “Probing topological spin structures using light-polarization and magnetic microscopy” and maps them directly to the advanced MPCVD diamond solutions offered by 6CCVD.

Executive Summary

The research presents a novel wide-field imaging platform combining ensemble Nitrogen-Vacancy (NV) magnetometry with Magneto-Optic Kerr Effect (MOKE) microscopy for concurrent, non-perturbative analysis of magnetic structures.

Dual Modality Imaging: Achieved simultaneous detection of sample magnetization (MOKE) and resulting stray magnetic fields (NV-based ODMR).
Diamond Sensor Requirements: Utilized a high-quality, near-surface diamond sensor featuring a 100 nm thick ¹⁴N doped, 99.9% ¹²C purified layer grown by Chemical Vapor Deposition (CVD).
Key Application: Successful imaging and characterization of magnetic stripe domains in multilayered ferromagnetic thin films (Ta/CoFeB/MgO stacks).
High Sensitivity: Demonstrated magnetometric sensitivity (dBNV) of approximately 2 µT µm/√Hz and polarimetric sensitivity (δΘκ) of 50 prad µm/√Hz.
Operational Versatility: The instrument operates over a broad temperature range and is magnetically non-perturbative, ideal for studying novel magnetic structures and their dynamics.
Future Resolution Improvement: Resolution is currently limited by the sensor-sample offset distance (~2 µm), highlighting the need for ultra-flat, high-purity diamond surfaces suitable for direct thin-film deposition.

Technical Specifications

The following hard data points were extracted from the experimental section of the paper, detailing the material properties and performance metrics of the concurrent imaging setup.

Parameter	Value	Unit	Context
Diamond NV Layer Thickness	100	nm	¹⁴N doped, 99.9% ¹²C purified CVD layer
Carbon Isotope Purity	99.9	%	Isotope purification level in the CVD layer
NV Ensemble Density	1.2 x 10¹⁷	cm^-3	Resulting density after He⁺ implantation and annealing
He⁺ Implantation Energy	25	keV	Used to create vacancies for NV formation
Vacuum Annealing Temperature	900	°C	2 hours, for NV formation
O₂ Annealing Temperature	425	°C	2 hours, for charge-state stabilization
Laser Excitation Wavelength	532	nm	Diode laser for NV spin polarization/readout
Continuous Wave (CW) Laser Power	80	mW	Used for NV Photoluminescence (PL) collection
Magnetometric Sensitivity (dBNV)	2	µT µm/√Hz	Average sensitivity within 40 x 40 µm² Field-of-View (FOV)
Polarimetric Sensitivity (δΘκ)	50	prad µm/√Hz	MOKE imaging sensitivity
Effective Spatial Resolution	~2	µm	Limited by diamond-sample offset distance
Magnetic Sample Saturation (M_s)	760	kA/m	Co₂₀Fe₆₀B₂₀ multilayer stack at room temperature

Key Methodologies

The concurrent MOKE/NV-based magnetic microscopy relies on precise material engineering and sophisticated optical control.

Diamond Sensor Fabrication:
- Growth of a 100 nm thick ¹⁴N doped layer via CVD on an electronic-grade diamond substrate.
- Use of 99.9% ¹²C isotope purification to enhance coherence times.
- Vacancy creation via 25 keV He⁺ implantation (10¹² ions/cm² dose).
- Two-step annealing process: 900 °C (vacuum) for NV formation, followed by 425 °C (O₂) for charge-state stabilization.
Optical Setup:
- Home-built inverted epifluorescence microscope utilizing a 532 nm intensity-stabilized diode laser.
- High-magnification (60x) objective with 0.2 mm working distance.
- Detection via scientific-CMOS (sCMOS) camera, yielding an effective pixel area of 108 x 108 nm².
Concurrent Imaging Protocol:
- NV Imaging (Magnetic Field): Uses a 650 nm longpass filter to collect NV PL (637-800 nm) while blocking the reflected 532 nm pump light. CW Optically Detected Magnetic Resonance (ODMR) is performed by scanning a frequency triplet (separated by 2.16 MHz) over the NV resonances.
- MOKE Imaging (Magnetization): The longpass filter is removed and replaced by a neutral density filter (OD=3) to collect the reflected 532 nm light. Polarization control is achieved using two linear polarizers (one for preparation, one for analysis/analyzer).
- MW Delivery: Omega-shaped stripline glued onto the coverslip, supplied by a 16 W amplifier, to deliver the MW fields required for ODMR.

