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Thin films of topological Kondo insulator candidate SmB6: Strong spin-orbit torque without exclusive surface conduction

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Science Advances  19 Jan 2018:
Vol. 4, no. 1, eaap8294
DOI: 10.1126/sciadv.aap8294
  • Fig. 1 Crystal structure characterization and transport properties of SmB6 thin films.

    (A) Crystal structure of SmB6 (top) and epitaxy relation of SmB6/Si(001). (B) Out-of-plane θ/2θ XRD of the 250-nm-thick SmB6/Si(001) film. (C) In-plane φ scan of the Si(220) peak (top) and the SmB6(110) peak (bottom). (D) Temperature dependence of the sheet resistance for the 250-nm-thick film of SmB6/Si(001). Inset: Representative Arrhenius plot of GmeasuredGplateau as a function of 1/T for the 250-nm-thick film, from 50 to 5 K. The red curve shows the linear fitting of ln Ginsulator versus 1/T. (E) Activation energy Δ derived from the Arrhenius plot for various film thicknesses. The dashed lines denote Δ values reported in previous bulk SmB6 studies (18, 19, 35).

  • Fig. 2 Thickness dependence of sheet resistance and sheet carrier density.

    (A) Sheet resistance for SmB6 films with various thicknesses as a function of temperature. Thickness dependence of the sheet conduction of SmB6 thin films at 1.8 K (B) and 100 K (C). Thickness dependence of the sheet carrier concentration n at 1.8 K (D) and 100 K (E).

  • Fig. 3 SOT-induced switching of CFB/W/SmB6 devices.

    (A) Schematic drawing of the device for SOT-induced switching experiment. (B) Hall resistance for the trilayer MgO(1.5)/CFB(1)/W(0.8) (blue) and MgO(1.5)/CFB(1)/W(0.8)/SmB6(50) (red). The latter is magnified 150 times. (C) Current distribution in SmB6 of CFB/W/SmB6 multilayers for various SmB6 film thicknesses. The red dashed line shows the calculation using longitudinal resistivity. SOT switching for CFB(1)/W(0.8)/SmB6(50) measured at 20 K (D) and 300 K (E), with external magnetic fields of 500 Oe (blue) and −500 Oe (red) applied along the x direction. (F) Switching phase diagram of CFB(1)/W(0.8)/SmB6(50) at 20 K.

  • Fig. 4 The SOT originated from the SmB6 film and its relative strength.

    (A) Critical switching current IC for MgO(1.5)/CFB(1)/W(0.8)/SmB6(50) (Embedded Image, red) and the trilayer MgO(1.5)/CFB(1)/W(0.8) (Embedded Image, blue), and the estimated IC for MgO(1.5)/CFB(1)/W(0.8)/SmB6(50) assuming that SmB6 only dilutes the current (black). (B) Percentage of SOT contributed by the SmB6 layer of MgO(1.5)/CFB(1)/W(0.8)/SmB6(t), where the thickness of SmB6 t takes 20 nm (black), 30 nm (blue), and 50 nm (red). (C) Critical current density of 0.8-nm W (purple), 5-nm W (black), and 50-nm SmB6 for switching a CFB layer if in direct contact with them.

  • Fig. 5 SOT-induced switching of Pt/Co/Pt/SmB6 devices.

    (A and B) SOT switching for Pt(1.7)/Co(0.5)/Pt(2)/SmB6(50) measured at 20 K (A) and 300 K (B), with external magnetic fields 300 Oe (blue) and −300 Oe (red) applied along the x direction. (C and D) SOT switching for Pt(1.7)/Co(0.5)/Pt(2) measured at 20 K (C) and 300 K (D), with external magnetic fields of 300 Oe (blue) and −300 Oe (red) applied along the x direction. (E) Nominal critical current density of Pt(1.7)/Co(0.5)/Pt(2)/SmB6(50) (blue) and Pt(1.7)/Co(0.5)/Pt(2) (red) multilayer films, where |Jc| is calculated by dividing the total current by the thickness of the multilayer film. The external in-plane field is 300 Oe. (F) Critical current density required for SmB6(50) that produces enough SOT to switch the Pt/Co/Pt PMA layer, under an in-plane field (Hx) of 300 Oe.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/1/eaap8294/DC1

    section S1. Comparison of the transport properties of SmB6 thin films and single-crystal bulk specimens

    section S2. Magnetoresistance and Hall effect

    section S3. Additional data on crystalline and chemical stoichiometry characterizations

    section S4. SOT-induced switching of perpendicularly magnetized CoFeB

    section S5. Current distribution in multilayer thin films

    fig. S1. Resistance ratio as a function of sample thicknesses.

    fig. S2. Magnetoresistance of SmB6 thin films.

    fig. S3. Hall resistance of SmB6 thin films.

    fig. S4. Additional XRD data of SmB6 thin films.

    fig. S5. XPS spectrum of SmB6 thin films.

    fig. S6. Additional SOT switching results of various temperatures.

    fig. S7. A schematic diagram of the anomalous Hall voltage measurement under the presence of a nonmagnetic shunting layer.

    fig. S8. Temperature dependence of resistance of SmB6 and CoFeB/W multilayers.

    fig. S9. Sheet resistance of CoFeB/W multilayer as a function of W thicknesses.

    References (43, 44)

  • Supplementary Materials

    This PDF file includes:

    • section S1. Comparison of the transport properties of SmB6 thin films and single-crystal bulk specimens
    • section S2. Magnetoresistance and Hall effect
    • section S3. Additional data on crystalline and chemical stoichiometry characterizations
    • section S4. SOT-induced switching of perpendicularly magnetized CoFeB
    • section S5. Current distribution in multilayer thin films
    • fig. S1. Resistance ratio as a function of sample thicknesses.
    • fig. S2. Magnetoresistance of SmB6 thin films.
    • fig. S3. Hall resistance of SmB6 thin films.
    • fig. S4. Additional XRD data of SmB6 thin films.
    • fig. S5. XPS spectrum of SmB6 thin films.
    • fig. S6. Additional SOT switching results of various temperatures.
    • fig. S7. A schematic diagram of the anomalous Hall voltage measurement under the presence of a nonmagnetic shunting layer.
    • fig. S8. Temperature dependence of resistance of SmB6 and CoFeB/W multilayers.
    • fig. S9. Sheet resistance of CoFeB/W multilayer as a function of W thicknesses.
    • References (43, 44)

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