Quantum Imaging of Ferromagnetic van der Waals Magnetic Domain Structures at Ambient Conditions

TL;DR

This work uses NV-center-based quantum magnetic imaging to directly map magnetization in Fe$_{5}$GeTe$_{2}$ flakes under ambient conditions, addressing how domain structures and the Curie temperature vary with thickness and external fields. The approach combines wide-field and scanning NV modalities to obtain $B_z$ maps and reconstruct $M_z$, revealing sub-micrometer ferromagnetic domains and near-room-temperature $T_C$ that remain largely thickness-independent down to 15 nm. A key finding is the identification of crystallographic stripe features linked to compositional modulations and oxidation, which affect local magnetic patterns and are characterized via autocorrelation analysis and correlative SEM/EDXS and AFM measurements. These results advance the understanding of 2D vdW magnets in devices and highlight the potential for NV-based imaging to study spin textures, interfacial effects, and oxide-driven variations in next-generation spintronic systems.

Abstract

Recently discovered 2D van der Waals magnetic materials, and specifically Iron-Germanium-Telluride ($\rm Fe_{5}GeTe_{2}$), have attracted significant attention both from a fundamental perspective and for potential applications. Key open questions concern their domain structure and magnetic phase transition temperature as a function of sample thickness and external field, as well as implications for integration into devices such as magnetic memories and logic. Here we address key questions using a nitrogen-vacancy center based quantum magnetic microscope, enabling direct imaging of the magnetization of $\rm Fe_{5}GeTe_{2}$ at sub-micron spatial resolution as a function of temperature, magnetic field, and thickness. We employ spatially resolved measures, including magnetization variance and cross-correlation, and find a significant spread in transition temperature yet with no clear dependence on thickness down to 15 nm. We also identify previously unknown stripe features in the optical as well as magnetic images, which we attribute to modulations of the constituting elements during crystal synthesis and subsequent oxidation. Our results suggest that the magnetic anisotropy in this material does not play a crucial role in their magnetic properties, leading to a magnetic phase transition of $\rm Fe_{5}GeTe_{2}$ which is largely thickness-independent down to 15 nm. Our findings could be significant in designing future spintronic devices, magnetic memories and logic with 2D van der Waals magnetic materials.

Figures (14)

• Figure 1: (a) Diamond lattice hosting a nitrogen (red) vacancy (white) defect. Black balls represent carbon atoms. An external bias magnetic field is applied along the z-axis and has equal magnitudes of the projections on each of the four possible NV center axes. (b) The spin-1 NV center energy level diagram depicting optical pumping with a 532 nm green laser from the ground triplet ($^3$A$_2$) to the excited triplet ($^3$E) which may decay via various channels. Dotted/dashed arrows represent optically forbidden transitions. Each spin state is labeled with the respective magnetic spin quantum number, m$_s$, $\Delta$ represents the Zeeman splitting. $\rm D_{gs}$ = 2.87 GHz is the ground state zero-field splitting, excited state zero-field splitting $\rm D_{es}$ = 1.42 GHz and MW represents the microwave control. (c) Schematic of the experimental setup of NV center quantum magnetometer. (d) A typical ODMR under an external bias magnetic field. Black diamonds ($\blacklozenge$) are the experimental points while the red line (-) is a Lorentzian fit.
• Figure 2: Magnetization characterization of bulk FGT single crystal. (a) Zero field cooled magnetic moment as a function of the temperature under 100 Oersted magnetic field. (b) A zoomed-in plot of the black dashed square in (a) showing two transitions at 95 K and 133 K. (c) A zoomed-in plot of the blue dashed rectangle in (a) showing ferromagnetic to paramagnetic phase transition at $\sim$305K. (d) A zoomed-in plot of the dotted red rectangle in (a) showing the paramagnetic phase fitted using Curie-Weiss law giving T$\rm_C$=305.5$\pm$2.5 K.
• Figure 3: (a) Optical image of an FGT flake transferred onto the diamond surface by mechanical exfoliation, see Supporting Information Section-S1 si-ACS-25 for detail. (b) AFM topography of the FGT flake. (c) The extracted stray magnetic field, sensed by QDM, of the FGT flake under a 1800 $\mu$T z-bias field at 288 K. (d) The reconstructed OOP magnetization ($\rm M_z$) thiel-sci-19tan-IEEE-96 from the OOP stray magnetic field ($\rm B_z$) of the flake shown in (c).
• Figure 4: Temperature-dependent quantum magnetic images and phase transition plots. (a) Experimentally extracted OOP stray magnetic field (B$\rm _z$) of 221 nm thick FGT flake under 3150 $\mu$T z-bias field for the temperature in range 279-328 K. In the magnetic image, one can observe the multiple domains revealing ferromagnetic ordering at low temperatures. As the temperature increases, domains start rearranging, eventually disappearing at around 300 K, displaying the magnetic phase transition. (b) Experimentally extracted OOP stray magnetic field (B$\rm _z$) of a 15 nm thin FGT flake under 3150 $\mu$T z-bias field for the temperature in range 279-313 K. (c) The stray magnetic field variance $\Delta$B$\rm ^2_{norm}$ versus temperature plot for the FGT flake shown in (a). The retrieved T$\rm_C$ is 296$\pm$1.5 K. (d) The stray magnetic field variance $\Delta$B$\rm ^2_{norm}$ versus temperature plot for the FGT flake shown in (b). The retrieved T$\rm_C$ is 295$\pm$2.1 K. Black rectangles in (a) and (b) are the background and the dashed line encapsulating the flakes are the regions considered for computing the normalized stray field variance $\Delta$B$\rm ^2_{norm}$.
• Figure 5: (a) T$\rm_C$ verses FGT flake thickness plot. (b) Average magnetic field of an FGT flake as a function of the flake thickness at temperatures 283 K (◆) and 294 K (◆).