Quantum Imaging of Magnetic Phase Transitions and Spin Fluctuations in Intrinsic Magnetic Topological Nanoflakes

At a Glance

Metadata	Details
Publication Date	2022-07-11
Journal	Nano Letters
Authors	Nathan J. McLaughlin, Chaowei Hu, Mengqi Huang, Hanyi Lu
Citations	1
Analysis	Full AI Review Included

Executive Summary

This research utilizes Nitrogen-Vacancy (NV) centers in diamond to perform nanoscale quantum imaging of static and dynamic magnetic properties in intrinsic magnetic topological nanoflakes of the MnBi₂Te₄(Bi₂Te₃)_n family.

Core Achievement: Direct, quantitative imaging of local magnetization and spin fluctuations in two-dimensional (2D) MnBi₄Te₇ nanoflakes, overcoming the sensitivity limitations of conventional magnetometry for weakly magnetized 2D materials.
Static Imaging Results: Spatially resolved NV wide-field magnetometry revealed the characteristic domain wall nucleation and propagation behaviors during the first-order antiferromagnetic-to-ferromagnetic spin-flip transition in MnBi₄Te₇.
Dynamic Sensing Results: NV relaxometry was used to probe longitudinal spin fluctuations, allowing the quantitative extraction of the intrinsic spin diffusion constant (D) and static longitudinal magnetic susceptibility (χ₀) as functions of temperature and frequency.
Spin Transport Mechanism: The measured spin diffusion constant (D ≈ 6.1 x 10^-6 m²/s at 4.5 K) across MnBi₂Te₄, MnBi₄Te₇, and MnBi₈Te₁₃ suggests that spin transport is primarily driven by intralayer exchange coupling and is confined within individual magnetic septuple layers.
Domain Wall Dynamics: A dramatic increase in magnetic noise was observed during the spin-flip transition, attributed to low-frequency spin fluctuations hosted by the spatially evolving magnetic domain walls.
Technological Impact: The NV quantum sensing platform provides a unique tool for investigating the fundamental interplay between topology and magnetism at the nanoscale in emergent quantum materials.

Technical Specifications

Parameter	Value	Unit	Context
Material Family Studied	MnBi₂Te₄(Bi₂Te₃)_n	N/A	Intrinsic magnetic topological insulator
Measured Temperature Range	4.5 to 25	K	Range for NV relaxometry and D/χ₀ extraction
MnBi₄Te₇ Flake Thickness	83	nm	Exfoliated sample dimension
Critical Spin Flip Field (MnBi₄Te₇)	~1400	G	First-order magnetic transition field
Néel Temperature (T_N) (MnBi₄Te₇)	~13	K	Antiferromagnetic phase transition point
Minimum Magnon Frequency (f_min)	~51	GHz	Bulk MnBi₄Te₇ spin excitation energy gap
NV ESR Frequencies (f_ESR)	1.0, 1.2, 2.7	GHz	Used for longitudinal spin fluctuation sensing
Intrinsic Spin Diffusion Constant (D)	6.1 ± 0.8 x 10^-6	m²/s	MnBi₄Te₇ measured at 4.5 K
Static Magnetic Susceptibility (χ₀) Peak	9.9 ± 0.6 x 10^-3	nm	Measured around T_N (~13 K)
Spatial Resolution (Wide-Field NV)	~500	nm	Restricted by the optical diffraction limit
Target Resolution (Scanning NV)	Tens of	nm	Potential resolution using scanning NV microscopy

Key Methodologies

The study employed two primary NV quantum sensing techniques—wide-field magnetometry and relaxometry—on exfoliated nanoflakes transferred onto a [111]-oriented diamond membrane.

