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Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit

Abstract

Since the discovery of graphene1, the family of two-dimensional materials has grown, displaying a broad range of electronic properties. Recent additions include semiconductors with spin–valley coupling2, Ising superconductors3,4,5 that can be tuned into a quantum metal6, possible Mott insulators with tunable charge-density waves7, and topological semimetals with edge transport8,9. However, no two-dimensional crystal with intrinsic magnetism has yet been discovered10,11,12,13,14; such a crystal would be useful in many technologies from sensing to data storage15. Theoretically, magnetic order is prohibited in the two-dimensional isotropic Heisenberg model at finite temperatures by the Mermin–Wagner theorem16. Magnetic anisotropy removes this restriction, however, and enables, for instance, the occurrence of two-dimensional Ising ferromagnetism. Here we use magneto-optical Kerr effect microscopy to demonstrate that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 kelvin is only slightly lower than that of the bulk crystal, 61 kelvin, which is consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase, highlighting thickness-dependent physical properties typical of van der Waals crystals17,18,19. Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect20, whereas in trilayer CrI3 the interlayer ferromagnetism observed in the bulk crystal is restored. This work creates opportunities for studying magnetism by harnessing the unusual features of atomically thin materials, such as electrical control for realizing magnetoelectronics12, and van der Waals engineering to produce interface phenomena15.

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Figure 1: Crystal structure, layer thickness identification, and MOKE of bulk CrI3.
Figure 2: MOKE measurements of monolayer CrI3.
Figure 3: Layer-dependent magnetic ordering in atomically-thin CrI3.

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Acknowledgements

Work at the University of Washington was mainly supported by the Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division (DE-SC0008145 and SC0012509), and a University of Washington Innovation Award. Work at the Massachusetts Institute of Technology was supported by the Center for Integrated Quantum Materials under NSF grant DMR-1231319 as well as the Gordon and Betty Moore Foundation’s EPiQS Initiative (grant GBMF4541 to P.J.-H.). Device fabrication was supported in part by the Center for Excitonics, an Energy Frontier Research Center funded by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under Award Number DESC0001088. D.H.C.’s contribution is supported by DE-SC0002197. Work at Carnegie Mellon University is supported by DOE BES DE-SC0012509. W.Y. is supported by the Croucher Foundation (Croucher Innovation Award), the RGC of Hong Kong (HKU17305914P), and the HKU ORA. Work at Oak Ridge National Laboratory (M.A.M.) was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. X.X. and D.X. acknowledge the support of a Cottrell Scholar Award. X.X. acknowledges the support from the Clean Energy Institute (funded by the State of Washington) and from a Boeing Distinguished Professorship in Physics.

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Authors and Affiliations

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Contributions

X.X. and P.J.-H. supervised the project. E.N.-M. and M.A.M. synthesized and characterized the bulk CrI3 crystal. E.N.-M. and D.R.K. fabricated the samples and analysed the layer thickness, assisted by G.C. and B.H. B.H. built the MOKE setup with help from E.S. and D.Z. G.C. and B.H. performed the MOKE measurements, assisted by K.L.S. and E.N.-M. R.C., D.X. and W.Y. provided theoretical support. B.H., G.C., E.N.-M., X.X., P.J.-H., D.X. and D.H.C. wrote the paper with input from all authors. All authors discussed the results.

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Correspondence to Pablo Jarillo-Herrero or Xiaodong Xu.

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Extended data figures and tables

Extended Data Figure 1 SQUID magnetometry in bulk CrI3.

a, Zero-field-cooled/field-cooled temperature dependence of the magnetization of a CrI3 bulk crystal with an applied magnetic field of 10 G perpendicular to the basal plane of the sample. The black line is a criticality fit (M = α(1 − TC/T)β) of the data with TC = 61 K and β = 0.125 (Ising universality class). b, Hysteresis loops of the same sample with the external magnetic field in perpendicular and parallel orientations with respect to the CrI3 layers. (emu, electromagnetic units.)

Extended Data Figure 2 Thickness dependence of the optical contrast of the CrI3 flakes.

