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6CCVD Research

Revealing room temperature ferromagnetism in exfoliated Fe 5 GeTe 2 flakes with quantum magnetic imaging

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2022-02-22
Journal2D Materials
AuthorsHang Chen, Shahidul Asif, Matthew P. Whalen, Jeyson TĂĄmara-Isaza, Brennan Luetke
InstitutionsOak Ridge National Laboratory, Universidad Nacional de Colombia
Citations29
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This study successfully validates room-temperature ferromagnetism (FM) in exfoliated Fe5GeTe2 (FGT) thin flakes using Quantum Magnetic Imaging (QMI) based on Nitrogen Vacancy (NV) centers in diamond.

  • Room-Temperature FM Confirmed: Ferromagnetic order was directly confirmed at room temperature (RT) in FGT flakes, validating previous indirect electrical transport measurements.
  • Ultra-Thin FM: FM was observed in Pt-protected FGT flakes down to 21 nm (7 unit cells), providing a basis for realizing room-temperature monolayer ferromagnets.
  • Curie Temperature (Tc): The Tc was determined to be approximately 300 K, showing independence from flake thickness in the measured range (21 nm to 100 nm).
  • Anisotropy Insight: Stray field patterns and their response to perpendicular magnetizing fields are consistent with a perpendicular easy-axis magnetic anisotropy.
  • High-Throughput Screening: The wide-field NV QMI setup enables rapid, parallel characterization and screening of multiple exfoliated flakes simultaneously.
  • Degradation Monitoring: Time-resolved imaging monitored magnetic degradation in ambient conditions, revealing domain motion and a 33% decrease in overall stray field intensity over the first 10 hours, even in Pt-protected samples.
  • Methodological Advantage: NV QMI provides a local, highly sensitive, and artifact-free method to directly probe magnetization, overcoming limitations associated with global electrical transport or configuration-dependent optical techniques (MOKE/RMCD).

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Minimum FM Thickness (Protected)21nmCorresponds to 7 unit cells (Pt protected Fe5GeTe2).
Minimum FM Thickness (Unprotected)40nmLess than 20 unit cells (unprotected Fe5GeTe2).
Curie Temperature (Tc)~300KDetermined via temperature-dependent QMI (consistent across 21 nm to 100 nm thickness).
NV Implantation Energy6keVUsed for creating NV centers in diamond.
Average NV Depth~20nmDistance of NV ensemble from the diamond surface.
NV Implantation Density1012-1013cm-2Range of nitrogen implantation density.
Laser Wavelength532nmUsed for NV photoluminescence excitation (cw-ODMR).
Optical Resolution (System Limit)570nmLimited by optical diffraction.
Camera Pixel Size (on sample)133nmWithout 2x2 binning.
Protection Layer Thickness5nmPlatinum (Pt) layer deposited via e-beam evaporation.
Initial Magnetization Field0.6TApplied perpendicular field for magnetic initialization.
ODMR Bias Field (B0)30 to 40GLow bias field used during QMI measurement.
Temperature Fluctuationless than 1KFluctuation within a single measurement cycle during temperature sweeps.
Observed Stray Field Range±40”TTypical range of measured Bz stray field amplitude.

Key Methodologies

Section titled “Key Methodologies”

The experiment relies on the integration of material synthesis, thin-flake exfoliation, and wide-field NV ensemble Quantum Magnetic Imaging (QMI).

  1. Material Synthesis: Bulk single crystals of Fe5-xGeTe2 (average composition near Fe4.7GeTe2) were grown, quenched from the growth temperature, and cooled to 10 K or less to achieve the high-Tc ‘Q-C’ metastable phase (~310 K bulk Tc).
  2. NV Diamond Preparation: Electronic grade {100}-front facet diamonds were implanted with nitrogen (6 keV energy) and subsequently annealed to create NV ensembles approximately 20 nm below the surface.
  3. Sample Preparation: FGT flakes were mechanically exfoliated onto the NV diamond surface using standard tape in air.
  4. Protection Layer: To prevent degradation, a 5 nm Platinum (Pt) layer was immediately deposited onto the exfoliated flakes via electron-beam evaporation.
  5. QMI Setup (Wide-Field ODMR):
    • A 532 nm laser illuminated the sample area (40 x 40 ”m2).
    • Microwave (MW) signals (up to 45 dBm) were delivered via a co-planar waveguide on a printed circuit board (PCB) to drive NV spin transitions.
    • The magnetic stray field was measured using continuous-wave Optically Detected Magnetic Resonance (cw-ODMR) by monitoring NV photoluminescence (PL).
  6. Magnetic Initialization: Flakes were initialized ex situ by applying a strong perpendicular magnetic field (0.6 T) from a permanent magnet, followed by QMI measurement under a low bias field (B0, tens of Gauss).
  7. Temperature Control: An electrical heater placed on the PCB allowed temperature sweeps (up to 330 K) to determine the Tc by monitoring the temperature dependence of the stray field (BNV).
  8. Data Acquisition and Processing: Data was acquired by sweeping the MW frequency and capturing images I(x,y,f). Sample drift was corrected by repeatedly acquiring, aligning, and summing images during post-processing to maintain sufficient SNR.

