Dirac-fermion-mediated ferromagnetism in a topological insulator

Journal name:
Nature Physics
Volume:
8,
Pages:
729–733
Year published:
DOI:
doi:10.1038/nphys2388
Received
Accepted
Published online

Topological insulators are a newly discovered class of materials in which helical conducting modes exist on the surface of a bulk insulator1, 2, 3, 4, 5, 6. Recently, theoretical works have shown that breaking gauge symmetry7 or time-reversal symmetry8 in these materials produces exotic states that, if realized, represent substantial steps towards realizing new magnetoelectric effects9, 10 and tools useful for quantum computing11. Here we demonstrate the latter symmetry breaking in the form of ferromagnetism arising from the interaction between magnetic impurities and the Dirac fermions12, 13. Using devices based on cleaved single crystals of Mn-doped Bi2Te3−ySey, the application of both solid-dielectric and ionic-liquid gating allows us to measure the transport response of the surface states within the bulk bandgap in the presence of magnetic ions. By tracking the anomalous Hall effect we find that the surface modes support robust ferromagnetism as well as magnetoconductance that is consistent with enhanced one-dimensional edge-state transport on the magnetic domain wall. Observation of this evidence for quantum transport phenomena demonstrates the accessibility of these exotics states in devices and may serve to focus the wide range of proposed methods for experimentally realizing the quantum anomalous Hall effect8, 10 and states required for quantum computing14, 15.

At a glance

Figures

  1. Ferromagnetism in a magnetically doped topological insulator.
    Figure 1: Ferromagnetism in a magnetically doped topological insulator.

    a, Schematic of a Dirac electron with spin σ and momentum k on the surface of a topological insulator with an average z component of local spins . In the TRS broken state, the competition between the exchange interaction and the Rashba-like surface texture yields a finite component of σ in the z direction. b, Breaking of TRS opens a gap in the surface state spectrum starting at the Dirac point located at the time reversal invariant momentum Γ. c, Bulk crystals of MnxBi2−xTe3−ySey show ferromagnetic ordering below TC=13K, as seen in M(T). Both σxx and Δσxy (the spontaneous component of σxy) show anomalies at TC. The inset shows scaling of M and the anomalous part of the Hall conductivity σxyA. d, Device structure for measurement of microcrystals. A back gate is formed by the SiO2/Si substrate and a top gate with the deposition of ionic liquid. e, Atomic force micrograph (false colour) of Ti/Au contacts made by electron beam lithography for device B. The crystal thickness is 7nm. f, Measurements of Δσxy for device A indicate a ferromagnetic transition. Application of the back-gate voltage V B is shown to tune the magnetic response with varying of the transition temperature TC.

  2. AHE and
TC tuned by carrier density.
    Figure 2: AHE and TC tuned by carrier density.

    a, Hall conductivity σxy at T=2K for device A at several back-gate voltages V B, offset vertically 0.04 e2/h. The observed anomalous Hall response grows on depletion of carriers. b, After application of V T=−3V, device B shows a further enhanced spontaneous σxy whereas the ordinary Hall response σxyN changes from n-type to p-type at the most negative V B. Here, a vertical offset of 0.4 e2/h is used. c, Temperature dependence of σxy in device C. There is a sharp onset of σxyA below 12K. See Fig. 1f for the case of device A. d, Estimates of TC for devices A–E. Contrary to conventional dilute magnetic semiconductors, the transition is suppressed on increasing the carrier density. The error bars reflect the discreteness of the measurement in T. The inset depicts the critical chemical potential position μc for the onset of ferromagnetic behaviour, as estimated from σxyN relative to the conduction band (CB), valence band (VB) and surface states (SS).

  3. Sign reversal in magnetoconductivity driven by gating for device A.
    Figure 3: Sign reversal in magnetoconductivity driven by gating for device A.

    a, Back-gate voltage V B dependence of σxx, showing the onset of enhanced conductivity at the reversal of M on depletion of carriers at T=2K. b, Virgin curve and trained behaviour for V B=−100V (n2D=1.5×1013e−2cm−2). c, Schematic depiction of the domain structure in a magnetic topological insulator; domain walls across the opposite M domains support a chiral mode (shown in green). dT dependence of σxx(B) through the ferromagnetic transition gated with V B=−100V. The enhanced conductivity can be seen to track the M reversal. eσxx(B) at high carrier density shows hysteresis of the conventional form, that is, suppressed conductivity attributed to carrier scattering at magnetic domain walls. f, The difference between the virgin and trained σxx (≡σxxv) exhibits a crossover from positive to negative values on increased carrier density.

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Affiliations

  1. Cross-Correlated Materials Research Group (CMRG) and Correlated Electron Research Group (CERG), RIKEN-ASI, Wako 351-0198, Japan

    • Joseph G. Checkelsky,
    • Yoshihiro Iwasa &
    • Yoshinori Tokura
  2. Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, Hongo, Tokyo 113-8656, Japan

    • Jianting Ye,
    • Yoshinori Onose,
    • Yoshihiro Iwasa &
    • Yoshinori Tokura
  3. Multiferroics Project, ERATO, Japan Science and Technology Agency (JST), c/o University of Tokyo, Hongo, Tokyo 113-8656, Japan

    • Yoshinori Onose &
    • Yoshinori Tokura

Contributions

J.G.C. and Y.O. performed the single-crystal growth and experiments. J.G.C. and J.Y. made devices and performed device experiments aided by Y.O. J.G.C. analysed the data and wrote the manuscript with contributions from all authors. Y.I. and Y.T. contributed to discussion of the results and guided the project. Y.T. conceived and coordinated the project.

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The authors declare no competing financial interests.

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