Large enhancement of Curie temperature in Fe3GeTe2 by Fe intercalation in the van der Waals gap

doi:10.21203/rs.3.rs-2611131/v1

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Large enhancement of Curie temperature in Fe3GeTe2 by Fe intercalation in the van der Waals gap

https://doi.org/10.21203/rs.3.rs-2611131/v1

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The magnetic and electrical transport properties of the two-dimensional ferromagnetic Fe3GeTe2 have proven to be a subject of intrigue, with a broad range of Curie temperatures (Tc) observed in recent studies, ranging from 160 K to 230 K, and even reaching room temperature by ionic gating. Despite these discoveries, the underlying mechanism behind the varying Tc values remains a puzzle. Here, we employed scanning transmission electron microscopy to explore the atomic structure of Fe3GeTe2 crystals exhibiting Tc values of 160, 210, and 230 K. The results of the elemental mapping reveal that Fe intercalation on the interstitial sites within the van der Waals gap is present in the high- Tc (210 and 230 K) samples. The electrical transport measurements demonstrate the presence of an exchange bias effect in these high- Tc samples, owing to the existence of ferromagnetic and antiferromagnetic interfaces, while this effect is absent in the low-Tc (160 K) samples. First-principles calculations further suggest that the Fe-intercalation layer may be responsible for the local antiferromagnetic coupling that gives rise to the exchange bias effect, and that the introduction of interlayer exchange paths greatly contributes to the enhancement of Tc. This discovery of the Fe-intercalation layer elucidates the mechanism behind the hidden antiferromagnetic ordering that underlies the enhancement of Tc in Fe3GeTe2.

Physical sciences/Physics/Condensed-matter physics/Electronic properties and materials

Physical sciences/Physics/Condensed-matter physics/Magnetic properties and materials

The recent discovery of two-dimensional (2D) magnets offers a promising opportunity to explore both the fundamental aspects of physics and practical applications, such as spintronic devices. Researchers first observed 2D magnets by exfoliating the bulk crystals of NiPS₃^1,2, FePS₃^3,4, CrI₃^5–7, Cr₂Si₂Te₆⁸, and Cr₂Ge₂Te₆⁹. However, as these crystals are semiconductors or insulators, it is impossible to directly evaluate their magnetism using transport measurements or apply them to electronic devices^10–12. Quantitatively probing the magnetism of monolayer or few-layer crystals is technically challenging. Indirect techniques, such as the low-temperature magneto-optical Kerr effect^9,13 or scanning single-spin magnetometer based on diamond nitrogen-vacancy centers¹⁴, have been applied to address this challenge. The recent observation of 2D ferromagnetic Fe₃GeTe₂ (FGT), thanks to its metallic nature with high carrier density up to 10²¹ cm^− 3, provides a path to study both the charge and spin degrees of freedom in 2D magnetic systems^15–17. FGT exhibits a high Curie temperature (T_c) that is representative from 200 K in bulk to 100 K in monolayer crystals^18,19. The T_c of an N-layer crystal follows an empirical formula as T_c(N) = [1-(C/N)^λ]T_c^3D, where C is a non-universal constant, and λ = 1/ν demonstrates the critical exponent ν of the spin dimensionality in the bulk 3D crystal¹¹. However, an ionic gating could significantly enhance the T_c of trilayer crystals up to room temperature²⁰. Later experiments demonstrate that ionic gating can also dramatically enhance the coercive field (H_c)²¹. Alternatively, the T_c and H_c can be tuned by Fe vacancy²². The tunable magnetic properties reinforce the potential application of FGT in electrically controlled magnetoelectronic devices^23–27.

In the lattice structure of FGT, each unit cell contains two quintuple layers, where Te, Fe, Fe-Ge, Fe, and Te layers are successively stacked. Fe atoms in the Fe and Fe-Ge layers exhibit two inequivalent Fe sites but with different valence, namely, Fe³⁺ (Fe-1 site) and Fe²⁺ (Fe-2 site). Both have partially filled d-orbitals, but they induce considerably different magnetic pictures as local magnetic moment and itinerant ferromagnetic order, respectively^{10–12,15−21,28}. Therefore, it is essential to explore the exact atomic structure of both Fe-1 and Fe-2 sites. Chemical doping or occupancy on either site can locally change the magnetic exchange interaction and induce modification of T_c and H_c in a wide range.

