Research ArticleMATERIALS SCIENCE

Manipulating surface magnetic order in iron telluride

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Science Advances  01 Mar 2019:
Vol. 5, no. 3, eaav3478
DOI: 10.1126/sciadv.aav3478
  • Fig. 1 Spin-polarized STM of Fe1.06Te.

    (A) Topographic STM image taken with a nonmagnetic tip (14.5 by 14.5 nm2). Protrusions are excess Fe atoms. Inset: Fourier transform (FT) image of (A). Peaks that arise from the Te lattice are highlighted with solid circles. (B) Topographic SP-STM image taken at the same position as (A) with a magnetic tip. Stripes arise from the AFM order. Inset: FT image of (B) showing additional peaks due to the AFM order. (C and D) Topographic SP-STM images taken at the same position with the tip polarized along two opposite in-plane directions (1.9 by 2.6 nm2). Tunneling parameters for (A) to (D): V = 100 mV, I = 50 pA. (E) Difference image of (C) and (D). The height difference is proportional to the spin polarization of the tunneling current. Inset at the bottom left: Structural model of the Fe1+xTe surface, showing the spin order in the Fe lattice (red). Inset at top right: DFT calculation of the magnetic contrast due to the spin polarization at the Fermi energy (see section S1).

  • Fig. 2 Spin spiral in Fe1.16Te.

    (A) Topographic SP-STM image (8.2 by 4.6 nm2, V = 50 mV, I = 200 pA). (B to D) Magnetic images taken at out-of-plane angles θ = −90°, −30°, and 30°. Close inspection reveals that the positions of the maxima of the magnetic order shift as a function of out-of-plane angle θ. (E) Line cuts through magnetic images as shown in (B) to (D) along a for different out-of-plane field angles. The line cuts show the shift of the maxima of the stripes. (F) Plot of the phase of the stripes shown in (E) as a function of field angle θ. The phase has been extracted using the maximum marked by an arrow in (E). Measurements were taken at an in-plane angle φ = 120° from the crystal a axis.

  • Fig. 3 Manipulation of surface excess iron concentration.

    Model of Fe1+xTe (A) before and (B) after removal of excess iron. Red (yellow) spheres are Fe (Te) atoms. Dashed open circles mark the interstitial/excess Fe atoms at the surface layer that are removed during surface manipulation, leading to a 50% reduction in the concentration of Fe interstitials in the surface layer (to x/2) compared to that of the bulk (x). (C) SP-STM image of Fe1.12Te (65.3 by 28.2 nm2, V = 150 mV, I = 50 pA) showing an area where surface excess iron has been removed by the tip (blue) next to one where the excess iron has been left untouched (green). (D) Fourier transform of (C) showing magnetic peaks due to the bicolinear order (blue arrow) and at an incommensurate position (green arrow, see fig. S5 for Fourier transforms of regions with high and low excess iron concentrations at the surface). (E) Line cut from the Fourier transform in (D) taken along the aTe direction. Peaks corresponding to the bicolinear order at q = (0.5, 0) and the incommensurate order q = (0.39, 0) are highlighted by a blue arrow and a green arrow, respectively. (F and G) Maps of the intensity of the magnetic order at the wave vector of the bicolinear order [q = (0.5, 0)] and of the incommensurate order [q = (0.39, 0)]. Both have been obtained through Fourier filtering at the respective wave vector and then low pass filtering of the modulus. The maps show that the bicolinear order predominantly exists in regions that have been cleaned of Fe, while the incommensurate order is dominant in regions where Fe is still present. a.u., arbitrary units.

  • Fig. 4 Magnetic order in a surface layer of Fe1.06Te on Fe1.12Te.

    (A) Image of the magnetic order (20.5 by 20.5 nm2) projected onto an in-plane direction of the magnetization as indicated by the arrows. (B) As in (A), taken with the same tip with an out-of-plane direction of the magnetization (V = 100 mV, I = 50 pA). (C) FT image of (B), with intensity at the center suppressed for clarity. Peaks due to the AFM order along a and b are marked with pink and cyan circles, respectively. (D) Normalized line cuts taken from the origin along a (red) and b (blue) in (C). Dashed lines indicate the positions qTe and qAFM along both a and b directions.

  • Fig. 5 Plaquette order in on Fe1.1Te on Fe1.2Te.

