Noninvasive measurements of spin transport properties of an antiferromagnetic insulator

At a Glance

Metadata	Details
Publication Date	2022-01-07
Journal	Science Advances
Authors	Hailong Wang, Shu Zhang, Nathan J. McLaughlin, Benedetta Flebus, Mengqi Huang
Institutions	University of California, Los Angeles, Boston College
Citations	44
Analysis	Full AI Review Included

Executive Summary

This research introduces a novel, non-invasive quantum sensing method utilizing Nitrogen-Vacancy (NV) centers to characterize the intrinsic spin transport properties of Antiferromagnetic Insulators (AFIs).

Core Innovation: NV relaxometry is used to probe the time-dependent fluctuations of the longitudinal spin density (spin noise) in alpha-Fe₂O₃, enabling the measurement of intrinsic spin transport without requiring an external spin bias (electrical or thermal gradient).
Key Achievement: The intrinsic spin diffusion constant (D) of alpha-Fe₂O₃ was experimentally determined to be (8.9 ± 0.5) x 10^-4 m²/s at 200 K, yielding a spin conductivity of (7.1 ± 0.4) x 10⁶ S/m.
Mechanism Probed: The longitudinal spin noise is linked to two-magnon scattering processes, which are accessible by NV centers in the low GHz regime, despite the AFI magnon band minimum being in the Terahertz (THz) regime.
Temperature Dependence: The spin diffusion constant D was measured across the Morin phase transition (T_M ~ 263 K), showing a smooth decrease from 200 K to 300 K, confirming that spin transport is primarily driven by thermal magnons governed by exchange interaction.
Material Characterization: The technique successfully diagnosed underlying spin transport properties, including a spin diffusion length of ~3 µm and a magnon mean free path of ~90 nm at 200 K.
Technological Advantage: This method offers a significant opportunity for diagnosing high-frequency magnetic materials (e.g., 2D magnets, spin liquids) where conventional magnetic resonance techniques struggle due to high resonance frequencies.

Technical Specifications

Parameter	Value	Unit	Context
Intrinsic Spin Diffusion Constant (D)	8.9 ± 0.5	10^-4 m²/s	Uniaxial AF state (200 K)
Spin Conductivity	7.1 ± 0.4	10⁶ S/m	Calculated from D (200 K)
Spin Diffusion Constant (D)	6.6 ± 0.4	10^-4 m²/s	Easy-plane AF state (300 K)
Spin Diffusion Length (lambda)	~3	µm	Estimated at 200 K
Magnon Mean Free Path	~90	nm	Calculated at 200 K
Magnon Velocity (v)	~30	km/s	Literature value used for calculation
Momentum Scattering Time (tau)	~3	ps	Calculated at 200 K
Morin Transition Temperature (T_M)	~263	K	Phase transition in alpha-Fe₂O₃
Measurement Temperature Range	200 to 300	K	Range tested across T_M
NV-to-Sample Distance (d₁)	250 ± 6	nm	NV1 sensor proximity
NV-to-Sample Distance (d₂)	185 ± 5	nm	NV2 sensor proximity
NV ESR Frequency (f_-)	0.96	GHz	Used for temperature dependence measurement (Fig. 3c)
Gold Stripline Thickness	200	nm	Used for microwave control

Key Methodologies

The intrinsic spin transport properties were determined using NV relaxometry, probing the longitudinal spin noise generated by the alpha-Fe₂O₃ crystal.

Sample Preparation and Integration:
- Patterned diamond nanobeams containing individually addressable NV centers were transferred onto the surface of an alpha-Fe₂O₃ single crystal.
- Nanobeam dimensions were 500 nm x 500 nm x 10 µm to ensure nanoscale proximity (NV-to-sample distances d₁ ≈ 250 nm, d₂ ≈ 185 nm).
- A 200-nm-thick Au stripline was fabricated on the crystal surface for microwave control of the NV spin states.
- An external magnetic field (H) was applied and aligned to the NV-axis.
NV Spin Initialization:
- The NV spin was initialized to the m_s = 0 state using a 3-µs-long green laser pulse.
NV Relaxometry Measurement Sequence:
- The NV spin was allowed to relax for a variable delay time (t). Longitudinal spin noise from the alpha-Fe₂O₃ (associated with two-magnon scattering) induces NV spin transitions (m_s = 0 ↔ ±1).
- After the delay, the occupation probabilities of the NV spin states were measured by applying a microwave pi pulse at the corresponding Electron Spin Resonance (ESR) frequencies (f₊) and measuring the spin-dependent photoluminescence (PL).
Data Extraction and Modeling:
- The integrated PL intensity was measured as a function of delay time (t) and fitted using a three-level model to extract the NV relaxation rates (Γ₊).
- The measured relaxation rate Γ₊ was related to the spectral density of the longitudinal spin noise, |δB_||(f_±)|², via perturbation theory (Equation 2).
- Using the fluctuation-dissipation theorem and the conventional diffusion equation (Equation 1), the relaxation rate was modeled as a function of the intrinsic spin diffusion constant D (Equation 3).
- The experimental frequency dependence of Γ₊ was fitted to the theoretical model to quantitatively determine D in the absence of external spin biases.

Commercial Applications

This technology provides critical diagnostic capabilities for materials central to next-generation information processing and storage, particularly in the field of spintronics.

Next-Generation Spintronic Devices: AFIs (like alpha-Fe₂O₃) are crucial for developing energy-efficient, ultrafast spintronic devices for long-range spin information communication and storage, leveraging their THz magnon band gaps.
High-Frequency Magnetic Material Diagnosis: The NV relaxometry technique is uniquely suited for characterizing spin transport in materials whose coherent spin resonances are in the THz regime (e.g., two-dimensional magnets, spin liquids, magnetic Weyl semimetals), which are inaccessible by conventional GHz-range techniques.
Quantum Sensing Platforms: NV centers serve as highly sensitive, nanoscale quantum sensors capable of probing magnetic and electric properties with unprecedented spatial resolution (potential for nanometer scale resolution via scanning NV microscopy).
Magnetic Memory and Data Processing: The ability to precisely measure intrinsic spin diffusion constants (D) and understand spin transport across phase transitions (like the Morin transition) is essential for optimizing AFI performance in high-density magnetic memory and ultrafast data processing applications.
Fundamental Materials Research: Provides a non-invasive tool for studying equilibrium spin dynamics and thermal magnon transport in a broad class of magnetic insulators.

View Original Abstract

Nitrogen-vacancy centers offer an alternative way to detect spin diffusive transport in an antiferromagnet.

Tech Support

Original Source

DOI: https://doi.org/10.1126/sciadv.abg8562