In this study, an investigation into the atomic structure of Fe ions was conducted on samples with high-T_c (230 K), intermediate-T_c (210 K), and low-T_c (160 K). The scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy (EDS) techniques were utilized to explore the atomic structure of the FGT crystals with varying T_c. In the high-T_c (230 K) samples, Fe-intercalation was observed on the interstitial sites in the van der Waals gap, while the low-T_c crystals lacked such intercalation. The Hall resistivity loops measurements showed the presence of an exchange bias effect in the high-T_c samples, whereas the low-T_c (160 K) samples exhibited no exchange bias effect. The exchange bias effect can be attributed to the coexistence of ferromagnetic and antiferromagnetic interface (center). To investigate the underlying antiferromagnetic coupling in the inter-layer ferromagnetic system, first-principles calculations were conducted to determine the contribution of the Fe-intercalation layer, which plays a role in local antiferromagnetic coupling for the exchange bias effect. The introduced interlayer exchange paths were found to significantly enhance T_c. The identification of the Fe-intercalation layer provides a means to explore the mechanism behind the enhancement of T_c and H_c of FGT.

Structure analysis. We utilized a transmission electron microscope that is double Cs-corrected and is equipped with energy-dispersive X-ray spectroscopy (EDS) to accurately reconstruct the lattice structure. The crystals were sectioned by focused ion beam (FIB) along three crystal planes indexed as (001), (10), and (100), as depicted in the schematic image in Supplementary Figs. 3a - 3d. Figure 1 displays the high-angle annular dark-field images captured using scanning transmission electron microscopy mode (HAADF-STEM) and the corresponding EDS atomic resolution mapping of FGT samples having different T_c values of 160, 210, and 230 K. The HAADF-STEM images are suitable for identifying all ions based on their size and electronic states since the Ge and Te anions are much larger than the Fe cations. Moreover, the HAADF-STEM images are in good agreement with the cartoon of the crystal structure along the (10) plane, and the planes of (100) and (001) are also consistent with the structure (see Supplementary Figs. 3(e)-(j)). Additionally, the Fe-1 and Fe-2 sites are identifiable, where Fe-2 is positioned on the same layer with the Ge ions.

Elementary distribution analysis. Elementary mappings obtained through EDS for the intermediate-T_c and high-T_c crystals (Fig. 1(b) and (c)) indicate the presence of three distinct kinds of defects. Firstly, an additional layer of weaker spots between two Te-sublayers was observed in the Te mapping towards (10) plane, and such Te ions were also observed along the (100) plane. Based on the lattice cartoon of Ge and Te ions along (100) and (10) planes, it was found that the substitution of Te occurred on the Ge sites. Secondly, the Ge mapping demonstrated the doping of a few Ge on the Te sites. Thirdly, a Fe intercalation layer was clearly observed within the van der Waals gap in the Fe mapping towards both (10) and (100) planes. The HAADF-STEM image also confirmed the presence of such intercalation layer within the van der Waals gap. However, no occupation of Ge or Te on Fe sites was detected.

In the low-T_c crystal as shown in Fig. 1(a), both HAADF-STEM and EDS images demonstrated the absence of Fe-intercalation, while the antisite defects of Ge and Te anions were observed. The antisite of Te and Ge was found in both high-T_c and low-T_c crystals, indicating that it was not the main factor that modulated T_c in different samples. On the other hand, the additional layer of Fe ion not only contributed to the carriers of FGT, but also modified the magnetic coupling, thus inducing peculiar magnetic ordering.

Density functional theory calculations on the defects. The application of density functional theory (DFT) was utilized to reveal the underlying cause of Fe intercalation and its effects on the Curie temperature in FGT. The primary origin of intercalated Fe can be attributed to the segregation of Fe ions from their original sites to the interlayer region, resulting in the creation of Fe vacancies at those sites. This process can be facilitated by the presence of additional atoms, such as Te or Ge, that fill these Fe vacancies, leading to the formation of antisite defects. Formation enthalpies of several vacancies, antisite defects, and their various combinations, including Fe³⁺ (V_Fe3+), Fe²⁺ (V_Fe2+), Ge (V_Ge), and Te (V_Te) vacancies, Te_Ge antisites, bare Fe intercalation (Fe_3+xGeTe₂), as well as combinations of Fe_3+xGeTe₂ with Ge_Fe3+, Ge_Fe2+, Te_Fe3+, and Te_Fe2+ antisites, were examined in our calculations. Figures 2a-2c and Supplementary Fig. 5 illustrate their formation enthalpies as a function of Fe, Ge, and/or Te chemical potentials. V_Fe2+ (red) and Fe_3+xGeTe₂ (blue) exhibit the lowest formation enthalpies among all considered defects in Fe deficient and rich limits, respectively, as shown in Fig. 2a. Nevertheless, the combination of V_Fe2+ and Fe_3+xGeTe₂ (V_Fe2+ + Fe_3+xGeTe₂) has a higher formation enthalpy under both Fe deficient and Fe rich limits, indicating that it is less likely to be the source of Fe intercalation. Although this combination can intrinsically introduce intercalated Fe atoms, it requires additional energy gain for stabilization.

