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Probing magnetism in 2D van der Waals crystalline insulators via electron tunneling

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Science  15 Jun 2018:
Vol. 360, Issue 6394, pp. 1218-1222
DOI: 10.1126/science.aar3617
  • Fig. 1 Experimental setup.

    (A) Optical micrograph of a tetralayer CrI3 tunnel junction device (device D1, false color). The dashed line encloses the tunnel junction area. The graphite contacts are themselves contacted by Au/Cr wires in a four-point geometry. Inset: Schematic of the van der Waals heterostructures studied in this work. Electrons tunnel between two graphite sheets separated by a magnetic CrI3 tunnel barrier. The entire stack is encapsulated in hexagonal boron nitride. (B) Schematic energy diagram of a metal/ferromagnetic insulator/metal junction. The red and blue lines in the barrier region represent the spin-up and spin-down energy barriers, respectively. The lower barrier for spin-up electrons leads to spin-polarized tunneling and reduced resistance for a ferromagnetic barrier. (C) Zero-bias junction resistance versus temperature for device D1 cooled with (purple) and without (black) an applied magnetic field. The curves begin to deviate around the bulk Curie temperature (61 K), giving evidence for magnetic order in the CrI3 barrier and for spin-polarized tunneling. The magnetic field was applied perpendicular to the CrI3 layers.

  • Fig. 2 Magnetoconductance of few-layer CrI3.

    (A) Conductance through a bilayer CrI3 tunnel barrier (device D2) as a function of an out-of-plane applied magnetic field with 500-μV AC excitation. The data were taken both for decreasing (purple line, left arrow) and increasing (black line, right arrow) magnetic field. The magnetoconductance traces are consistent with previous magnetometry data (4) for bilayer CrI3 showing that the two layers are antiparallel at zero field but can be aligned with an external field below 1 T. (B and C) Schematic of barriers experienced by spin-up and spin-down electrons tunneling through bilayer CrI3 in the low-field (B) and high-field (C) states. In the low-field state, the two layers are antiparallel, and both spins see a high barrier. In the high-field state, the layers are aligned and up spins see a low-energy barrier, leading to increased conductance. (D to F) Analogous data and schematics for a tetralayer CrI3 barrier device (device D3). In both cases, the sample temperature was 300 mK.

  • Fig. 3 Origin of magnetoresistance in CrI3.

    (A) Magnetoresistance ratio (black circles) versus CrI3 layer number for multiple devices at 300 mK. We also plot a fit to the spin filter model (purple stars). The only fitting parameter, TP/TAP = 3.5, gives the ratio of spin-up to spin-down transmission through a CrI3 monolayer. (B) Resistance-area product versus CrI3 layer number for multiple devices. The resistances are measured in the fully aligned magnetic configuration and were taken at zero bias. (C) Electronic structure of a trilayer graphite/trilayer CrI3 heterostructure calculated with density functional theory. The CrI3 is in the fully ferromagnetic configuration, and its bands are projected on the spin-up and spin-down channels. Although the minority spins do not show states close to the Fermi energy, there are a large number of states in the majority channel. The difference establishes a microscopic basis for the large TP/TAP that we observe.

  • Fig. 4 Inelastic tunneling spectroscropy.

    (A) Top panel: Differential conductance versus a DC bias voltage for a bilayer CrI3 barrier device (D2) at zero applied magnetic field. The AC excitation was 200 μV and the temperature was 300 mK. Bottom panel: Absolute value of d2I/dV2 versus a DC bias voltage, obtained via numerical differentiation of the data in the top panel. According to the theory of inelastic tunneling spectroscopy, the peaks in d2I/dV2 correspond to phonon or magnon excitations of the barrier or electrodes. (B) |d2I/dV2| (color scale at right) versus applied magnetic field and DC bias voltage. All three inelastic peaks increase in energy as the applied field is increased. (C) Energy of the two lowest-energy inelastic peaks versus applied magnetic field. The zero-field energy is subtracted from both peaks for clarity. The peak locations were determined by Gaussian fits to the data. The error bars represent estimated standard deviations calculated from the least-squares fitting procedure. The dashed gray line shows the Zeeman energy shift of a 2μB magnetic moment (0.12 meV/T), which roughly matches the evolution of the 3-meV peak. (D) Calculated magnon density of states (DOS) for CrI3. The details of the calculations are described in the supplementary text. (E) Calculated dispersion of magnons with applied magnetic field at zero temperature. (F) Calculated renormalized magnon dispersion with magnetic field at finite temperature (T = 0.033J, where J is the nearest-neighbor exchange).

Supplementary Materials

  • Probing magnetism in 2D van der Waals crystalline insulators via electron tunneling

    D. R. Klein, D. MacNeill, J. L. Lado, D. Soriano, E. Navarro-Moratalla, K. Watanabe, T. Taniguchi, S. Manni, P. Canfield, J. Fernández-Rossier, P. Jarillo-Herrero

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Supplementary Text
    • Figs. S1 to S8
    • Table S1
    • References

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