Sample Preparation and Setup:
- Exfoliated MnBi₂Te₄(Bi₂Te₃)_n nanoflakes (e.g., MnBi₄Te₇) were transferred onto a diamond sample containing shallowly implanted NV ensembles.
- A freestanding Au wire delivered microwave currents to control the NV spin state.
- An external magnetic field (B_ext) was applied along the out-of-plane (OOP) direction (z-axis).
Static Magnetometry (Wide-Field ODMR):
- Protocol: Used 1 µs green laser pulses for NV initialization (m_s = 0) and readout. Microwave π pulses (~100 ns) were swept in frequency (f) to measure the Optically Detected Magnetic Resonance (ODMR) spectrum.
- Field Extraction: The magnitude of the local static magnetic field (B_tot) was extracted from the Zeeman splitting (Δf_OOP) of the OOP-oriented NV centers.
- Magnetization Mapping: The magnetic stray field (B_m) was calculated by subtracting B_ext. Quantitative magnetization (4πM) maps were then reconstructed using established reverse-propagation protocols.
Dynamic Sensing (NV Relaxometry):
- Protocol: The NV spin relaxation rate (Γ) was measured by monitoring the occupation probabilities of the NV spin states (m_s = 0 and m_s = ±1) as a function of delay time (t).
- Noise Source Identification: In the low-field regime (B_ext < 800 G), the measured relaxation was driven by longitudinal spin fluctuations (two-magnon processes), as the NV ESR frequencies (1.0-2.7 GHz) were much lower than the minimum magnon energy (51 GHz).
- Spin Transport Parameter Extraction: The measured Γ(f_ESR, T) data were fitted using the fluctuation-dissipation theorem, allowing the quantitative extraction of the intrinsic spin diffusion constant (D) and static longitudinal magnetic susceptibility (χ₀).
Domain Wall Imaging:
- NV relaxometry was performed in the high-field regime (during the spin-flip transition, B_ext ≈ 1300-1600 G).
- The enhanced relaxation rate (Γ^D) was attributed to low-frequency spin dynamics generated by the magnetic domain walls formed during the phase transition, highlighting spatially evolving magnetic textures.

Commercial Applications

The findings and methodologies presented in this research are highly relevant to several emerging fields in quantum technology and advanced materials engineering:

Quantum Sensing and Metrology:
- The NV wide-field imaging platform provides a robust, nanoscale local probe for characterizing magnetic phase transitions and spin transport in 2D materials, crucial for quality control and fundamental research in quantum material development.
- The technique can be extended to a broad family of layered van der Waals crystals (49-52) for investigating local spin dynamics.
Spintronics and Data Storage:
- Magnetic topological materials like MnBi₂Te₄(Bi₂Te₃)_n are candidates for next-generation spintronic devices due to their nontrivial band topology and tunable magnetism.
- Direct imaging of magnetic domain walls, which host chiral edge conduction (15, 67), offers a pathway to engineer domain wall-based memory or logic devices.
Quantum Information Sciences (QIS):
- The dynamic coupling observed between topological materials (spin transport channels) and NV spin qubits points toward developing NV-based hybrid architectures for quantum information processing (60).
Topological Quantum Devices:
- The ability to correlate local magnetization (imaged by NV centers) with macroscopic quantum transport behaviors (like the quantum anomalous Hall effect or layer Hall effect) is essential for designing transformative quantum electronic technologies.

View Original Abstract

Topological materials featuring exotic band structures, unconventional current flow patterns, and emergent organizing principles offer attractive platforms for the development of next-generation transformative quantum electronic technologies. The family of MnBi2Te4 (Bi2Te3)n materials is naturally relevant in this context due to their nontrivial band topology, tunable magnetism, and recently discovered extraordinary quantum transport behaviors. Despite numerous pioneering studies, to date, the local magnetic properties of MnBi2Te4 (Bi2Te3)n remain an open question, hindering a comprehensive understanding of their fundamental material properties. Exploiting nitrogen-vacancy (NV) centers in diamond, we report nanoscale quantum imaging of magnetic phase transitions and spin fluctuations in exfoliated MnBi2Te4 (Bi2Te3)n flakes, revealing the underlying spin transport physics and magnetic domains at the nanoscale. Our results highlight the unique advantage of NV centers in exploring the magnetic properties of emergent quantum materials, opening new opportunities for investigating the interplay between topology and magnetism.

Tech Support

Original Source

DOI: https://doi.org/10.1021/acs.nanolett.2c01390