A potential method for stabilization involves occupying the Fe vacancy with Ge or Te atoms, resulting in the formation of antisite defects. In the Te-rich regime, the formation enthalpy of the Te/Ge antisite (Te substituting Ge) lies within a range of -0.3 to 2.0 eV (as shown in Fig. 3b), which is the most energetically favored among all considered antisite defects. This is consistent with the most frequently observed Te substitution at Ge sites in our experiments (as depicted in Fig. 1). The substituted Ge atoms have a propensity to occupy Fe³⁺ sites instead of Fe²⁺ sites, as the filling of the former site (Fe³⁺) provides an additional energy gain of at least 0.8 eV (as shown in Fig. 2c and Supplementary Fig. 5). Therefore, we may infer that the introduction of intercalated Fe atoms is most likely facilitated by the formation of Te_Ge and Ge_Fe3+ antisite defects in FGT. This process could be regulated, in principle, by controlling the supplying ratio of Te.

Incorporating the Te_Ge antisite defect and/or Fe intercalation presents an opportunity to further adjust the magnetic properties and spin exchange couplings of FGT. Upon considering the Te_Ge antisite defects, an intralayer and interlayer ferromagnetic (FM) state becomes the most energetically favored magnetic configuration (Fig. 2d-g and Supplementary Fig. 6). This state is more stable by 14 meV/Fe than the antiferromagnetic (AFM) order, and is consistent with the observed FM behavior in FGT samples where Te_Ge antisite is commonly present. As illustrated in Fig. 2g, the FM state remains the ground state when the intercalated Fe is situated in the interlayer region sandwiched by FGT free of Te_Ge antisite defects. However, in the presence of both Te_Ge antisite and Fe intercalation, a ferrimagnetic order (FiM, Fig. 2e) emerges, where the intercalated Fe sublayers are AFM coupled with adjacent FGT layers, and this configuration is energetically favored. This local AFM coupling can potentially give rise to an exchange bias effect in the high-T_c samples with both Te_Ge antisite and Fe intercalation.

Fe intercalation modified exchange couplings. Incorporation of Fe intercalation would furnish additional magnetic moments and exchange paths, thus reinforcing magnetic couplings along the interlayer direction in FGT. To assess the tunability of the Curie temperature, we computed the spin-exchange coupling (SEC) parameters based on a Heisenberg model, which are indicated with dashed arrows in different colors in Fig. 3a and listed in Supplementary Table 1. Monte Carlo simulations were executed with an anisotropic Heisenberg (AH) model (refer to the Methods for elaboration). Local magnetic moments of Fe atoms along the easy magnetization axis, as disclosed by Monte Carlo simulations, were depicted in Fig. 3b for FGT with defects. Based on the derived SEC parameters and the easy axis single-ion anisotropy, a T_c value of 148 K was obtained in FGT with Te_Ge antisite defects, which is in close proximity to the experimental value of 160 K of low-T_c ferromagnetic samples. As the magnetic couplings within FGT layers are predominantly determined by the intrinsic FGT exchange couplings and Te_Ge antisite defects, we further considered a J₇ to describe the spin exchange between the intercalated Fe and the Fe atoms in the adjacent FGT layers. Our calculations demonstrated that the value of J₇ is highly adjustable, ranging from -3.0 to 15.9 meV/Fe, depending on the density of the intercalated Fe atoms (Supplementary Table 2). Due to the augmented number of exchange paths and enlarged J₇ value, T_c elevates from 160 to 250 K with an increase in Fe intercalation density (Fig. 3b), indicating that Fe intercalation is an exceptionally efficient method of enhancing T_c in FGT.

