Control and local measurement of the spin chemical potential in a magnetic insulator

At a Glance

Metadata	Details
Publication Date	2017-07-14
Journal	Science
Authors	Chun-Hui Du, Toeno van der Sar, Tony X. Zhou, Pramey Upadhyaya, Francesco Casola
Institutions	Center for Astrophysics Harvard & Smithsonian, Harvard University
Citations	244
Analysis	Full AI Review Included

Technical Documentation and Analysis: Nanoscale Control of Spin Chemical Potential Using NV Diamond Sensors

Documentation Prepared for 6ccvd.com

Executive Summary: NV Diamond for Nanoscale Spintronics

This research successfully demonstrates the use of Nitrogen-Vacancy (NV) center single-spin magnetometry in diamond nanobeams to achieve non-perturbative, nanoscale characterization of the spin chemical potential ($\mu$) in magnetic insulators. This platform is critical for the advancement of low-dissipation spintronic devices.

Core Technology: Single-Crystal Diamond (SCD) nanobeams containing individually addressable NV centers serve as high-sensitivity, nanoscale magnetic field sensors.
Measurement Target: Magnon chemical potential ($\mu$) was successfully probed in a 20 nm-thick Yttrium Iron Garnet (YIG) film at room temperature and nanoscale proximity (d $\approx$ 100 nm).
Control Mechanism 1 (FMR): The magnon chemical potential was efficiently controlled and saturated by driving the system’s Ferromagnetic Resonance (FMR) using an on-chip microwave stripline.
Control Mechanism 2 (SHE): Quantitative extraction of the spin-Hall induced chemical potential ($\mu$) was achieved using an adjacent Platinum (Pt) stripline, validating the NV platform for spin caloritronics studies.
Fundamental Physics: First experimental determination of the local thermomagnonic torque ($\eta$), found to be of the same order of magnitude ($\approx 10^{-4}$) as the YIG Gilbert damping parameter ($\alpha$).
6CCVD Relevance: Replication and extension of this research critically require high-purity, optical-grade SCD wafers with precisely controlled NV-center depth and crystallographic orientation, alongside advanced custom metalization capabilities.

Technical Specifications: Magnon Chemical Potential Measurement

The following parameters were extracted from the experimental measurements, demonstrating the precision required for high-fidelity NV magnetometry and spintronics integration.

Parameter	Value	Unit	Context
Magnetic Insulator Thickness	20	nm	Yttrium Iron Garnet (YIG) film thickness
Substrate Material	N/A	N/A	Gd${3}$Ga${5}$O$_{12}$ (GGG)
Sensor Type	NV Center	S=1 electron spin	Magnetic field sensor embedded in diamond
NV-YIG Distance (d)	109 $\pm$ 10	nm	Distance extracted by fitting NV relaxation rates (NV1)
Magnetic Field ($B_{ext}$)	14.4	mT	External DC field used to extract FMR linewidth
FMR Linewidth	8	MHz	Typical linewidth of YIG ferromagnetic resonance
Therm. Magnonic Torque ($\eta$)	$\approx 10^{-4}$	Unitless	Value extracted from fit to $B_{AC}$ dependence
Gilbert Damping ($\alpha$)	$\approx 10^{-4}$	Unitless	YIG damping parameter; comparable to $\eta$
Charge Current Density ($J_{c}$) in Pt	$1.2 \times 10^{11}$	A/m$^{2}$	Current used for Spin Hall Effect (SHE) injection
SHE-Induced Chemical Potential ($\mu$)	$\approx 0.1$	GHz	Extracted chemical potential at maximum $J_c$
MW Stripline Thickness	600	nm	Gold (Au) stripline
SHE Stripline Thickness	10	nm	Platinum (Pt) stripline

Key Methodologies: Diamond-Based Spintronics Recipe

The experiment relied on highly precise material fabrication and characterization techniques to integrate the nanoscale sensor with the magnetic thin film.

