Measuring the Magnetic Moment Density in Patterned Ultrathin Ferromagnets with Submicrometer Resolution
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2015-07-13 |
| Journal | Physical Review Applied |
| Authors | T. Hingant, Jean-Philippe Tetienne, L.âJ. MartĂnez, K. Garcia, D. Ravelosona |
| Institutions | Université Paris-Sud, Centre National de la Recherche Scientifique |
| Citations | 38 |
| Analysis | Full AI Review Included |
Technical Documentation: Quantitative Nanoscale Magnetometry using NV-Diamond Probes
Section titled âTechnical Documentation: Quantitative Nanoscale Magnetometry using NV-Diamond ProbesâThis documentation analyzes the research detailing a novel, high-resolution method for measuring the magnetic moment density ($I_s$) in ultrathin ferromagnetic films using a scanning Nitrogen-Vacancy (NV) center in diamond magnetometer.
Executive Summary
Section titled âExecutive SummaryâThe reported research establishes an advanced methodology for nanoscale magnetic characterization, leveraging the atomic-size sensitivity of a diamond-based NV center magnetometer.
- Four Orders of Magnitude Improvement: The method achieves an unprecedented spatial resolution in the range of $(100 \text{ nm})^2$, improving upon existing SQUID and VSM techniques by at least four orders of magnitude.
- Quantitative $I_s$ Measurement: Surface density of magnetic moments ($I_s$) is inferred locally from stray field measurements with high precision, yielding uncertainties typically within a few percent.
- Ambient Operation: The technique operates under ambient conditions and requires no external magnetic field, minimizing parasitic signals from extrinsic magnetic impurities.
- Key Materials & Geometry: Experiments focused on ultrathin ferromagnetic stacks (Ta/CoFeB/MgO) patterned into 1 ”m-wide wires and 500 nm square dots.
- Material Sensitivity Demonstrated: The technique successfully measured the modification of $I_s$ induced by localized $\text{He}^{+}$ ion irradiation, quantifying a relative decrease of approximately 40% in the irradiated region.
- Diamond Probe Requirements: The magnetometer utilizes a single NV center hosted in a diamond nanocrystal ($\sim 50 \text{ nm}$ size) grafted onto an AFM tip, underscoring the demand for high-quality, ultra-small diamond material engineered for probe applications.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Spatial Resolution ($I_s$) | $< (100 \text{ nm})^2$ | Improvement over state-of-the-art methods. | |
| $I_s$ Uncertainty | $\sim 1%$ to $3%$ | Based on statistical fit outcomes. | |
| Probe-Sample Distance ($d$) | $50$ to $100$ | nm | Limiting factor for spatial resolution. |
| Diamond Nanocrystal Size | $\sim 50$ | nm | Size of the NV-hosting diamond on the AFM tip. |
| CoFeB Thickness ($t$) | $1$ | nm | Ultrathin ferromagnetic layer thickness. |
| Typical $I_s$ (As Deposited) | $97.7 \pm 3.0$ | ”B/nm2 | Co${20}$Fe${60}$B$_{20}$ sample. |
| Irradiation Energy | $15.5$ | keV | $\text{He}^{+}$ ion irradiation for local modification. |
| Irradiation Fluence | $1.6 \times 10^{15}$ | ions/cm2 | Dose used for $I_s$ modification study. |
| $I_s$ Reduction (Measured) | $\sim 40%$ | Relative decrease in irradiated CoFeB region. | |
| Measurement Environment | Ambient | $\text{N/A}$ | NV magnetometry measurement condition. |
Key Methodologies
Section titled âKey MethodologiesâThe core methodology involves utilizing a scanning NV magnetometer integrated with an AFM tip to accurately map the stray magnetic fields above patterned ultrathin magnetic films.
Sample Preparation and Layer Stack
Section titled âSample Preparation and Layer Stackâ- Substrate & Deposition: Multilayer stacks were deposited by PVD (Singulus Tech) onto a $\text{Si}|\text{SiO}_2 (100 \text{ nm})$ substrate.
- Layer Stack Geometry: $\text{Ta}(5 \text{ nm})|\text{CoFeB}(1 \text{ nm})|\text{MgO}(2 \text{ nm})|\text{Ta}(5 \text{ nm})$.
- CoFeB Stoichiometry: Samples used $\text{Co}{20}\text{Fe}{60}\text{B}{20}$ or $\text{Co}{40}\text{Fe}{40}\text{B}{20}$.
- Annealing: Specific samples (for irradiation) were annealed at $300^\circ\text{C}$ for 2 hours.
- Patterning: Magnetic wires (1 ”m wide) or square dots (500 nm) were defined using e-beam lithography followed by ion beam etching (etching depth $\delta d$: 10-50 nm).
- MW Antenna: A $100 \text{ nm}$ thick Au stripe was defined via a second lithography step to serve as a microwave antenna for NV spin excitation.
NV Magnetometry and Data Extraction
Section titled âNV Magnetometry and Data Extractionâ- Probe Setup: A single NV center in a diamond nanocrystal was grafted onto the apex of an AFM tip (tapping mode operation).