The magnetic characteristics of three distinct samples with varying T_c were assessed through measurements of the anomalous Hall effect. The magnetic field was applied along the c-axis, and the FGT flakes were transferred onto pre-patterned Ti/Au electrodes under an inert atmosphere, as illustrated in Fig. 3(c) and 3(d). The temperature dependent normalized anomalous Hall resistivity (ρ_AHE) was determined from the ρ_AHE-H curves in Supplementary Figure 7, as shown in Fig. 3(e). The ρ_AHE-T curves approached zero at temperatures of 160 K, 210 K, and 230 K for the low-T_c, intermediate-T_c, and high-T_c samples, respectively, indicating the same T_c as the bulk crystals.

Exchange bias effect. Our density functional theory (DFT) calculations of FGT, incorporating both Te_Ge antisite and Fe intercalation, indicate that intercalated Fe atoms could induce local antiferromagnetic (AFM) coupling between intercalated Fe sublayers and adjacent FGT layers, resulting in an exchange bias effect in the high-T_c samples. To investigate this effect, we analyzed the Hall resistance hysteresis loops for both high and low-T_c samples, as shown in Supplementary Fig. 8. The low-T_c crystal exhibited a typical anomalous Hall effect (AHE) of ferromagnetism under zero-field-cooling (ZFC), as displayed in Supplementary Fig. 8a, while the ordinary Hall effect was relatively weak as a flat Hall resistance (R_xy) at large magnetic field (H). The coercive fields H_c were almost the same at 160 mT for both positive and negative sweeping directions, comparable to the previous report on FGT²⁹. To study the effect of magnetization history on the hysteresis loops, we also examined nine consecutive Hall resistivity loops at 2 K immediately after field-cooling (FC) by applying a magnetic field of 14 T along the c-axis above the T_c (see Supplementary Fig. 8b). The Hall resistance loops of the low-T_c samples were independent of sweeping history or cooling field, and the H_c was fixed at about 160 mT.

In the case of the high-T_c samples, as demonstrated in Fig. 4a-4d and Supplementary Fig. 8e-8g, the hysteresis loops of Hall resistance exhibit a perfectly square shape with no intermediate state, consistent with the Stoner-Wohlfarth model. Notably, the magnetization reversal process is observed to have an asymmetric shape, with the center of the hysteresis loop shifted from zero field by a certain value, resulting in different coercive fields H_c1 and H_c2. This asymmetric R_xy hysteresis loop is a typical behavior of the exchange bias effect, which is essentially attributed to the interfacial coupling between a ferromagnetic (FM) layer and an adjacent antiferromagnetic (AFM) layer³⁰.

The exchange bias effect exhibited by the high-T_c samples is noteworthy for its FC dependent behavior. When the system is subjected to ZFC, a fixed coercive field is observable on the negative side of H_c2 for all loops. Conversely, the positive side of H_c1 fluctuates with sweeping history, leading to a variable H_E within a broad range from -162 to 89 mT. The ZFC exchange bias effect is atypical, yet consistent with previous research into gating induced exchange bias effect in FGT²¹. Once exposed to FC of a high magnetic field (±14 T), as illustrated in Fig. 4c and 4d, the exchange bias field displays greater stability than those obtained from ZFC. With a negative field cooling (-14 T), the coercive fields are almost biased to the positive side, whereas applying a positive FC of 14 T causes the coercive fields to shift to the negative side, in line with a negative exchange bias effect. As a result, the exchange field (H_E) is primarily fixed at 89 mT for -14 T FC and -72 mT for 14 T FC.

Moreover, the antiferromagnetic (AFM) coupling has the ability to increase the coercive fields of the high-T_c sample to approximately 640 mT, which is significantly higher than that of the low-T_c sample (±160 mT). This intensification of magnetic coupling from the low-T_c crystals is evident in the square-shaped hysteresis loops of Hall resistance and the enhanced H_c of the high-T_c FGT. However, unlike the conventional training effect observed in FM/AFM bilayer systems³¹, there is always a fluctuation of HE, as discussed in detail in Supplementary note V. Therefore, identifying the relationship between the local atomic structure and the AFM coupling within the FM ordering is crucial to uncover the source of the exchange bias effect.

