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# Gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2

## Abstract

Materials research has driven the development of modern nano-electronic devices. In particular, research in magnetic thin films has revolutionized the development of spintronic devices1,2 because identifying new magnetic materials is key to better device performance and design. Van der Waals crystals retain their chemical stability and structural integrity down to the monolayer and, being atomically thin, are readily tuned by various kinds of gate modulation3,4. Recent experiments have demonstrated that it is possible to obtain two-dimensional ferromagnetic order in insulating Cr2Ge2Te6 (ref. 5) and CrI3 (ref. 6) at low temperatures. Here we develop a device fabrication technique and isolate monolayers from the layered metallic magnet Fe3GeTe2 to study magnetotransport. We find that the itinerant ferromagnetism persists in Fe3GeTe2 down to the monolayer with an out-of-plane magnetocrystalline anisotropy. The ferromagnetic transition temperature, Tc, is suppressed relative to the bulk Tc of 205 kelvin in pristine Fe3GeTe2 thin flakes. An ionic gate, however, raises Tc to room temperature, much higher than the bulk Tc. The gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2 opens up opportunities for potential voltage-controlled magnetoelectronics7,8,9,10,11 based on atomically thin van der Waals crystals.

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## Data availability

The datasets generated and analysed during the current study are available from the corresponding author on reasonable request.

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## Acknowledgements

We thank X. Jin, B. Lian and G. Chen for discussions. Part of the sample fabrication was conducted at Fudan Nano-fabrication Laboratory. Y.D., Y. Yu, Y.S., J.W. and Y.Z. acknowledge support from National Key Research Program of China (grant nos. 2016YFA0300703, 2018YFA0305600), NSF of China (grant nos U1732274, 11527805, 11425415 and 11421404), Shanghai Municipal Science and Technology Commission (grant no. 18JC1410300), and Strategic Priority Research Program of Chinese Academy of Sciences (grant no. XDB30000000). Y. Yu also acknowledges support from China Postdoc Innovation Talent Support Program. N.Z.W. and X.H.C. acknowledge support from the ‘Strategic Priority Research Program’ of the Chinese Academy of Sciences (grant no. XDB04040100) and the National Basic Research Program of China (973 Program; grant no. 2012CB922002). X.H.C. also acknowledges support from the National Natural Science Foundation of China (grant no. 11534010) and the Key Research Program of Frontier Sciences, CAS (grant no. QYZDY-SSW-SLH021). J. Zhu acknowledges financial support from Chinese University of Hong Kong (CUHK) under grant no. 4053084, from the University Grants Committee of Hong Kong under grant no. 24300814, and the Start-up Funding of CUHK. J.W. also acknowledges support from the NSF of China (grant no. 11774065). Z.S., Y. Yi and S.W. acknowledge support from the National Basic Research Program of China (grant no. 2014CB921601).

### Reviewer information

Nature thanks I. Zutic and the other anonymous reviewer(s) for their contribution to the peer review of this work.

## Author information

Authors

### Contributions

Y.Z. conceived the project. N.Z.W. and X.H.C. grew the bulk FGT crystal. Y.D., Y.S. and Y. Yu developed device fabrication method. Y.D. fabricated devices and performed electric measurements with the help of Y. Yu. Z.S., Y. Yi and S.W. performed optical measurements. Y.D., Y. Yu, Y.Z.W. and Y.Z. analysed the data. J. Zhang and J. Zhu carried out DFT calculations; J.W. carried out theoretical calculations and modelling; and all three performed theoretical analysis. Y.D., Y. Yu, J.W. and Y.Z. wrote the paper and all authors commented on it.

### Corresponding author

Correspondence to Yuanbo Zhang.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Extended data figures and tables

### Extended Data Fig. 1 Characterization of FGT bulk crystal.

a, Powder X-ray diffraction pattern of bulk FGT crystal. b, Temperature-dependent magnetization of bulk FGT measured during zero-field cooling (ZFC, blue curve) and field cooling (FC, red curve) with an external magnetic field of 0.1 T. (a.u., arbitrary units; emu, electromagnetic unit for magnetic moment).

### Extended Data Fig. 2 FGT thin-flake device fabrication.

a, A representative optical image of FGT thin flakes exfoliated on top of Al2O3 film supported on a sapphire substrate. The image was captured with a CCD camera mounted on an optical microscope operating in transmission mode. b, Cross-sectional profile of optical transmission $${G}_{{\rm{sample}}}^{{\rm{T}}}/{G}_{{\rm{substrate}}}^{{\rm{T}}}$$ along the black line in a. Scale bar, 10 μm. c, Layer-dependent optical transmission extracted from the image shown in a. The black line is a fit to the data (blue squares) using the Beer–Lambert law. d, e, Optical images obtained in transmission mode (d) and reflection mode (e) for a typical monolayer sample. f, Device fabricated from the monolayer sample shown in d and e. Electrical contacts to the sample were made with indium microelectrodes. Scale bar, 100 μm. g, h, Optical images obtained in transmission mode (g) and reflection mode (h) for a typical trilayer sample. i, Device fabricated from the trilayer sample shown in g and h. Electrical contacts to the sample were made with thermally evaporated Cr/Au electrodes. Scale bar, 100 μm. The optical transmission of the monolayer and trilayer sample shown in d and g are presented as red dots in c.

