Research ArticleMATERIALS SCIENCE

Atomic-scale spin-polarization maps using functionalized superconducting probes

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Science Advances  20 Jan 2021:
Vol. 7, no. 4, eabd7302
DOI: 10.1126/sciadv.abd7302
  • Fig. 1 Concept of SP-STM and YSR-SP-STM.

    (A) Schematic drawing of the concept of a conventional SP-STM experiment. The spin-resolved tip DOS and sample LDOS are assumed to be energy independent for simplicity. The thickness of the vertical arrows indicate the strength of the main contribution to the tunnel current. Between the top and the bottom panel, the sample magnetization has been reversed, which enables a measurement of the tunneling magnetoresistance effect. (B) Schematic drawing of the concept of a YSR-SP-STM experiment. A single YSR state in the gap of a superconducting tip that is magnetized, e.g., by an external magnetic field, is assumed for simplicity. Between the top and the bottom panel, the bias polarity has been reversed, which enables the measurement of the absolute value of the sample’s spin polarization. Note that there is an intrinsic asymmetry in the intensity of the ↑ YSR state with respect to the ↓ YSR state that is unrelated to magnetism.

  • Fig. 2 Conventional SP-STM on chains along different crystallographic directions using a Cr bulk tip.

    (A) Sketch of the uppermost Nb(110) layer (brown) with Mn adatoms (red) on hollow sites. Using STM tip–based atom manipulation, the adatoms can be arranged with different options for interatomic spacings and crystallographic directions, which strongly affects their magnetic coupling (FM, ferromagnetic; AFM: antiferromagnetic). (B) Constant-current STM image of a 2a − [001] Mn5 chain, the corresponding dI/dV maps in opposite external magnetic fields Bz, and the calculated ASP. dI/dV is measured in arbitrary units (a.u.). Scale bar, 500 pm. (C) Constant-current STM image of a 3a2[11¯1] Mn13 chain, the corresponding dI/dV maps in opposite external magnetic fields Bz, and the calculated ASP. (D) Constant-current STM image of a 1a − [001] Mn15 chain, the corresponding dI/dV maps in opposite external magnetic fields Bz, and the calculated ASP. All images share the same scale. Parameters: V = −5 mV, I = 1 nA, and Vmod = 2 mV for (B) and (C) and V = −6 mV, I = 1 nA, and Vmod = 2 mV for (D).

  • Fig. 3 Picking up two Fe atoms onto a Nb tip and demonstrating out-of-plane magnetic contrast.

    (A) Constant-current STM images before (left two images) and after picking up the two indicated Fe atoms (right image) onto the tip (V = 6 mV and I = 2 nA). Scale bar, 2 nm. (B) dI/dV spectra taken on the normal conducting Nb(110) substrate in an external field of Bz = 0.4 T with (red) and without (gray) Fe atoms attached to the tip (Vstab = −10 mV, Istab = 2 nA, and Vmod = 100 μV for the YSR tip and Vstab = −6 mV, Istab = 1 nA, and Vmod = 20 μV for the clean Nb tip). (C) Constant-current STM topography (left) and the corresponding dI/dV map (right) of an antiferromagnetic 2a − [001] Mn5 chain measured at Bz = 0.5 T and stabilized outside of the superconducting gap (Vstab = −10 mV, Istab = 2 nA, and Vmod = 3 mV). Scale bar, 0.5 nm. (D) dI/dV spectra taken on the substrate and on the five atoms of the chain (Vstab = −10 mV, Istab = 2 nA, and Vmod = 100 μV) with the YSR tip in an external field of Bz = 0.5 T. The spectra are vertically offset for clarity. (E) Asymmetry AYSR,norm(rt) of the YSR peaks in (D) calculated from Eq. 3 (top) compared to the spin asymmetry ASP(rt) obtained with the Cr tip (bottom) using the data from Fig. 2B and averaging across the atoms and a substrate position. The off-zero dashed gray lines mark the average YSR asymmetry measured on ↑ and ↓ atoms.

  • Fig. 4 Magnetic YSR contrast on densely packed adatom structures.

    (A) Constant-current STM image of a 3a2[11¯1] Mn15 chain and the YSR asymmetry measured along the chain. The bottom panel shows the contrast obtained with a Cr tip on the similar Mn13 chain of Fig. 2C. Scale bar, 1 nm. (B) Constant-current STM image of a 1a − [001] Mn15 chain and the YSR asymmetry measured along the chain. The bottom panel shows the contrast obtained with a Cr tip on the same Mn15 chain in Fig. 2D. The nonzero dashed gray lines mark the average YSR asymmetry measured on ↑ and ↓ atoms from Fig. 3E. All data were measured with Vstab = −10 mV, Istab = 2 nA, Vmod = 100 μV, and Bz = 0.5 T.

Supplementary Materials

  • Supplementary Materials

    Atomic-scale spin-polarization maps using functionalized superconducting probes

    Lucas Schneider, Philip Beck, Jens Wiebe, Roland Wiesendanger

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