Spin-torque oscillation in a magnetic insulator probed by a single-spin sensor
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2020-07-02 |
| Journal | Physical review. B./Physical review. B |
| Authors | H. Zhang, Mark Ku, Francesco Casola, Chunhui Du, Toeno van der Sar |
| Institutions | University of California, Los Angeles, University of Maryland, College Park |
| Citations | 27 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled āExecutive Summaryā- Core Achievement: Demonstrated quantitative, nanoscale characterization of microwave magnetic fields generated by a Spin-Torque Oscillator (STO) built on a Platinum (Pt) / Yttrium Iron Garnet (YIG) magnetic insulator hybrid.
- Sensing Technology: Utilized the spin of a single Nitrogen-Vacancy (NV) defect in a diamond nanobeam, positioned approximately 100 nm from the device, providing high spatial and spectral resolution.
- Spin-Wave Analysis: The NV sensor resolved multiple spin-wave modes (including the fundamental Ferromagnetic Resonance, FMR, and higher-order modes) and characterized their damping modification via spin current injection.
- Auto-Oscillation Confirmation: STO auto-oscillation was confirmed using three independent metrics: suppression of effective damping, divergence of the magnetic noise power spectral density (up to three orders of magnitude increase), and synchronization to an external microwave source.
- Threshold Measurement: The onset of auto-oscillation was precisely determined, yielding threshold currents of Ith1 ā 3.5 mA for the fundamental mode and Ith2 ā 4.4 mA for the higher-order mode.
- Future Potential: The technique opens the way for sub-Hz spectral resolution and nanoscale spatial mapping of STO signals, enabling their use as local probes for mesoscopic spin systems and quantum control.
Technical Specifications
Section titled āTechnical Specificationsā| Parameter | Value | Unit | Context |
|---|---|---|---|
| NV Sensor Proximity | ~100 | nm | Distance from single NV spin to Pt/YIG microstructure. |
| YIG Film Thickness | 17 | nm | Epitaxially grown on a GGG substrate. |
| Pt Film Thickness | 10 | nm | Sputtered layer on top of YIG. |
| STO1 Threshold Current (Ith1) | 3.5 | mA | Onset of auto-oscillation for fundamental FMR mode. |
| STO2 Threshold Current (Ith2) | 4.4 | mA | Onset of auto-oscillation for higher-order mode. |
| NV Gyromagnetic Ratio (Ī³) | 2.8 | MHz/G | Used for calculating phase shift in magnetometry. |
| NV Zero-Field Splitting (Dgs) | 2.87 | GHz | Intrinsic NV property. |
| Estimated Mode Coupling Strength | ~10 | MHz | Interaction between STO1 and STO2 modes. |
| Pt Cleaning Pressure | less than 5x10-8 | Torr | Ar+ plasma cleaning prior to Pt sputtering for purity. |
| Nanobeam Resist Stack (PMMA) | ~30 | nm | PMMA (495A2) layer thickness in the EBL resist stack. |
| Nanobeam Resist Stack (HSQ) | ~250 | nm | HSQ (XR-1541-006) layer thickness in the EBL resist stack. |
Key Methodologies
Section titled āKey MethodologiesāThe experiment combined advanced nanofabrication of hybrid spintronic devices with high-sensitivity quantum sensing techniques.
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Pt/YIG Device Fabrication:
- YIG Growth: A 17 nm YIG film was epitaxially grown on a (111) orientation GGG substrate using Pulsed Laser Deposition (PLD).
- Pt Deposition: The YIG surface was cleaned using Ar+ plasma (pressure below 5x10-8 Torr) before sputtering a 10 nm Platinum (Pt) layer.
- Patterning: The Pt/YIG stripe was defined using Electron-Beam Lithography (EBL) with a multi-layer resist stack (PMMA/HSQ/FOX-16), followed by Ar+ ion milling to transfer the pattern.
- Leads: Au electrical leads for DC current (Idc) injection and microwave driving were defined using EBL and e-beam evaporation.
-
NV Sensor Platform:
- Nanobeam Fabrication: Bulk diamond containing NVs was patterned into a nanobeam structure using established EBL and etching techniques.
- Positioning: The nanobeam was placed approximately 100 nm from the Pt/YIG microstructure using a home-built laser scanning confocal microscope setup.
-
Spin-Wave Spectroscopy (Stray-Field Magnetometry):
- The NV spin state was initialized (Green laser) and measured using a spin-echo sequence (Ļ/2 pulse, two Ļ pulses, final Ļ/2 pulse).
- A microwave drive field (b1) was applied during the central 2Ļ period to excite spin-wave resonances (FMR).
- The change in the YIG stray static magnetic field (ĪB||) was detected via the phase shift (Ļ = Ī³ĪB||2Ļ) imparted on the NV spin state.
-
Spin-Wave Noise Spectroscopy (NV Relaxometry):
- The NV spin was initialized (ms = 0) and allowed to relax for a hold time (Ļ).
- The spin-dependent photoluminescence (PL) was measured to determine the spin relaxation rate (Ī).
- The rate Ī is directly proportional to the magnetic-noise power spectral density B2(Ļ) generated by the STOs, allowing quantitative measurement of auto-oscillation onset and magnitude.
-
Synchronization Measurement:
- An external microwave source (fmw) was added to the NV relaxometry sequence.
- The external magnetic field (Bext) was tuned so the NV transition coincided with the free-running STO frequency.
- Synchronization was observed as a locking interval (Īfs) where the STO frequency was pulled to match fmw, resulting in a change in the measured PL signal.
Commercial Applications
Section titled āCommercial Applicationsā| Industry/Sector | Application | Relevance to Technology |
|---|---|---|
| Spintronics & Magnonics | On-chip Spin-Wave Sources | STOs in magnetic insulators (YIG) are ideal for generating coherent spin waves, used for local excitation of spin-wave resonances and fundamental studies of magnon dynamics. |
| Integrated Microwave Circuits | Nanoscale Microwave Generators | STOs provide a compact, tunable source for microwave magnetic fields, enabling highly integrated, energy-efficient radio frequency (RF) components. |
| Quantum Computing & Control | Spin Qubit Manipulation | The localized, high-frequency magnetic fields generated by STOs can be used for precise, local control and excitation of nearby spin qubits (e.g., other NV centers or solid-state spins). |
| Neuromorphic Computing | Building Blocks for Neural Networks | STOs are proposed as fundamental components for energy-efficient, biologically inspired computing architectures due to their non-linear dynamics and synchronization capabilities. |
| Advanced Sensing & Metrology | Ultra-High Resolution Magnetometry | The NV sensing technique, capable of nanometer spatial resolution and potential sub-Hz spectral resolution, is critical for quantitative mapping and characterization of complex magnetic systems. |
| Materials Science Research | Probing Magnetic Phase Transitions | STO dynamics provide a sensitive tool for investigating phenomena such as magnon thermodynamics and strongly-correlated many-body physics in magnetic materials. |
View Original Abstract
We locally probe the magnetic fields generated by a spin-torque oscillator (STO) in a microbar of ferrimagnetic insulator yttrium-iron-garnet using the spin of a single nitrogen-vacancy (NV) center in diamond. The combined spectral resolution and sensitivity of the NV sensor allows us to resolve multiple spin-wave modes and characterize their damping. When damping is decreased sufficiently via spin injection, the modes auto-oscillate, as indicated by a strongly reduced linewidth, a diverging magnetic power spectral density, and synchronization of the STO frequency to an external microwave source. These results open the way for quantitative, nanoscale mapping of the microwave signals generated by STOs, as well as harnessing STOs as local probes of mesoscopic spin systems.