    (A) Nonmagnetic image (for details, see section S5 and fig. S6). Inset: FT image showing peaks due to the Te lattice. (B) Image of the magnetic structure for φ = −27°. AFM order can be seen along both lattice directions. Inset: FT image of (B), with peaks due to AFM order in the a (b) direction marked with pink (blue) circles. (C and D) As in (B), with (C) φ = 27° and (D) out-of-plane magnetic field (θ = 90°). Insets show the corresponding FT images. Images (A) to (D) were recorded in the same area (24.1 by 24.1 nm2), V = −40 mV, I = 100 pA. (E) Integrated intensities of the magnetic peaks in the FT image as a function of in-plane angle φ. The horizontal dashed line indicates the integrated intensity of a point away from the magnetic peaks. Blue (red) markers are the intensities of the qb (qa) peak. Solid lines are numerical fits of I = I0|cos (φ − φ0)| + c to the data. Vertical dashed lines indicate the in-plane field directions parallel to the a and b axes, as well as that of maximum intensity (φ = 117°). (F) As in (E), plotted as a function of out-of-plane angle θ at in-plane angle of maximum intensity of the magnetic order, φ = 117°. Data shown here were recorded in the same location and with the same tip.

  • Fig. 6 Magnetic structures and phase diagram of a surface layer of Fe1+x/2Te.

    (A) Model structures of surface layers of Fe1.03Te/Fe1.06Te, Fe1.06Te/Fe1.12Te, and Fe1.1Te/Fe1.2Te that are consistent with the SP-STM results. The magnetic unit cell in each case is highlighted (for details of the model, see section S7). Arrows indicate Fe spins and are colored blue if they have a positive component along b and red if they have a negative component. Green and gray spheres represent the upper and lower Te atoms. (B) Phase diagram of the magnetic order in the surface layer after removal of excess iron (with concentration Fe1+x/2Te) as a function of bulk excess Fe concentration x. Red dots represent the ratio of the intensities I(qb)/I(qa) of the magnetic order along the lattice directions a and b taken from the Fourier transforms of the STM data. The blue diamonds show the wave vector of the magnetic order in terms of the lattice spacing from neutron scattering from (2); the black dotted lines depict the “mixed spin density wave (SDW)” phase defined there. The gray background highlights how the lattice parameters change with excess Fe concentration and is defined by Embedded Image, using values for a and b given in (2) (see also table S1). (C to E) SP-STM images (7 by 7 nm2) of out-of-plane polarization of a surface layer of (C) Fe1.03Te in the monoclinic phase and (D) Fe1.06Te and (E) Fe1.1Te in the orthorhombic phase.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/3/eaav3478/DC1

    Section S1. DFT calculation of magnetic contrast

    Section S2. SP-STM study of the magnetic structure of Fe1.06Te

    Section S3. Incommensurate order in Fe1.16Te

    Section S4. Manipulating the surface excess Fe concentration

    Section S5. Alternative method to determine sample spin polarization

    Section S6. Model for the magnetic structure at x = 0.12

    Section S7. Model for the magnetic structure at x = 0.2

    Fig. S1. Spin-polarized imaging at low excess iron concentrations x < 0.12.

    Fig. S2. Spin-polarized imaging at high excess iron concentrations x > 0.12.

    Fig. S3. Manipulation of surface excess iron with aggressive tunneling parameters.

    Fig. S4. Manipulation of surface excess iron with moderate tunneling parameters.

    Fig. S5. Manipulating surface magnetic order.

    Fig. S6. Extracting surface spin polarization.

    Fig. S7. Simulated SP-STM images for x = 0.12.

    Table S1. Crystal structure of Fe1+xTe at different excess iron concentrations x.

    References (3237)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. DFT calculation of magnetic contrast
    • Section S2. SP-STM study of the magnetic structure of Fe1.06Te
    • Section S3. Incommensurate order in Fe1.16Te
    • Section S4. Manipulating the surface excess Fe concentration
    • Section S5. Alternative method to determine sample spin polarization
    • Section S6. Model for the magnetic structure at x = 0.12
    • Section S7. Model for the magnetic structure at x = 0.2
    • Fig. S1. Spin-polarized imaging at low excess iron concentrations x < 0.12.
    • Fig. S2. Spin-polarized imaging at high excess iron concentrations x > 0.12.
    • Fig. S3. Manipulation of surface excess iron with aggressive tunneling parameters.
    • Fig. S4. Manipulation of surface excess iron with moderate tunneling parameters.
    • Fig. S5. Manipulating surface magnetic order.
    • Fig. S6. Extracting surface spin polarization.
    • Fig. S7. Simulated SP-STM images for x = 0.12.
    • Table S1. Crystal structure of Fe1+xTe at different excess iron concentrations x.
    • References (3237)

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