In essence, it can be concluded that Fe intercalation is responsible for the emergence of the exchange bias effect in the high-T_c samples. In the low-T_c crystal, all magnetic moments at Fe-1 and Fe-2 sites are ferromagnetically coupled, whereas the intercalated Fe may locally induce antiferromagnetic coupling to adjacent layers. The pinning effect of the FM/AFM interface on the FM domain is crucial in achieving the exchange bias effect. It is worth noting that the intercalated Fe is not uniformly filled but randomly distributed at interstitial sites. Thus, the FM/AFM interface is more likely to be the pinning sites. By gradually ramping up the high magnetic fields to 14 T while cooling, the magnetic moment in the AFM component overcomes its exchange energy to align along H, and the AFM coupling at the FM/AFM interface can be well-defined. Consequently, the exchange bias effect is turned from negative to positive, as shown in Fig. 4(e) and 4(f) for 14 T and -14 T cooling, respectively. When under ZFC, FM/AFM interfaces (center) can be spontaneously generated after the FM transition, although the polarization is randomly distributed for each interface due to the random formation of the ferromagnetic domain. The polarization direction of the FM domain can be well-aligned under relatively low field sweeping (900 mT), while the AFM can hardly be reoriented. Thus, the pinning effect still exists, resulting in the exchange bias effect.

To summarize, we have established a connection between the magnetism and structure of Fe₃GeTe₂ with varying Curie temperature using electrical transport measurements, STEM, and first-principles calculations. Our results demonstrate the presence of an exchange bias effect in high-T_c samples, indicating the existence of a ferromagnetic and antiferromagnetic interface at the center, while this effect is absent in low-T_c samples. Atomic-resolution scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy imaging provide element-resolved atomic structure information of Fe₃GeTe₂ in real-space and confirm the presence of Fe intercalation on interstitial sites in the van der Waals gap in high-T_c samples. First-principles calculations further support the notion that the Fe-intercalation layer can lead to local antiferromagnetic coupling for the exchange bias effect, thus significantly enhancing both antiferromagnetic coupling and T_c. The discovery of the Fe-intercalation layer provides a mechanism for enhancing T_c and H_c in Fe₃GeTe₂, namely, the hidden antiferromagnetic ordering due to the Fe-intercalation. It also offers a path to control T_c and H_c in Fe₃GeTe₂ by manipulating the external magnetic coupling within the inter-layer ferromagnetic system, which is promising for the development of spintronic devices.

Crystal growth. The bulk crystal of high-T_c and low-T_c FGT was synthesized by using the chemical vapor transport (CVT) and self-flux method, respectively. For CVT method, Fe (99.95%, Aldrich), Ge (99.999%, Aladdin) blocks and Te (99.99%, Aladdin) granules in a molar ratio of 3:1:2 were mixed using a mortar for 20 minutes. The starting materials were placed into an alumina crucible with iodine. The crucible was sealed into a quartz tube and was subsequently placed in a two-temperature zone tube furnace. One end of the quartz tube loading the mixture was kept at 750 °C and another end for the crystal growth was kept at 600 °C. After 150 hours, large and thin pieces of black Fe₃GeTe₂ plates with metallic cluster were obtained at the cold end of the tube. For self-flux method, Fe powder, Ge blocks and Te powder in a molar ratio of 3:3:6 were mixed and placed into an alumina crucible. The crucible was sealed into a quartz tube under a vacuum of 10⁻⁴ Pa and subsequently heated in a furnace up to 1000 °C in 10 h. After reaction at this temperature for 20 h, the assembly was slowly cooled down to 550 °C at a temperature decreasing rate of 3 °C h⁻¹ and stayed at 550 °C for 15 h. The assembly was finally naturally cooled down to room temperature by shutting down the furnace and thin pieces of black crystals were obtained.

Structure analysis. The crystal structure was confirmed by single crystal X-ray diffraction (XRD, Bruker D8), the X-ray diffraction patterns shows the clear diffraction patterns (see Supplementary note 1), guaranteeing the quality of the crystal. The double Cs-corrected scanning transmission electron microscopy (STEM, Grand JEM-ARM300F) was applied to analysis the atomic structure, the microscopy was equipped with a cold field-emission gun and operated at the accelerating voltage of 300 kV. To confirm the exact type of the defects, we cut the crystal by a focused ion beam (FIB, Grand JIB-4700F) along three crystal planes with the index of (001), (1¯20), and (100) as illustrated in Supplementary Fig. 3. Here the thickness of the thin specimens is around 50-100 nm. The energy-dispersive X-Ray spectroscopy (EDS) mapping was applied for the element distribution study.

