Broadband multi-magnon relaxometry using a quantum spin sensor for high frequency ferromagnetic dynamics sensing
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-10-16 |
| Journal | Nature Communications |
| Authors | Brendan McCullian, Ahmed M. Thabt, Benjamin Gray, Alex L. Melendez, Michael Wolf |
| Institutions | United States Air Force Research Laboratory, The Ohio State University |
| Citations | 64 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Broadband Sensing Achievement: Demonstrated the first broadband multi-magnon Nitrogen-Vacancy (NV) relaxometry, enabling the detection of high-frequency ferromagnetic dynamics (magnons) at frequencies greater than the NV spin resonance frequency (2.87 GHz).
- Nonlinear Mechanism Probed: The technique utilizes the NV sensor to detect magnetic field noise generated by spinwaves excited via a second-order nonlinear instability process (resonance saturation) in a Nickel Zinc Aluminum Ferrite (NZAFO) thin film.
- High Wavevector Access: The method successfully probed spinwaves with wavevectors (k) up to 3 x 106 m-1, extending deep into the exchange-dominated region of the magnon spectrum.
- Multi-Magnon Noise Source: In the high-frequency regime (e.g., 4.0 GHz pump), NV relaxation occurs despite the absence of individual NV-resonant magnons, confirming that multiple magnons participate in creating difference-frequency magnetic noise that couples to the NV spin.
- Enhanced Sensitivity: The NV relaxation signal was found to be maximal at the critical magnetic field corresponding to the onset of the spinwave instability, suggesting utility for high-sensitivity imaging of nonlinear switching processes.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Ferromagnetic Film Material | Ni0.65Zn0.35Al0.8Fe1.2O4 | N/A | Nickel Zinc Aluminum Ferrite (NZAFO) |
| NZAFO Film Thickness | 23 | nm | Grown via Pulsed Laser Epitaxy (PLE) |
| NV Sensor Material | Nanodiamond Powder | N/A | Diameter ~100 nm, NV concentration ~30 ppm |
| NV-NZAFO Separation | 300 - 600 | nm | Estimated separation based on nanodiamond layer |
| NV Ground State Splitting (D) | 2.87 | GHz | Zero-field splitting frequency |
| NV Excitation Wavelength | 532 | nm | Continuous green laser source |
| Maximum Driven Spinwave Wavevector (k) | 3 x 106 | m-1 | Achieved in the exchange-dominated regime |
| Microwave Pump Frequency (fMW) - Low | 2.2 | GHz | Used for single-magnon relaxometry |
| Microwave Pump Frequency (fMW) - High | 4.0 | GHz | Used for multi-magnon relaxometry |
| FMR Field (Uniform Mode, 2.2 GHz) | 94 | Gauss | Ferromagnetic Resonance condition |
| FMR Field (Uniform Mode, 4.0 GHz) | 206 | Gauss | Ferromagnetic Resonance condition |
| Instability Cutoff Field (4.0 GHz) | 175 | Gauss | Field where instability-driven spinwaves are maximal |
| Lock-in Modulation Frequency (fmod) | ~1 | kHz | Used for simultaneous PL and MW absorption detection |
Key Methodologies
Section titled âKey Methodologiesâ- Sample Preparation: A 23 nm thick NZAFO film was grown on a MgAl2O4 substrate using Pulsed Laser Epitaxy (PLE).
- Microwave Structure Fabrication: A 15-Âľm wide tapered microstrip antenna (Ti/Ag/Au stack) was fabricated on the NZAFO surface using lithography to excite ferromagnetic dynamics.
- Quantum Sensor Deposition: Nanodiamond powder (~100 nm diameter, containing ensemble NV centers) was drop-cast onto the microstrip and NZAFO surface, establishing a sensor-sample separation of 300-600 nm.
- Magnetic Field Application: An electromagnet applied a static, in-plane magnetic field (H) along the NZAFO [100] crystalline axis, which was swept during measurements.
- Parametric Excitation: Amplitude Modulated (AM) microwaves were applied via the microstrip antenna at constant frequencies (e.g., 2.2 GHz or 4.0 GHz) and varying powers (up to +20 dBm) to drive the second-order spinwave instability.
- Simultaneous Lock-in Detection:
- NV centers were optically polarized and excited using a 532 nm laser.
- NV photoluminescence (PL) and microwave transmission were simultaneously measured using lock-in amplifiers referenced to the AM frequency (~1 kHz).
- NV relaxation (decrease in PL) was correlated with the magnetic field noise generated by the parametrically driven spinwaves.
- Regime Analysis: Measurements were conducted in two regimes:
- One-Magnon Regime: Driven spinwave frequency < NV frequency (2.2 GHz pump). Relaxation relies on four-magnon scattering creating NV-resonant magnons.
- Multi-Magnon Regime: Driven spinwave frequency > NV frequency (4.0 GHz pump). Relaxation relies on multi-magnon processes creating difference-frequency noise at the NV resonance.
Commercial Applications
Section titled âCommercial Applicationsâ- Advanced Magnonics and Spintronics: Provides a critical tool for characterizing energy dissipation and scattering rates in magnetic insulators (e.g., ferrites, YIG) used in next-generation spin-wave computing and logic devices.
- High-Frequency Quantum Sensing: Extends the operational frequency range of NV-based quantum sensors, allowing them to probe magnetic noise in components operating in the 4 GHz and potentially higher regimes, previously inaccessible due to the NV resonance limit.
- Materials Science Characterization: Enables local, nanoscale mapping of exchange interactions and damping parameters in novel magnetic thin films by measuring the relaxation dependence on high-k spinwaves.
- Nonlinear Device Engineering: Useful for understanding and optimizing the switching dynamics and stability thresholds in magnetic memory and microwave devices where nonlinear effects (like spinwave instability) are critical or detrimental.
- RF/Microwave Component Testing: Allows for the localized detection and quantification of magnetic field noise generated by ferromagnetic components (e.g., circulators, filters) under high-power operating conditions.
View Original Abstract
Abstract Development of sensitive local probes of magnon dynamics is essential to further understand the physical processes that govern magnon generation, propagation, scattering, and relaxation. Quantum spin sensors like the NV center in diamond have long spin lifetimes and their relaxation can be used to sense magnetic field noise at gigahertz frequencies. Thus far, NV sensing of ferromagnetic dynamics has been constrained to the case where the NV spin is resonant with a magnon mode in the sample meaning that the NV frequency provides an upper bound to detection. In this work we demonstrate ensemble NV detection of spinwaves generated via a nonlinear instability process where spinwaves of nonzero wavevector are parametrically driven by a high amplitude microwave field. NV relaxation caused by these driven spinwaves can be divided into two regimes; one- and multi-magnon NV relaxometry. In the one-magnon NV relaxometry regime the driven spinwave frequency is below the NV frequencies. The driven spinwave undergoes four-magnon scattering resulting in an increase in the population of magnons which are frequency matched to the NVs. The dipole magnetic fields of the NV-resonant magnons couple to and relax nearby NV spins. The amplitude of the NV relaxation increases with the wavevector of the driven spinwave mode which we are able to vary up to 3 Ă 10 6 m â1 , well into the part of the spinwave spectrum dominated by the exchange interaction. Increasing the strength of the applied magnetic field brings all spinwave modes to higher frequencies than the NV frequencies. We find that the NVs are relaxed by the driven spinwave instability despite the absence of any individual NV-resonant magnons, suggesting that multiple magnons participate in creating magnetic field noise below the ferromagnetic gap frequency which causes NV spin relaxation.