Quantum Sensing of Spin Fluctuations of Magnetic Insulator Films with Perpendicular Anisotropy
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-03-11 |
| Journal | Physical Review Applied |
| Authors | Eric Lee-Wong, Jinjun Ding, Xiaoche Wang, ChuanâPu Liu, Nathan J. McLaughlin |
| Institutions | Colorado State University, University of California, San Diego |
| Citations | 8 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research demonstrates the use of Nitrogen Vacancy (NV) centers in diamond as a local quantum probe to measure intrinsic spin fluctuations (magnetic noise) in nanometer-thick magnetic insulator films with Perpendicular Magnetic Anisotropy (PMA).
- Core Achievement: Successful noninvasive measurement of non-coherent thermal magnon fluctuations in epitaxial Yttrium Iron Garnet (YIG) thin films, a capability inaccessible by conventional magnetometry (FMR, SQUID).
- PMA Mechanism: The YIG films, grown on GSGG substrates via RF sputtering, exhibit strong PMA induced by tensile epitaxial strain, enabling out-of-plane magnetization.
- Sensing Mechanism: NV relaxometry measures the spin relaxation rate (magnetic noise), which is highly sensitive to the local dipolar stray fields generated by YIG thermal magnons when the magnon frequency matches the NV Electron Spin Resonance (ESR) frequency.
- Key Correlation: The measured field dependence of the NV relaxation rates is well correlated with the variation of the magnon density and the magnon band structure (specifically the FMR frequency minimum).
- Nanoscale Resolution: The platform achieves nanoscale spatial resolution, with NV-to-sample distances determined by fitting the relaxation data, ranging from 114 nm to 239 nm.
- Technological Impact: The results highlight NV centers as a powerful diagnostic tool for characterizing the noise environment of functional magnetic elements, crucial for designing next-generation, high-density, and scalable spintronic devices.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| YIG Film Thicknesses | 8, 10, 12, 30 | nm | Grown on GSGG (111) substrates. |
| GSGG Substrate Lattice Constant | 12.554 | A | Provides tensile strain for PMA induction. |
| Bulk YIG Lattice Constant | 12.376 | A | Comparison value. |
| Surface Roughness (8 nm YIG) | 0.12 | nm | Measured via Atomic Force Microscopy (AFM). |
| Effective Magnetization (4ÏMeff) (8 nm YIG) | -456 ± 7 | Oe | Negative sign confirms strong PMA. |
| Effective Magnetization (4ÏMeff) (12 nm YIG) | -1489 ± 10 | Oe | Enhanced PMA compared to 8 nm film. |
| Gilbert Damping (α) (8 nm YIG) | 5.2 x 10-3 | Unitless | Extracted from FMR linewidth fitting. |
| Gilbert Damping (α) (12 nm YIG) | 2.5 x 10-3 | Unitless | Lower damping observed in thicker film. |
| Inhomogeneous Linewidth (ÎHinh) (8 nm YIG) | 13.8 ± 1.1 | Oe | Contribution to FMR linewidth broadening. |
| NV-to-Sample Distance (d) (8 nm YIG) | 239 ± 11 | nm | Extracted from fitting NV relaxation rates. |
| NV-to-Sample Distance (d) (12 nm YIG) | 114 ± 10 | nm | Closer proximity achieved in this measurement set. |
| NV ESR Frequency (Zero Field) | 2.87 | GHz | NV center ground state splitting (f0). |
| Minimal Magnon Energy (fFMR) (12 nm YIG, H=0) | 3.9 | GHz | Magnon band gap, greater than NV f0. |
Key Methodologies
Section titled âKey Methodologiesâ- YIG Film Fabrication: Epitaxial Y3Fe5O12 (YIG) thin films were grown on single-crystal (111) Gd3(Sc2Ga3)O12 (GSGG) substrates using radio-frequency (RF) sputtering.
- PMA Induction: The lattice mismatch between YIG and GSGG provides tensile strain, which induces the required out-of-plane magnetocrystalline anisotropy (PMA).
- Magnetic Characterization: Ferromagnetic Resonance (FMR) measurements (angle and frequency dependent) were used to extract dynamic properties, including the effective magnetization (4ÏMeff) and Gilbert damping (α).
- NV Center Integration: Single NV spins contained within a patterned diamond nanobeam (500 nm x 500 nm x 10 ”m equilateral triangular prism) were mechanically transferred onto the YIG surface, achieving van der Waals contact.
- NV Spin Initialization: A green laser pulse was applied to initialize the NV electron spin to the ms = 0 state.
- NV Relaxometry Measurement: The NV spin relaxation rate (α) was measured by tracking the decay of the spin-dependent photoluminescence (PL) intensity as a function of delay time (t) after initialization.
- Magnetic Field Tuning: An external magnetic field (H) was applied and aligned to the NV axis (at an angle ΞNV relative to the surface normal) to tune the NV ESR frequency (f±) relative to the YIG magnon band structure.
- Noise Correlation: The measured relaxation rates (α) were correlated to the magnon spectral density D(f, k) and the NV transfer function f(k, d) (which filters noise based on wavevector k ~ 1/d) to diagnose the intrinsic spin fluctuations.
Commercial Applications
Section titled âCommercial Applicationsâ- Quantum Sensing and Imaging: Utilizing NV centers for high-resolution, nanoscale mapping of magnetic fields and noise in complex magnetic structures, critical for quality control and failure analysis in miniaturized devices.
- Spintronic Device Optimization: Providing noninvasive diagnostics of intrinsic spin fluctuations (noise) in magnetic insulators, enabling engineers to optimize material parameters (e.g., thickness, strain) for reduced energy dissipation and improved reliability in spintronic elements.
- High-Density Magnetic Memory: Application in diagnosing noise sources in PMA materials used for high-density magnetic random access memory (MRAM) and domain wall racetrack memory, where thermal stability and scalability are paramount.
- Magnonics and Spin Wave Computing: Characterizing non-coherent thermal magnons, which are key sources of damping and decoherence in proposed magnonic logic circuits and long-distance spin transport applications.
- Hybrid Quantum Technologies: Developing solid-state quantum architectures by leveraging the strong coupling demonstrated between NV qubits and magnetic excitations (magnons) for potential use in quantum information storage and transduction.
View Original Abstract
Nitrogen vacancy (NV) centers, optically active atomic defects in diamond,\nhave been widely applied to emerging quantum sensing, imaging, and network\nefforts, showing unprecedented field sensitivity and nanoscale spatial\nresolution. Many of these advantages derive from their excellent\nquantum-coherence, controllable entanglement, and high fidelity of operations,\nenabling opportunities to outperform the classical counterpart. Exploiting this\ncutting-edge quantum metrology, we report noninvasive measurement of intrinsic\nspin fluctuations of magnetic insulator thin films with a spontaneous\nout-of-plane magnetization. The measured field dependence of NV relaxation\nrates is well correlated to the variation of magnon density and band structure\nof the magnetic samples, which are challenging to access by the conventional\nmagnetometry methods. Our results highlight the significant opportunities\noffered by NV centers in diagnosing the noise environment of functional\nmagnetic elements, providing valuable information to design next-generation,\nhigh-density, and scalable spintronic devices.\n