Research ArticleSPINTRONICS

Observation of current-induced, long-lived persistent spin polarization in a topological insulator: A rechargeable spin battery

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Science Advances  14 Apr 2017:
Vol. 3, no. 4, e1602531
DOI: 10.1126/sciadv.1602531
  • Fig. 1 Detection of current (Id)–independent spin signal in a Bi2Te2Se (BTS221) thin flake by spin potentiometry.

    (A) Schematic 3D device structure used in the potentiometric measurement, showing a three-terminal electrical connection. The TI surface defines the x-y plane, and the surface normal defines the z direction. The two outside nonmagnetic (for example, Au) contacts (the left is grounded) are used to inject a DC bias current, and the middle FM (for example, Py) contact is magnetized by an in-plane magnetic B field (labeled) along its easy axis (y direction, with −y defining the positive B and M direction). The middle FM contact is a tunneling probe (with a thin Al2O3 tunnel barrier underneath in our devices). (B) Optical image of device A used in the potentiometric measurement. (C to H) Voltage measured by the FM spin detector (Py) on device A as a function of in-plane magnetic field for representative bias currents (Id) of 100 pA (C), −100 pA (D), 10 nA (E), −10 nA (F), 1 μA (G), and −1 μA (H). The directions of Id (red arrow), the inferred channel (top surface) spin polarization S (dashed green arrow), and Py magnetization M (black arrow) are labeled in (G) and (H). (I) The voltage change δV = V+MV−M [marked by the arrow in (E) and defined as the spin signal indicating the presence of channel spin polarization] as a function of the applied DC bias current. Here, δV is extracted from forward sweep traces (backward sweeps yield similar results). All the measurements were performed at the temperature T = 0.3 K.

  • Fig. 2 Writing current effect on the spin signal.

    (A and B) Schematic four-terminal device structures and measurement setups for writing current and spin potentiometry. (C to H) Voltage detected by the FM contact as a function of in-plane magnetic field measured on device B at the relatively small Id of 0.5 μA (C, E, and G) and −0.5 μA (D, F, and H) after applying a large Iw of −40 μA for 2 hours (C and D), 50 μA for 0.5 hours (E and F), and −40 μA for 3 hours (G and H). The trend of the B field–induced voltage change, which measures the direction of the channel spin polarization, is independent of the relatively small Id but is set by the direction of the large Iw (applied at B = 0 T and before the spin potentiometric measurement). The directions of Id, Iw, channel spin polarization S as determined by the spin signal, and Py magnetization M are labeled by the corresponding arrows. All the measurements are performed at T = 1.6 K.

  • Fig. 3 Spin signal transitioning from being largely independent on the bias current (Id) to linearly dependent on Id when Id increases.

    (A to J) The voltage detected by the FM contact as a function of in-plane magnetic field measured on device C for representative bias currents (Id) of 0.01 μA (A), −0.01 μA (B), 5 μA (C), −5 μA (D), 10 μA (E), −10 μA (F), 20 μA (G), −20 μA (H), 80 μA (I), and −80 μA (J). The upper and lower sets of traces in (E) show two repeated sets of measurements. The directions of the bias current Id, channel spin polarization S as determined by the spin signal, and Py magnetization M are labeled by the corresponding arrows in (A), (B), (I), and (J). (K) The spin signal δV as a function of the bias current Id. All measurements were performed with a four-terminal configuration at T = 1.6 K.

  • Fig. 4 Dependences of the spin signal on out-of-plane B field component, gate voltage, and temperature.

    (A and B) Effect of perpendicular (out-of-plane) magnetic field on spin signals. In this experiment, the magnetic B field was tilted away from the sample (device A) surface plane (x-y) by an angle θ [depicted in the inset in (A)]. Data are measured at Id = −100 nA and T = 0.3 K. (A) Magnetic field dependence of the voltage at different angles. (B) The coercive field Bc [marked in (A) for the θ = 54° (trace)], Bc·cosθ (in-plane component of Bc), and spin signal δV as functions of θ. (C and D) Gate dependence of the spin signal measured on device D. (C) The measured voltage V as a function of the in-plane magnetic field measured at T = 1.6 K and Id = −1 μA (top) and 1 μA (bottom) at various back-gate voltages Vg. (D) The extracted spin signal δV (left axis; black symbols) and the voltage V0 (right axis; red dashed line; measured at B = 0 T before performing the spin potentiometry) as a function of Vg. The measurements were performed at T = 1.6 K. (E) Temperature T dependence of the measured spin signal from device E up to 76 K. Devices D and E have a four-terminal configuration (Fig. 2B)

  • Fig. 5 Persistent spin signal and its time dependence.