6CCVD Solutions & Capabilities

The successful replication and advancement of this concurrent imaging technique are critically dependent on the quality, purity, and geometric precision of the diamond sensor. 6CCVD is uniquely positioned to supply the necessary materials and engineering services.

Applicable Materials

To replicate the high-performance sensor described in the paper, researchers require ultra-high purity Single Crystal Diamond (SCD) with precise layer control.

6CCVD Material	Specification Match	Customization Potential
Electronic Grade SCD Substrates	Provides the base material (up to 10 mm thick) with low defect density, essential for high-quality CVD overgrowth.	Substrates available up to 10 mm thickness, polished to Ra < 1 nm for optimal epitaxial growth.
High-Purity SCD (Isotope Purified)	We offer SCD with ultra-low nitrogen background and high ¹²C enrichment (required for long coherence times, T₂), crucial for achieving high magnetometric sensitivity.	We can supply SCD wafers ready for specific ¹⁴N or ¹⁵N doping via subsequent implantation or during the CVD process.
Custom Thin-Film SCD	We can grow SCD layers with thicknesses ranging from 0.1 µm to 500 µm, perfectly matching the 100 nm (0.1 µm) near-surface layer requirement.	Precise thickness control (± 5%) is guaranteed, enabling optimization of the NV layer depth relative to the sample surface.

Customization Potential

The paper notes that minimizing the diamond-sample offset distance (~2 µm) is the primary limitation to spatial resolution. This suggests a move toward direct deposition of magnetic samples onto the diamond surface.

Ultra-Polished Surfaces: 6CCVD provides SCD polishing down to Ra < 1 nm. This atomic-scale flatness is essential for subsequent deposition of high-quality magnetic thin films (like the Ta/CoFeB/MgO stack) directly onto the diamond sensor, eliminating the air gap and maximizing spatial resolution.
Custom Dimensions and Shaping: The experimental setup requires precise sensor dimensions for integration with the MW stripline. 6CCVD offers custom laser cutting and shaping of SCD plates and wafers up to 125 mm (PCD equivalent) to fit specific optical and microwave geometries.
Integrated Metalization for MW Striplines: The experiment requires an omega-shaped stripline for MW delivery. 6CCVD offers internal metalization services (Au, Pt, Ti, Cu, Pd, W) to deposit high-conductivity contacts directly onto the diamond surface, simplifying integration and improving MW coupling efficiency.

Engineering Support

The development of advanced quantum sensors requires deep expertise in both material science and application physics.

6CCVD’s in-house PhD team specializes in MPCVD growth parameters, defect engineering, and surface preparation. We can assist researchers working on similar NV-based Quantum Sensing and Spintronics projects by:

Optimizing Material Selection: Consulting on the ideal ¹²C purity, nitrogen concentration, and crystal orientation (e.g., <100> or <111>) for specific vector magnetometry protocols.
Surface Preparation Guidance: Ensuring the diamond surface is optimally prepared (e.g., oxygen or hydrogen termination) for subsequent thin-film deposition or implantation processes.
Global Logistics: Providing reliable Global Shipping (DDU/DDP) for sensitive, high-value diamond materials worldwide.

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

View Original Abstract

We present an imaging modality that enables detection of magnetic moments and\ntheir resulting stray magnetic fields. We use wide-field magnetic imaging that\nemploys a diamond-based magnetometer and has combined magneto-optic detection\n(e.g. magneto-optic Kerr effect) capabilities. We employ such an instrument to\nimage magnetic (stripe) domains in multilayered ferromagnetic structures.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevapplied.15.024040