Optical micrographs of a CrI3 flake illuminated with white light (a) and with 631-nm-(10-nm FWHM bandpass)-filtered light (b). c, AFM topography image of the same sample. d, Optical contrast map extracted from the 631-nm micrograph in b. Scale bars are 5 μm.

Extended Data Figure 3 Computed index of refraction of bulk CrI3.

Real (n) and imaginary (κ) components are plotted as a function of photon energy in the visible range.

Extended Data Figure 4 Fresnel model for the optical contrast C of CrI3 flakes on Si/285 nm SiO2 substrates.

a, Dependence of C with the number of layers for a CrI3 flake as a function of the illumination wavelength. b, Comparison of the experimental data with the computed thickness dependence of C for a red-light-illuminated sample (line cut at 631 nm as shown by the dashed line in a). The different shape markers indicate data coming from different exfoliated samples.

Extended Data Figure 5 AFM and MOKE measurements of graphite-encapsulated few-layer CrI3.

a, Optical microscope image of a bilayer CrI3 flake on 285-nm-thick SiO2. b, AFM data for the CrI3 flake in a encapsulated in graphite, showing a line cut across the flake with a step height of 1.5 nm. c, Optical microscope image of a trilayer CrI3 flake on 285-nm-thick SiO2. d, AFM data for the CrI3 flake shown in c encapsulated in graphite. A line cut taken across the flake shows a step height of 2.2 nm. Both scale bars are 2 μm. e and f show the MOKE signal as a function of applied magnetic field for the encapsulated bilayer in b and the encapsulated trilayer in d respectively.

Extended Data Figure 6 Magneto-optical Kerr effect experimental setup.

Schematic of the optical setup used to measure the MOKE effect in CrI3 samples. 633 nm optical excitation is provided by a power-stabilized HeNe laser. A mechanical chopper and photoelastic modulator provide intensity and polarization modulation, respectively. The modulated beam is directed through a polarizing beam splitter to the sample, housed in a closed-cycle cryostat at 15 K. A magnetic field is applied at the sample using a 7-T solenoidal superconducting magnet in Faraday geometry. The reflected beam passes through an analyser onto a photodiode, where lock-in detection measures the reflected intensity (at fC) as well as the Kerr rotation (at fPEM).

Extended Data Figure 7 Thin-film interference ray diagram of CrI3 on a silicon oxide/silicon substrate.

Light incident on CrI3 undergoes reflections at the CrI3–SiO2 interface (green–blue boundary) as well as the SiO2–Si interface (blue–grey boundary). These reflections interfere with the initial reflection off the CrI3 flake to produce thin-film interference that depends on the CrI3 layer thickness, d1, as well as the SiO2 thickness, d2. The underlying silicon wafer is assumed to be semi-infinite. The indices of refraction for CrI3, SiO2, and Si are n1, n2 and n3 respectively.

Extended Data Figure 8 Additional data for 1–3-layer samples showing Kerr rotation as a function of the applied magnetic field.

The insets show optical microscope images of the CrI3 flakes at 100× magnification. a, Additional monolayer data showing ferromagnetic hysteresis and remanent Kerr signal as a function of applied magnetic field. b, Additional bilayer data showing field-dependent behaviour consistent with an antiferromagnetic ground state. c, Additional trilayer data showing field-dependent behaviour consistent with a ferromagnetic ground state, as well as a larger remanent Kerr signal than the monolayer in a. All scale bars are 5 μm.

Extended Data Figure 9 Additional data for monolayer and bilayer samples under 780-nm excitation and 633-nm excitation.

Insets show optical microscope images of the CrI3 flakes at 100× magnification. a, Kerr rotation as a function of applied magnetic field for a monolayer a (or c), and a bilayer b (or d) under 780-nm (or 633-nm) excitation. All scale bars are 4 μm.

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Huang, B., Clark, G., Navarro-Moratalla, E. et al. Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit. Nature 546, 270–273 (2017). https://doi.org/10.1038/nature22391

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