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	Czech Academy of Sciences, Institute of Physics, Centre National de la Recherche Scientifique
Citations	25
Analysis	Full AI Review Included

Executive Summary

This research introduces a novel, wide-field imaging platform that concurrently measures both the magnetization (M) and the resulting stray magnetic fields (B) of magnetic structures, addressing a critical challenge in spintronics research.

Core Innovation: Developed a combined imaging modality integrating Magneto-Optic Kerr Effect (MOKE) microscopy for magnetization detection and Nitrogen-Vacancy (NV) diamond magnetometry for stray magnetic field detection.
Non-Perturbative Sensing: The NV-based magnetometry is magnetically non-perturbative and operates over a broad range of environmental conditions (cryogenic to above room temperature).
Addressing the Inverse Problem: Simultaneous MOKE/NV imaging provides complementary data, resolving the under-constrained inverse problem of reconstructing complex magnetic spin topologies solely from stray field measurements.
Sensor Performance: The wide-field NV ensemble sensor achieves an average magnetometric sensitivity of approximately 2 µT µm/√Hz.
Sample Demonstration: Successfully imaged magnetic stripe domains in a multilayered ferromagnetic stack (Ta/CoFeB/MgO), demonstrating the ability to observe magnetic topology and its associated field distribution simultaneously.
Operational Flexibility: The instrument is built upon a home-built inverted epifluorescence microscope, allowing for operation across a wide temperature range and under external magnetic and electrical fields.

Technical Specifications

Parameter	Value	Unit	Context
Imaging Modality	Concurrent MOKE / NV-ODMR	N/A	Wide-field, epi-fluorescence microscope basis
NV Sensor Type	¹⁴N-doped ensemble	N/A	100 nm layer on ¹²C substrate
NV Density	~1.2 x 10¹⁷	cm^-3	Concentration in the sensor layer
NV Excitation Wavelength	532	nm	Diode laser (CW operation)
NV Excitation Power	≈ 80	mW	Continuous Wave illumination
NV Zero-Field Splitting	≈ 2.87	GHz	Ground state ³A₂ (m_s = 0 to m_s = ±1)
¹⁴N Hyperfine Splitting	≈ 2.16	MHz	Used for simultaneous MW driving
Average Magnetometric Sensitivity (NV)	~2 µT µm/√Hz	µT µm/√Hz	Within 40 x 40 µm² Field-of-View (FOV)
Polarimetric Sensitivity (MOKE)	~50 prad µm/√Hz	prad µm/√Hz	Limited by mechanical instabilities
Spatial Resolution (NV)	≈ 2	µm	Limited by diamond-sample offset distance
MOKE Acquisition Time	10	ms/cycle	Typical exposure time per cycle
ODMR Sequence Time	100	ms/sequence	Typical exposure time per frequency point
Sample Saturation Magnetization (M_s)	≈ 760	kA/m	Measured via SQUID at room temperature
Sample Stack Thickness (Total)	13.08	nm	Ta(5)/CoFeB(1)/Ta(0.08)/MgO(2)/Ta(5)
Objective	Olympus UplanFL N 60X	N/A	Working distance 0.2 mm
Effective Pixel Area	≈ 108 x 108	nm²	On the sample focal plane (using sCMOS camera)

Key Methodologies

The experiment relies on precise material synthesis and a custom-built optical setup enabling concurrent, switchable imaging modalities.

Magnetic Sample Synthesis:
- Multilayer ferromagnetic stack (Ta/Co₂₀Fe₆₀B₂₀/Ta/MgO/Ta) grown on a Si/SiO₂ substrate (500 µm thickness) using DC magnetron sputtering (Singulus Rotaris system).
- The ultra-thin Ta insertion layer (0.08 nm) is used to fine-tune the perpendicular magnetic anisotropy.
NV Sensor Fabrication:
- A 100 nm thick ¹⁴N-doped layer was grown on an electronic-grade diamond substrate using Chemical Vapor Deposition (CVD).
- Vacancies were created via 25 keV He⁺ implantation (dose 10¹² ions/cm²).
- NV formation was completed via vacuum anneal (900 °C, 2 hours), followed by O₂ anneal (425 °C, 2 hours) for charge-state stabilization.
Optical and MW Setup:
- A home-built inverted epifluorescence microscope was used, employing a 532 nm diode laser (intensity stabilized via AOM/PID controller) for excitation.
- MW fields required for NV-ODMR were delivered via an omega-shaped stripline placed on the coverslip, amplified to 16 W (+43 dB).
- A bias magnetic field (B_z) was applied using a coil (radius 3 cm, producing ~6 G/A) supplied by a computer-controlled DC power supply.
NV Magnetic Imaging (ODMR):
- A Continuous Wave (CW) technique was employed, where optical polarization, MW drive, and spin-state readout occur simultaneously.
- To maximize sensitivity and drive all hyperfine resonances, three MW frequency components (separated by the 2.16 MHz hyperfine splitting) were applied simultaneously.
- The resulting NV Photoluminescence (PL) (650 nm longpass filtered) was collected by a scientific-CMOS camera.
MOKE Imaging (Magnetization):
- Polar MOKE configuration was used, sensing the magnetization component perpendicular to the sample surface.
- The setup incorporated two linear polarizers: one to prepare the incident green light and a second (analyzer) placed in front of the camera.
- The analyzer was optimized to be nearly crossed with the polarizer to achieve the best Signal-to-Noise Ratio (SNR).
Concurrent Operation:
- Switching between NV (magnetic field) and MOKE (magnetization) imaging was achieved using a computer-controlled flip mount to swap the longpass filter (for PL detection) with a neutral density filter (for reflected green light detection).

Commercial Applications

This concurrent imaging technology is highly relevant for the research and development of next-generation magnetic and quantum technologies.

Spintronics and Topological Computing:
- Direct visualization and control of magnetic textures like skyrmions, domain walls, and vortices, which are foundational elements for high-density, non-volatile memory (MRAM) and logic devices.
- Measurement of interfacial effects, such as the Dzyaloshinskii-Moriya Interaction (DMI), critical for stabilizing topological spin structures.
Advanced Materials Characterization:
- Non-invasive, ambient-condition study of static and dynamic spin configurations in novel magnetic thin films and heterostructures.
- Understanding the interplay between spin-spin and spin-orbit interactions in correlated electron systems.
Quantum Sensing and Metrology:
- Development of robust, wide-field NV-diamond magnetometers for industrial applications requiring high sensitivity and spatial resolution outside of cryogenic environments.
Integrated Circuit (IC) Failure Analysis:
- Mapping current flow and localized magnetic fields within integrated circuits for defect detection and performance analysis, offering higher resolution than traditional methods.
Magnetic Storage Media Development:
- Characterization and quality control of magnetic nanoparticles and thin films used in high-density data storage.

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