Atomically resolved magnetic structure of a Gd-Au surface alloy

The magnetic structure of a monolayer-thick $\mathrm{GdA}{\mathrm{u}}_2$ surface alloy on Au(111) has been investigated down to the atomic level by spin-polarized scanning tunneling microscopy. Spin-resolved tunneling spectroscopy combined with density-functional theory calculations reveal the local spin polarization of both Gd and Au atomic sites within the surface alloy. Moreover, the impact of dislocation lines on the atomic-scale magnetic structure as well as on the local coercive field strength is demonstrated.

Figure 1

(a) Overview STM image of a $\mathrm{GdA}{\mathrm{u}}_2$ surface alloy. Arrows mark dislocation lines. (b), (c) Atomically resolved STM constant-current images and simultaneously measured differential tunneling conductance maps (e), (f) at bias voltages of 3 and 0.75 V corresponding to the energies of the Gd- and Au-derived electronic states, respectively. A model of the surface alloy is shown in both images as well as on the bottom: red corresponds to Gd sites, green for Au sites. A rhombus indicates atoms visible in (b) and (e) and a hexagon shows atoms visible in (c) and (f). A blue spot marks the position of spectra presented in Fig. 2. (d) The 2D-FFT of a $50\times 50-\mathrm{n}{\mathrm{m}}^2$ STM image. A Bragg spot and a second-order moiré spot are marked. Isoheight lines of a 2D Gauss fit are shown in the insets in (d).

Figure 2

(a) Spin-polarized tunneling spectroscopy data obtained at the center of the protrusion within the moiré pattern. Below the corresponding spin asymmetry derived from the measured spin-resolved differential tunneling conductance curves are presented. (b) Calculated PDOS for different surface atoms and their electronic states. Each plot shows violet and green curves for the opposing spin channels and the corresponding spin asymmetry (filled blue). (c) Spin-asymmetry maps for energies corresponding to the Gd- and Au states, as derived from the measured spin-resolved differential tunneling conductance maps shown in Figs. 1 and 1. A model of the surface alloy is overlaid on top of the maps.

Figure 3

(a) SP-STM constant-current image of the $\mathrm{GdA}{\mathrm{u}}_2$ surface alloy with two Au(111) substrate steps. Uncovered Au(111) surface regions are visible at the top and lower left part of the image. (b) A spin-resolved differential tunneling conductance map recorded at −500 mT in-plane field. The signal is homogeneous except for dislocation lines and terrace edges showing additional electronic states. (c) Magnetic hysteresis curve as derived from the spin-resolved differential tunneling conductance signal spatially averaged over the whole area of the SP-STM measurement as a function of external magnetic field. (d) A series of spin-resolved differential tunneling conductance maps recorded as a function of in-plane magnetic-field strength.

Figure 4

(a) SP-STM image of a defect line in the $\mathrm{GdA}{\mathrm{u}}_2$ surface alloy. A model of the alloy is overlaid. (b) Spin-resolved differential tunneling conductance map recorded simultaneously with the constant-current image in (a). Same overlay is used in this panel. (c) Two types of defect lines with shorter and longer Gd-Gd nearest-neighbor distances, respectively.