Varied Magnetic Phases in a van der Waals Easy-Plane Antiferromagnet Revealed by Nitrogen-Vacancy Center Microscopy
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-07-22 |
| Journal | ACS Nano |
| Authors | Alexander J. Healey, Sharidya Rahman, Sam C. Scholten, Islay O. Robertson, Gabriel Abrahams |
| Institutions | The University of Melbourne, Centre for Quantum Computation and Communication Technology |
| Citations | 13 |
| Analysis | Full AI Review Included |
Varied Magnetic Phases in CuCrP2S6 Revealed by NV Center Microscopy
Section titled âVaried Magnetic Phases in CuCrP2S6 Revealed by NV Center MicroscopyâExecutive Summary
Section titled âExecutive SummaryâThis research utilizes widefield Nitrogen-Vacancy (NV) center magnetic microscopy to characterize the magnetic phases of individual flakes of the van der Waals (vdW) antiferromagnet CuCrP2S6 (CCPS) at cryogenic temperatures.
- Core Value Proposition: CCPS is confirmed as a promising candidate for vdW heterostructures, exhibiting robust, easily-switchable magnetic order (Hc ~40 mT) down to the trilayer limit.
- A-type Antiferromagnetism: The layered A-type antiferromagnetic structure (ferromagnetic within layers, antiferromagnetic between layers) is maintained down to 3L thickness.
- Monolayer Magnetization: The spontaneous magnetization (Ms) of the uncompensated CCPS monolayer is quantified as a lower bound of 3.5 ”B/nm2.
- Bulk Spin-Flop Transition: Thick flakes exhibit bulk behavior characterized by a low-field spin-flop transition, allowing quantification of the interlayer exchange coupling (JAFM).
- Anomalous Out-of-Plane Phase: An unexpected, predominantly out-of-plane ferromagnetic phase was observed near zero field in thick flakes, attributed to surface anisotropy effects arising from sample preparation or ambient exposure.
- Implications for vdW Materials: The fragility of the weak easy-plane anisotropy highlights the challenge of maintaining intrinsic magnetic properties in ultra-thin, weakly anisotropic vdW materials when exposed to external perturbations.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Material System | CuCrP2S6 (CCPS) | N/A | vdW A-type antiferromagnet |
| NĂ©el Temperature (TN) | 31 | K | Bulk material transition temperature |
| Measurement Temperature (Base) | ~5 | K | Closed-cycle cryostat conditions |
| Maximum Applied Field | Up to 1 | T | Superconducting vector magnet capability |
| NV Spatial Resolution (Min) | ~500 | nm | Limited by optical diffraction and standoff |
| NV Sensing Layer Depth | 100-200 | nm | From diamond surface |
| Monolayer Ms (Lower Bound) | 3.5 | ”B/nm2 | Spontaneous magnetization of uncompensated layer |
| Coercive Field (Hc) | 35-40 | mT | Typical switching field for odd-layered flakes (at 5 K) |
| Interlayer Exchange Coupling (JAFM) | (1.2 ± 0.6) x 10-4 | J/m2 | Fitted parameter from micromagnetic model |
| Single Layer Thickness (AFM) | ~0.8 | nm | Interlayer step height |
| Laser Wavelength | 532 | nm | NV initialization and readout |
| Laser Power Density | ~1 | kW/cm2 | Estimated density at the NV layer |
Key Methodologies
Section titled âKey MethodologiesâThe magnetic characterization relied on integrating mechanical exfoliation, advanced NV sensor fabrication, and cryogenic widefield magnetometry.
- NV Sensor Fabrication: A dense NV sensing layer was created in a (100)-oriented Type-Ib diamond substrate via 100 keV 12C- ion implantation (1012 ions/cm2 fluence) and high-temperature annealing (1100 °C).
- Substrate Preparation: The diamond surface was coated with an 80 nm Al grid and an 80 nm Al2O3 layer to facilitate flake location, protect the diamond, and shield against laser heating.
- Flake Exfoliation and Transfer: CCPS flakes were mechanically exfoliated and transferred onto the diamond substrate using a three-axis transfer stage. The stage was mildly heated (60-65 °C) during transfer to minimize trapped air molecules and ensure clean interfaces.
- Cryogenic Widefield Magnetometry: Measurements were conducted at ~5 K using a widefield NV microscope integrated into a closed-cycle cryostat with a 1 T vector magnet.
- Pulsed ODMR Protocol: Magnetic images were obtained using pulsed Optically Detected Magnetic Resonance (ODMR) to measure the stray field (BNV) projected along the NV axis.
- Vector Field Reconstruction: Fourier reconstruction (upward propagation) was applied to BNV maps taken along different NV axes to reconstruct the full vector magnetic field, enabling differentiation between in-plane and out-of-plane magnetization components.
- Micromagnetic Modeling: A two-sublattice micromagnetic model was used to simulate the thickness-dependent spin-flop transition behavior and fit experimental magnetization curves to extract the interlayer exchange coupling (JAFM).
Commercial Applications
Section titled âCommercial ApplicationsâThe findings regarding the robust, easy-switchable magnetism in ultra-thin CCPS flakes, coupled with the advanced NV sensing technique, have implications across several high-tech sectors:
- Spintronics and Antiferromagnetic Memory: CCPS can serve as a near-atomically thin A-type antiferromagnet, appealing for use in synthetic antiferromagnetic spintronic devices (e.g., spin current generation) where negligible stray fields are desirable.
- Magnetoelectric Devices: Exploiting the multiferroic nature (coexisting antiferroelectric and magnetic order) allows for the possibility of low-energy magnetic switching via electrostatic gating, crucial for next-generation, non-volatile memory.
- vdW Heterostructures: CCPS is a candidate for interfacial magnetic exchange interactions in stacked heterostructures, enabling the design of novel 2D magnetic devices that rely on robust magnetic order at the few-layer limit.
- Quantum Sensing and Characterization: The widefield NV microscopy technique provides a critical platform for quantitative, directional characterization of novel 2D magnets, essential for quality control and material optimization in the development of new electronic materials.
- Miniaturized Electronics: The robust magnetic order in ultra-thin flakes supports the miniaturization of magnetic components for use in highly integrated electronic circuits.
View Original Abstract
Interest in van der Waals materials often stems from a desire to miniaturize existing technologies by exploiting their intrinsic layered structures to create near-atomically thin components that do not suffer from surface defects. One appealing property is an easily switchable yet robust magnetic order, which is only sparsely demonstrated in the case of in-plane anisotropy. In this work, we use widefield nitrogen-vacancy (NV) center magnetic imaging to measure the properties of individual flakes of CuCrP<sub>2</sub>S<sub>6</sub>, a multiferroic van der Waals magnet known to exhibit weak easy-plane anisotropy in the bulk. We chart the crossover between the in-plane ferromagnetism in thin flakes down to the trilayer and the bulk behavior dominated by a low-field spin-flop transition. Further, by exploiting the directional dependence of NV center magnetometry, we are able to observe an instance of a predominantly out-of-plane ferromagetic phase near zero field, in contrast with our expectation and previous experiments on the bulk material. We attribute this to the presence of surface anisotropies caused by the sample preparation process or exposure to the ambient environment, which is expected to have more general implications for a broader class of weakly anisotropic van der Waals magnets.