Diamond Nanobeam Fabrication:
- Single-crystal diamond material containing individually addressable NV centers was fabricated into free-standing nanobeams (Reference 20).
- The nanobeams were transferred and precisely positioned onto the YIG/GGG structure to ensure nanometer proximity ($d$).
Thin Film Deposition & Structure:
- A 20 nm-thick YIG film was grown epitaxially on a GGG substrate.
- Metal striplines were fabricated: 600 nm Au for Microwave (MW) FMR excitation and 10 nm Pt for DC current injection (Spin Hall Effect).
Optical and Spin Initialization:
- A scanning confocal microscope was used to optically locate the NV centers.
- Green laser pulses were used for NV spin initialization and readout (photoluminescence measurement).
Magnon Excitation and Control:
- Magnons were excited either by applying a MW drive field ($B_{AC}$) near the FMR frequency (pumping via precession of coherent spin order parameter n).
- Alternatively, magnons were excited via electrical spin injection through the Pt stripline (Spin Hall Effect).
Chemical Potential Measurement:
- The magnon density of states (and thus the chemical potential $\mu$) was probed by measuring the magnetic field fluctuations at the NV site.
- The key observable was the NV spin relaxation rate ($\Gamma_{\pm}$), which is directly proportional to the magnon occupation number $n(\omega, \mu)$.
- The chemical potential $\mu$ was extracted using the ratio of relaxation rates at $\mu=0$ and $\mu > 0$, decoupling the measurement from sensor-specific transfer functions.

6CCVD Solutions & Capabilities: Enabling Advanced Magnon Physics

This research showcases the necessity of highly specialized, engineered diamond materials for pushing the limits of spintronics and spin caloritronics. 6CCVD is uniquely positioned to supply the materials required for replicating or advancing these nanoscale measurement platforms.

Applicable Materials

The foundation of this advanced sensing technique is the diamond host material and its contained quantum defect.

6CCVD Material	Application in This Research	Key Capability Match
Optical Grade SCD (High Purity)	Host material for NV centers, ensuring long coherence times and minimal background defects.	6CCVD delivers SCD wafers with high purity (low background N/O/Si), crucial for NV stability.
Custom SCD Thickness	The starting material must be suitable for complex nanobeam fabrication (Ref 20).	We provide SCD thickness ranging from 0.1 µm up to 500 µm, offering flexibility for deep etching and structuring.
Engineered NV Density/Depth	Requires precise control over NV depth and concentration near the sensing surface (109 nm proximity).	6CCVD offers defect engineering services, allowing precise control over NV incorporation for optimal quantum sensing performance.

Customization Potential for Research Replication

To fully replicate the high-density integration described in this paper (YIG, Pt, Au, and the diamond sensor), researchers require materials pre-processed to exacting standards.

Wafer Sizing and Custom Dimensions: The experiment utilizes nanoscale structures patterned from larger components. 6CCVD provides SCD wafers up to 125mm in diameter, suitable for large-scale microfabrication campaigns. Custom laser cutting and dicing services ensure precise dimensional requirements are met for nanobeam precursor structures.
Integrated Metalization Services: The research uses 600 nm Au and 10 nm Pt striplines for FMR drive and SHE injection, respectively. 6CCVD offers in-house metal deposition capabilities, including Au, Pt, Ti, W, and Cu. We can provide diamond substrates pre-patterned or metalized on specific crystal faces ($\text{Ra} < 1\text{ nm}$ polishing available) to ensure clean interfaces with thin film magnetic materials like YIG.
Ultra-Smooth Polishing: NV magnetometry relies on placing the sensor extremely close to the sample surface. 6CCVD guarantees surface roughness of Ra < 1 nm for SCD, ensuring maximum sensitivity and minimal sensor-to-sample distance ($d$).

Engineering Support

This study falls squarely within the critical areas of Spintronics, Magnonics, and Spin Caloritronics. 6CCVD’s in-house PhD team provides specialized consultation to assist engineering groups in selecting the optimal SCD substrate, NV defect structure, and surface preparation required for similar non-equilibrium spin transport and nanoscale thermal measurement projects.

Call to Action: For custom specifications or material consultation regarding high-purity diamond substrates, engineered NV centers, or integrated metalization for spintronic devices, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Diamonds to the rescue Keeping track of spin transport inside a spintronic device is challenging. Du et al. came up with a method involving diamond nitrogen-vacancy (NV) centers, which can act like tiny, very sensitive magnetometers. The authors placed diamond nanobeams containing the NV centers in close proximity to the sample. This allowed them to measure the spin chemical potential of spin waves—so-called magnons—with nanometer resolution in the material yttrium iron garnet. Because NV centers are also sensitive to temperature, the method may be of use in spin caloritronics. Science , this issue p. 195

Tech Support

Original Source

DOI: https://doi.org/10.1126/science.aak9611