Off-resonant detection of domain wall oscillations using deterministically placed nanodiamonds

At a Glance

Metadata	Details
Publication Date	2023-12-13
Journal	npj Spintronics
Authors	Jeffrey Rable, Jyotirmay Dwivedi, Nitin Samarth
Institutions	Pennsylvania State University
Citations	6
Analysis	Full AI Review Included

Technical Documentation & Analysis: Off-Resonant Detection of Domain Wall Oscillations

Executive Summary

This research establishes a critical methodology for detecting and manipulating nanoscale magnetic dynamics using Nitrogen-Vacancy (NV) centers in diamond, directly supporting the development of next-generation quantum spintronic devices.

Core Achievement: Demonstration of off-resonant detection of GHz-scale Domain Wall (DW) oscillations in patterned Permalloy (Py) nanowires using spin relaxometry (pulsed ODMR) of deterministically placed NV-nanodiamonds.
Mechanism: Detection relies on the enhanced relaxation of NV spins due to broadband stray field noise generated by the oscillating DW, confirming the feasibility of using DWs as localized magnetic field sources.
Observed Dynamics: DW oscillation frequencies were experimentally measured in the 1.8 GHz to 2.3 GHz range, validating the platform’s sensitivity to high-frequency magnetic textures.
Quantum Potential: Micromagnetic simulations predict that achieving resonant NV-DW coupling could increase the NV driving field by over 30x, leading to a drastic reduction in the $\pi$ pulse time required for qubit control.
Material Challenge Identified: Discrepancies between simulated and experimental results highlight the extreme sensitivity of DW dynamics to nanofabrication imperfections (e.g., edge roughness), emphasizing the need for ultra-high-quality, low-strain diamond substrates.
6CCVD Value Proposition: 6CCVD provides the precision MPCVD diamond substrates (SCD and highly polished PCD) and custom metalization services necessary to overcome fabrication limitations and realize the predicted resonant coupling enhancement.

Technical Specifications

Parameter	Value	Unit	Context
DW Oscillation Frequency (Measured)	1.8, 1.9, 2.3	GHz	Pulsed ODMR, Device 1
DW Oscillation Frequency (Simulated)	2.4	GHz	For 300 nm wide nanowire
NV Ground State Transition Frequency	2.87	GHz	Zero-field transition frequency
DW Nucleation Field (Measured)	11.25 to 13	mT	Field required to nucleate DW
Permalloy (Py) Thickness	10	nm	Fabricated nanowire thickness
Nanowire Width (Device 1)	300	nm	Semicircular geometry
Nanodiamond Diameter	100	nm	Commercial NV host material (3 ppm NV)
Simulated Driving Field Enhancement	>30	Factor	Potential increase upon resonant coupling
Simulated Stray Field Amplitude	>3	mT	Generated by DW oscillation (0.1 mT drive)
Microwave Delivery Wire Diameter	25	µm	Gold wire used for excitation
Py Gilbert Damping Parameter ($\alpha$)	0.0063	Unitless	Used in Mumax3 simulations

Key Methodologies

The experiment relied on precise nanofabrication and advanced pulsed magnetic resonance techniques:

Substrate Preparation: Semicircular Permalloy (Py, 10 nm thick) nanowires were fabricated via electron beam lithography, thin film deposition, and a liftoff process.
Defect Engineering: A notch-shaped defect (half-circle, radius $\sim$40% of wire width) was patterned into the nanowire center to serve as the deterministic Domain Wall (DW) pinning site.
NV Placement: 100 nm diameter nanodiamonds (3 ppm NV concentration) were deterministically positioned directly over the pinning site using an AFM pick-and-place protocol.
DW Control: DWs were nucleated by applying a magnetic field perpendicular to the wire (using an N52 permanent magnet) and denucleated by applying a tangential magnetic field.
Microwave Excitation: A microwave magnetic field was applied via a 25 µm diameter gold wire placed across the sample, driven by a signal generator and a +43 dBm amplifier.
Detection: Pulsed Optically Detected Magnetic Resonance (ODMR) was used for spin relaxometry. The NV centers were polarized using a 532 nm CW laser, followed by a microwave excitation pulse (5 µs), and subsequent readout.
Modeling: Micromagnetic simulations (Mumax3) were performed using 5 nm x 5 nm x 10 nm cells to model DW dynamics, nucleation fields, and the time-dependent AC stray field generated by the oscillating DW.

6CCVD Solutions & Capabilities

This research demonstrates the feasibility of using NV-diamond platforms for quantum spintronics, but the observed sensitivity to fabrication imperfections (edge roughness, strain) necessitates the highest quality diamond materials and precision integration capabilities offered by 6CCVD.

Applicable Materials for Replication and Extension

Application Requirement	6CCVD Material Recommendation	Technical Rationale
High-Coherence NV Host	Optical Grade Single Crystal Diamond (SCD)	SCD offers superior purity and ultra-low strain (Ra < 1 nm), crucial for minimizing inhomogeneous broadening and maximizing NV coherence time, essential for achieving the predicted resonant coupling.
Scalable Platform Substrate	Polycrystalline Diamond (PCD) Wafers	We offer PCD plates up to 125mm in diameter, providing a robust, thermally stable platform for scaling the patterned Py nanowire arrays and integrated microwave circuitry.
Integrated Electrical Control	Boron-Doped Diamond (BDD) Substrates	BDD provides a conductive platform, enabling future experiments involving current-driven DW oscillations or integrated electrical readout, as suggested by the authors.

Customization Potential & Engineering Support

6CCVD’s in-house capabilities directly address the integration and material quality challenges identified in this research:

Precision Polishing: The paper noted that DW dynamics are extremely sensitive to edge roughness. 6CCVD guarantees Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD, providing the atomically smooth surfaces required for high-fidelity nanofabrication of the 10 nm thick Py nanowires.
Custom Dimensions: While the paper used small chips, 6CCVD can supply SCD or PCD substrates up to 500 µm thick (or up to 10 mm thick for substrates) in custom dimensions, allowing researchers to transition from proof-of-concept devices to larger, integrated quantum systems.
Integrated Metalization: The experiment required a 25 µm gold wire for microwave delivery. 6CCVD offers internal metalization services (including Au, Pt, Ti, Cu) to deposit and pattern these microwave structures directly onto the diamond substrate, ensuring precise alignment and optimal coupling to the NV centers.
Engineering Support: 6CCVD’s in-house PhD team specializes in MPCVD diamond growth and material selection for quantum applications. We can assist researchers in optimizing material specifications (e.g., NV creation method, substrate orientation) to achieve the predicted 30x reduction in $\pi$ pulse time for similar nanoscale microwave generator projects.

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

Tech Support

Original Source

DOI: https://doi.org/10.1038/s44306-023-00002-3