- Detection Principle: The stray magnetic field ($\vec{B}{\text{NV}}$) projection along the NV axis induces a Zeeman shift ($\Delta f{\text{NV}}$) in the NV centerâs ESR frequency.
- Data Acquisition: Scanning the NV probe across the sample edge records the $\Delta f_{\text{NV}}$ profile and the corresponding AFM topography ($\text{topo}(x)$).
- Quantitative Fitting: Experimental $\Delta f_{\text{NV}}$ profiles were fitted using analytical equations derived for the stray field of a 1D magnetic wire (thin-film approximation), incorporating the measured topography function $z(x) = d + \text{topo}(x)$.
- Parameter Extraction: The fit yields the surface magnetic moment density ($I_s$) and the stand-off distance ($d$).
Local Modification (He$^{+}$ Irradiation)
Section titled âLocal Modification (He$^{+}$ Irradiation)â- Masking: A $400 \text{ nm}$ thick PMMA layer was used as a masking layer with open 1 ”m-wide windows defined by e-beam lithography.
- Irradiation: The sample was irradiated with $\text{He}^{+}$ ions (15.5 keV) through the PMMA mask to locally modify the magnetic properties of the CoFeB layer, achieving high spatial control ($\sim 100 \text{ nm}$).
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe success of this research hinges directly on accessing high-quality diamond material and advanced nanofabrication capabilities. 6CCVD is uniquely positioned to supply and engineer the critical diamond components required for replicating, extending, and industrializing this NV-based metrology.
Applicable Materials
Section titled âApplicable MaterialsâTo replicate the NV magnetometry probe used in this study, researchers require diamond with exceptionally low defects and precise characteristics.
| Material Requirement | 6CCVD Solution | Technical Advantage for NV Centers |
|---|---|---|
| NV Probe Material | Optical Grade SCD (Single Crystal Diamond) | Required for controlled NV center creation and maintaining long coherence times for sensitive measurements. |
| All-Diamond Tips (Future) | Custom SCD Substrates/Tips | 6CCVD can grow SCD material suitable for forming integrated scanning probes, enabling lower stand-off distances ($d < 10 \text{ nm}$) for enhanced resolution. |
| Potential Substrates | SCD Substrates (up to $500 \text{ ”m}$ thickness) | Providing large, uniform, high-purity surfaces for advanced diamond synthesis and subsequent tip/probe fabrication. |
Customization Potential
Section titled âCustomization PotentialâThe experimental setup required precise microfabrication, including complex multilayer thin films, etching, and metal antenna structures. 6CCVDâs capabilities directly address these engineering needs.
| Research Requirement | 6CCVD Customization Service | Value Proposition |
|---|---|---|
| Antenna Fabrication | Custom Metalization (Au, Ti, Pt, W, Cu) | We provide internal capability to deposit the required $100 \text{ nm}$ thick Au microwave antennae directly onto customized diamond substrates or polished wafers. |
| Probe Geometry | Laser Micromachining and Custom Dimensions | 6CCVD offers laser cutting for complex geometries, crucial for shaping diamond probes, ensuring compatibility with specific AFM cantilever systems and tight integration tolerances. |
| Surface Quality | Ultra-Polishing (Ra < 1 nm for SCD) | High-quality polishing is essential for minimizing surface noise and ensuring stable probe-sample distance ($d$) during scanning measurements. |
| Scale-Up Potential | Large-Area PCD/SCD Wafers | While the experiments used small microstructures, 6CCVD can supply SCD or PCD wafers up to $125 \text{ mm}$ for developing scalable fabrication processes. |
Engineering Support
Section titled âEngineering SupportâThis work demonstrates the crucial role of MPCVD diamond in advanced quantum sensing for solid-state physics and materials science. 6CCVD provides unparalleled expertise in this domain.
> 6CCVDâs in-house PhD engineering team specializes in diamond material selection and optimization for quantum sensing and nanoscale metrology applications, including NV center fabrication and integration. We can assist researchers and engineers with designing custom diamond components specifically tailored to achieve the lowest possible probe-to-sample distance ($d$) in magnetic imaging projects, thereby maximizing spatial resolution.
Call to Action
Section titled âCall to ActionâFor custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We ship globally, offering DDU (Delivery Duty Unpaid) as default, with DDP (Delivery Duty Paid) available upon request.
View Original Abstract
We present a new approach to infer the surface density of magnetic moments\n$I_s$ in ultrathin ferromagnetic films with perpendicular anisotropy. It relies\non quantitative stray field measurements with an atomic-size magnetometer based\non the nitrogen-vacancy center in diamond. The method is applied to\nmicrostructures patterned in a 1-nm-thick film of CoFeB. We report measurements\nof $I_s$ with a few percent uncertainty and a spatial resolution in the range\nof $(100$ nm)$^2$, an improvement by several orders of magnitude over existing\nmethods. As an example of application, we measure the modifications of $I_s$\ninduced by local irradiation with He$^+$ ions in an ultrathin ferromagnetic\nwire. This method offers a new route to study variations of magnetic properties\nat the nanoscale.