Transport measurement. For transport measurements, a thin FGT flake was transferred on pre-patterned Ti/Au electrodes under inert atmosphere in Fig. 3(c) and 3(d), using standard mechanical exfoliation and polymer transfer method. The device was wire bonded on the puck and loaded into Physical Property Measurement System (PPMS, Quantum Design) as soon as possible. AHE was measured by Keithley 2400 and 2182A as the current source and voltage meter. A low current of 10 µA was applied to prevent from local heating and current driven effect. The thickness of the FGT was measured by atomic force microscopy and the corresponding color code after the transport measurement. The thicknesses of the measured samples are from 25 to 50 nm.

DFT calculations. We performed density functional theory calculations using the same methods described in our previous publications. In brief, we used a van der Waals density functional (vdW-DF) method^,, with the optB86b functional for the exchange part (optB86b-vdW), to optimize atomic structures of bulk FGT. The choice of optB86b-vdW usually enables us to well reproduce experimentally measured structure-related properties of two-dimensional materials^,. Given fully relaxed atomic structures, we employed the standard Perdew-Burke-Ernzerhof (PBE) functional, with a proper on-site Coulomb interaction value (PBE+U), to compare energy differences of all considered magnetic configurations. This scheme was found to be capable of revealing magnetic properties highly comparable with those of the Heyd-Scuseria-Ernzerhof (HSE06) functionalin 2D magnetic layers, e.g. CrI₃and CrSCl. On-site Coulomb interactions on Fe 3d orbitals are considered with two different pairs of U and J values, namely U = 3.6 eV and J = 0.2 eV for Fe³⁺ and U = 4.6 eV and J = 1.0 eV for Fe²⁺, as revealed by a linear response method. A uniform Monkhorst–Pack k-mesh of 15×15×3 was adopted for integrating over the Brillouin zone of a FGT unit cell. An orthorhombic 4×2×2 magnetic supercell was used for predicting magnetic properties of FGT bulk crystals with low defect density, in which a MP k-mesh of 4 × 6 ×1 was used. A kinetic energy cutoff of 400 eV for the plane-wave basis set was used for both structural relaxations and electronic structure calculations of defective FGT bulk crystals. All atoms could relax until the residual force per atom was less than 0.01 eV/Å. Metropolis Monte Carlo simulations were carried out to predict Curie temperatures. A 2D anisotropic Heisenberg model containing both the easy axis single ion anisotropy and the isotropic exchange was adopted with a 50 × 50 lattice and the periodic boundary condition.

Data availability

All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Information. Additional data are available from the corresponding author upon reasonable request.

Acknowledgements

This research was supported in part by the Ministry of Science and Technology (MOST) of China (No. 2018YFE0202700, 2022YFA1603903), the National Natural Science Foundation of China (Grants No. 12004251, 12027804, 11974422, 12104302, 12104303, 61761166009, 11974422, 12104504), the Fundamental Research Funds for the Central Universities and the Research Funds of Renmin University of China (No. 22XNKJ30), the Natural Science Foundation of Shanghai (Grant No. 20ZR1436100), the Science and Technology Commission of the Shanghai Municipality (21JC1405100), the Strategic Priority Research Program (Chinese Academy of Sciences, CAS) (No. XDB30000000), the start-up funding from ShanghaiTech University, and Beijing National Laboratory for Condensed Matter Physics, the Interdisciplinary Program of Wuhan National High Magnetic Field Center (WHMFC202124). Growth of hexagonal boron nitride crystals was supported by JSPS KAKENHI (Grant Numbers 19H05790, 20H00354 and 21H05233) and A3 Foresight by JSPS. C.W. was supported by the China Postdoctoral Science Foundation (2021M693479). Calculations were performed at the Physics Lab of High-Performance Computing of Renmin University of China, Shanghai Supercomputer Center.

Author contributions

Y.W. and J.L. conceived the experiment. Y.W. and X.Z. carried out transport measurements. W.X., Y.G., S. B., and J.W. grew the crystals. Y.H. and Z.-J.W. carried out the TEM characterization. C.W. and W.J. performed theoretical calculations. J.W., Z.-J.W. and J.L. performed the analysis and interpretation of the data. K.W. and T.T. provided high quality h-BN bulk crystals. All authors assisted in the interpretation of data and contributed to the writing of the manuscript.

Competing interests

The authors declare no competing interests.

Additional information

Supplementary information

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Large enhancement of Curie temperature in Fe3GeTe2 by Fe intercalation in the van der Waals gap

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