### Extended Data Fig. 3 Determining Tc by Arrott plot analysis.

ac, Rxy as a function of external magnetic field μ0H obtained in a monolayer (a), bilayer (b) and trilayer (c) FGT device at varying temperatures. d, Arrott plots of the Hall resistance data of the bilayer sample shown in b. The temperature is varied from 72 K to 124 K with a 4 K interval. Black lines are line fits at high magnetic fields (from μ0H = 2.25T to μ0H = 3T). e, Spontaneous Hall resistance $${R}_{xy}^{{\rm{s}}}$$ as a function of temperature obtained from FGT samples with varying number of layers. $${R}_{xy}^{{\rm{s}}}$$ are normalized by their values at T = 1.5K. Tc is determined by the temperature where $${R}_{xy}^{{\rm{s}}}$$ approaches zero.

### Extended Data Fig. 4 Determining Tc from RMCD measurements.

a, Optical image of monolayer, bilayer and trilayer FGT samples. Scale bar, 10 μm. be, Polar RMCD signal as a function of magnetic field recorded in monolayer (b), bilayer (c), trilayer (d) and bulk (e) samples at various temperatures. The bulk sample refers to an 18-layer sample with the thickness determined from optical transmission. f, Remanent RMCD signal at zero magnetic field as a function of temperature obtained from datasets shown in be. The Tc of each sample is determined by the temperature where remanent RMCD signal vanishes. Vertical error bars represent the measurement uncertainty of the remanent RMCD signal.

### Extended Data Fig. 5 Finite-size scaling of Tc in FGT thin flakes.

Tc shown here is obtained from anomalous Hall measurements. We used the highest Tc at each sample thickness for the analysis. Error bars are defined as in Fig. 2d.

### Extended Data Fig. 6 Angle-dependent Hall resistance of a four-layer FGT flake.

a, Rxy as a function of magnetic field recorded at various tilt angles. Data were obtained at T = 1.5 K. b, Rxy as a function of magnetic field recorded at θH = 90° (blue) and θH = 20° (red) at T = 150 K, about 5 K above the Tc determined by anomalous Hall measurements in the same sample. c, θM as a function of θH. θM was extracted from the Rxy data at μ0H = 3 T in a. The solid line is a fit to the Stoner–Wohlfarth model. A Ku value of 5.1 × 105 J m−3 is obtained from the fit, if Ms takes the value of about 1.8μB per Fe atom. The broken line marks θM = θH that corresponds to Ku = 0.

### Extended Data Fig. 7 Ionic gating of an FGT trilayer ionic field-effect transistor.

a, Temperature-dependent RAH at representative Vg under a small external magnetic field of μ0H0 = 0.01 T. The ferromagnetic transition temperature Tc is extracted by extrapolating RAH to zero. b, Sample resistance Rxx as a function of Vg during the up-sweep (blue) and down sweep (red) of Vg. Data were obtained at T = 330 K. c, Tc as a function of Vg during the up-sweep (blue) and down sweep (red) of Vg. Tc exhibits reasonably good reversibility under gate modulation, even though Rxx acquires a large background, possibly due to sample degradation. Data were obtained from the same device discussed in Fig. 3. The error bars are also defined the same way as in Fig. 3. d, Line fits of Rxy as a function of μ0H in the range of 3 T < μ0H < 5 T at three representative gate voltages. RH is obtained as the slope of the line fits. e, Inverse of eRH as a function of Vg during the up-sweep (blue) and down-sweep (red) of the gate voltage. Data were obtained at T = 240 K. Error bars represent the standard deviations of the line fits in d.

### Extended Data Fig. 8 Thickness dependence of the gate modulation in a FGT ionic field-effect transistor.

Sample resistance measured as a function of Vg in trilayer (black), 10-layer (red) and 70-layer (blue) FGT devices. Resistances are normalized by their values at Vg = 0 V.

### Extended Data Fig. 9 Calculated exchange parameters.

a, The top view shows Jin1 to Jin6 for the intralayer (in plane) coupling. b, The side view shows Jz1 to Jz4 for the interlayer (out of plane) coupling. Only Fe ions are shown in the structure. The purple and red atoms are FeI and FeII ions, respectively. c, The left column shows the calculated exchange parameters for bulk FGT, Jini for intralayer and Jzi for interlayer, in a 3 × 3 × 1 bulk supercell. The right column shows the spin exchange parameters in a monolayer 3 × 3 × 1 supercell, denoted as Jxxi (Jyyi) and Jzzi. The units of J are millielectronvolts. The results indicate that the exchange coupling J is largely isotropic.

### Extended Data Fig. 10 LDA calculation of DOS and average magnetic moment as a function of electron doping level.

a, Calculated DOS for trilayer FGT as a function of electron doping level. b, Calculated average magnetic moment in bulk FGT as a function of electron doping level.

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Deng, Y., Yu, Y., Song, Y. et al. Gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2. Nature 563, 94–99 (2018). https://doi.org/10.1038/s41586-018-0626-9

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• DOI: https://doi.org/10.1038/s41586-018-0626-9

### Keywords

• Fegeta (FGT)
• Itinerant Ferromagnetism
• Ga Tion
• Thin Flakes
• Anomalous Hall Effect

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