Commercial Applications

Section titled “Commercial Applications”

The findings and the demonstrated methodology are critical for advancing next-generation magnetic and quantum technologies.

  • Room-Temperature Spintronics: FGT flakes operating at 300 K provide an essential material platform for implementing novel spintronic devices (e.g., spin valves, magnetic tunnel junctions) that are compatible with ambient operating conditions.
  • 2D Magnetic Memory: The ability to miniaturize FM order down to 7 unit cells (21 nm) supports the development of ultra-dense, highly tunable magnetic memory and storage devices.
  • Quantum Sensing Platforms: The NV QMI technique itself is a powerful enabling tool for:
    • High-Throughput Screening: Rapidly characterizing and validating new near-room-temperature 2D magnets, accelerating material discovery.
    • Microscopic Domain Engineering: Studying and controlling magnetic domains, domain walls, and spin textures (e.g., skyrmions) in 2D materials.
  • Degradation Science: Time-resolved QMI provides crucial insights into material stability, allowing engineers to monitor and mitigate degradation mechanisms (like oxidation) in air-sensitive 2D magnets, which is vital for device reliability.
  • Spin-Orbit Torque (SOT) Devices: The use of a Pt protection layer facilitates future studies on spin transfer and SOT effects, enabling electrical control over magnetization in FGT for energy-efficient switching applications.
View Original Abstract

Abstract Van der Waals (vdW) material Fe 5 GeTe 2 , with its long-range ferromagnetic ordering near room temperature, has significant potential to become an enabling platform for implementing novel spintronic and quantum devices. To pave the way for applications, it is crucial to determine the magnetic properties when the thickness of Fe 5 GeTe 2 reaches the few-layers regime. However, this is highly challenging due to the need for a characterization technique that is local, highly sensitive, artifact-free, and operational with minimal fabrication. Prior studies have indicated that Curie temperature T C can reach up to close to room temperature for exfoliated Fe 5 GeTe 2 flakes, as measured via electrical transport; there is a need to validate these results with a measurement that reveals magnetism more directly. In this work, we investigate the magnetic properties of exfoliated thin flakes of vdW magnet Fe 5 GeTe 2 via quantum magnetic imaging technique based on nitrogen vacancy centers in diamond. Through imaging the stray fields, we confirm room-temperature magnetic order in Fe 5 GeTe 2 thin flakes with thickness down to 7 units cell. The stray field patterns and their response to magnetizing fields with different polarities is consistent with previously reported perpendicular easy-axis anisotropy. Furthermore, we perform imaging at different temperatures and determine the Curie temperature of the flakes at ≈300 K. These results provide the basis for realizing a room-temperature monolayer ferromagnet with Fe 5 GeTe 2 . This work also demonstrates that the imaging technique enables rapid screening of multiple flakes simultaneously as well as time-resolved imaging for monitoring time-dependent magnetic behaviors, thereby paving the way towards high throughput characterization of potential two-dimensional (2D) magnets near room temperature and providing critical insights into the evolution of domain behaviors in 2D magnets due to degradation.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2018 - Two-dimensional itinerant ferromagnetism in atomically thin Fe3GeTe2 [Crossref]
  2. 2018 - The van der Waals ferromagnets Fe5-ή GeTe2 and Fe5-ή-x Ni x GeTe2—crystal structure, stacking faults, and magnetic properties [Crossref]
  3. 2018 - Hard magnetic properties in nanoflake van der Waals Fe3GeTe2 [Crossref]
  4. 2019 - First-principles study of ferromagnetic metal Fe5GeTe2 [Crossref]
  5. 2021 - Van der Waals heterostructures for spintronics and opto-spintronics [Crossref]
  6. 2017 - Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals [Crossref]
  7. 2017 - Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit [Crossref]
  8. 2016 - Magnetic structure and phase stability of the van der Waals bonded ferromagnetFe3−x GeTe2 [Crossref]
  9. 2017 - Wafer-scale two-dimensional ferromagnetic Fe3GeTe2 thin films grown by molecular beam epitaxy [Crossref]
  10. 2019 - Ferromagnetism near room temperature in the cleavable van der Waals crystal Fe5GeTe2 [Crossref]

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