    (A) Open-circuit voltage detected by the FM contact as a function of the in-plane magnetic B field measured on device F with the source-drain electrical connection open, showing a spin signal δV ~1.3 μV. (B) Open-circuit voltage versus B measured after t = 45 hours. (C) Spin signal δV versus elapsed time t, where δV gradually decreases to ~0.5 μV in the first 3 hours and stays around ~0.6 μV in the next 42 hours. The measurements in (A) to (C) were performed at T = 1.6 K. Inset shows the schematic of the open-circuit measurement in device F. (D) Open-circuit voltage versus B measured at T = 45 K. (E) Open-circuit voltage versus B measured at T = 45 K after t = 10 hours. (F) δV versus t measured at T = 45 K. The directions of Py magnetization M (black arrow) and the inferred channel (top surface) spin polarization S (dashed green arrow) are labeled in (A), (B), and (D).

  • Fig. 6 Comparison of the spin polarization lifetime that we measured with the lifetime of the NSP reported in different systems from previous literature.

    (A) Spin polarization lifetime as a function of temperature and (B) spin polarization lifetime as a function of magnetic B field. The red superscripts refer to the corresponding references. Previous data measured at B = 0 T are highlighted in green. QD, quantum dot.

  • Fig. 7 Schematics depicting a proposed mechanism of spin polarization transfer between the electrons and nuclei via the spin flip-flop process due to the hyperfine interaction.

    (A and B) Current-induced NSP: A large writing current Iw (A) induces (via SML of TSS) an ESP, which dynamically polarizes nuclear spins by the hyperfine interaction–enabled electron-nuclear spin flip-flop process. Reversing Iw (B) can reverse the NSP due to the reversed TSS ESP. (C and D) An existing NSP induces an ESP through hyperfine interaction–enabled spin flip-flop process, depicted as spin-down (C) and spin-up (D) polarized cases. Because of the long lifetime of nuclear spins, the induced ESP (and the measured spin signal) can persist for a long time (even in the absence of any driving current) but can be reversed by a large Iw (A and B) that reverses the nuclear spins.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/4/e1602531/DC1

    note S1. More discussions on the possible mechanisms for the persistent spin polarization, dynamical spin polarization between TSS electrons and nuclear spins, and the interplay of the SML of TSS electrons and hyperfine interaction between the TSS electron spins and nuclear spins.

    fig. S1. Characterization of the magnetic (Py) electrodes.

    fig. S2. Complete data set on the current dependence of the measured spin signal in device A.

    fig. S3. Spin signal largely independent on Id measured in additional devices.

    fig. S4. The writing current effect on the spin signal during cooling.

    fig. S5. The writing current effect on the spin signal measured at 26 K.

    fig. S6. Effect of the magnitude of writing current Iw on the spin signal.

    fig. S7. Effect of current writing time on the spin signal.

    fig. S8. Temperature dependence of the measured spin signal.

    table S1. Lifetime of the NSP reported in different systems from previous literature.

    References (5368)

  • Supplementary Materials

    This PDF file includes:

    • note S1. More discussions on the possible mechanisms for the persistent spin polarization, dynamical spin polarization between TSS electrons and nuclear spins, and the interplay of the SML of TSS electrons and hyperfine interaction between the TSS electron spins and nuclear spins.
    • fig. S1. Characterization of the magnetic (Py) electrodes.
    • fig. S2. Complete data set on the current dependence of the measured spin signal in device A.
    • fig. S3. Spin signal largely independent on Id measured in additional devices.
    • fig. S4. The writing current effect on the spin signal during cooling.
    • fig. S5. The writing current effect on the spin signal measured at 26 K.
    • fig. S6. Effect of the magnitude of writing current Iw on the spin signal.
    • fig. S7. Effect of current writing time on the spin signal.
    • fig. S8. Temperature dependence of the measured spin signal.
    • table S1. Lifetime of the NSP reported in different systems from previous literature.
    • References (53–68)

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