Investigating the differences between Co adatoms states on surfaces of selected bismuth chalcogenides

We report on the electronic and magnetic properties of Co atoms adsorbed on topological insulator (TI) surfaces, such as ${\mathrm{Bi}}_2{\mathrm{Se}}_3, {\mathrm{Bi}}_2{\mathrm{Te}}_3$, and on magnetically doped ${\mathrm{Bi}}_{1.9}{\mathrm{Mn}}_{0.1}{\mathrm{Te}}_3$ and ${\mathrm{Bi}}_{1.98}{\mathrm{Fe}}_{0.02}{\mathrm{Se}}_3$ in comparison to a nontopological insulator surface, ${\mathrm{Bi}}_2{\mathrm{S}}_3$. Co atoms are used as probes of impurity affected surface electronic properties of TI and non-TI tetradymite semiconductors. X-ray spectroscopy, x-ray magnetic circular dichroism, and scanning tunneling microscopy and spectroscopy were employed. We found that the electronic properties of Co adatoms do not depend much on the substrate. In particular, no TI specific phenomenon was found. Magnetic dichroism measurements reveals an in-plane easy axis of adsorbed Co atoms on all topological insulators and ${\mathrm{Bi}}_2{\mathrm{S}}_3$, but not on the ferromagnetic substrate ${\mathrm{Bi}}_{1.9}{\mathrm{Mn}}_{0.1}{\mathrm{Te}}_3$, for which an out-of-plane easy axis was found.

Figure 1

(a) STM image of the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ surface covered by Co adatoms measured at $U=400$ mV and $I=0.4$ nA. The inset presents the high-resolution image of Co atoms (${\mathrm{Co}}_A$ and ${\mathrm{Co}}_B$) occupying two different adsorption sites. (b) STM image of the ${\mathrm{Bi}}_{1.9}{\mathrm{Mn}}_{0.1}{\mathrm{Te}}_3$ surface covered by Co adatoms measured at $I=0.3$ nA; $U=400$ mV. Yellow circles mark areas, where Co atoms surround Mn dopants. (c),(d) STS measurements performed on ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and ${\mathrm{Bi}}_{1.9}{\mathrm{Mn}}_{0.1}{\mathrm{Te}}_3$ after Co deposition, respectively. The current feedback was opened at $U=400$ mV, $I=0.4$ and 0.3 nA. The inset shows $dI/dU$ and $I$($U$) curves (feedback opened at $U=-1.5$ V and $I=0.25$ nA) revealing the charging effect of a Co atom adsorbed on the Mn dopant site.

Figure 2

Panels (a) and (d) present STM constant-current images of Co atoms adsorbed onto cleaved ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_{1.98}{\mathrm{Fe}}_{0.02}{\mathrm{Se}}_3$ surfaces at $I=0.5$ nA, $U=400$ mV and $I=0.4$ nA, $U=550$ mV, respectively. (c) STM image presenting a large overview of the Co covered ${\mathrm{Bi}}_{1.98}{\mathrm{Fe}}_{0.02}{\mathrm{Se}}_3$ surface ($I=0.3$ nA, $U=500$ mV). (b),(e) Tunneling spectra taken on Co atoms at two different hollow sites on ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_{1.98}{\mathrm{Fe}}_{0.02}{\mathrm{Se}}_3$. The spectra were acquired at $U=200$ mV and $I=0.1$ nA; $U=500$ mV and $I=0.3$ nA, respectively.

Figure 3

X-ray-absorption spectroscopy and magnetic circular dichroism at the Co $L_{3,2}$ edges for Co single impurities deposited on (a) ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, (b) ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, and (c) ${\mathrm{Bi}}_2{\mathrm{S}}_3$. The measurements were performed at $T=10$ K. The Co coverage is 0.01, 0.02, and 0.015 monolayer (ML), respectively. The spectra in (a)–(c) are shifted by an offset of approximately 0.75, 0.5, and 0.6 arbitrary units with respect to each other.

Figure 4

Magnetically doped TI surfaces covered by Co adatoms. (a) Co $L_{2,3}$ XAS/XMCD spectra of Co impurities deposited on ${\mathrm{Bi}}_{1.9}{\mathrm{Mn}}_{0.1}{\mathrm{Te}}_3$. (b) Co $L_{2,3}$ XAS/XMCD spectra of Co impurities on ${\mathrm{Bi}}_{1.98}{\mathrm{Fe}}_{0.02}{\mathrm{Se}}_3$. The spectra were recorded at an external magnetic field of 50 kOe, and at $T=10$ K. The spectra in (a) and (b) are shifted by an offset of approximately 0.5 and 0.9 arbitrary units with respect to each other.

Figure 5

(a) $L_{2,3}$ XAS spectra of Fe adatoms (0.17 ML) on ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. (b) $L_{2,3}$ XAS spectra of Co impurities (0.11 ML) deposited on ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. All spectra were recorded at $T=10$ K, $H=50$ kOe with an alignment of the photon helicity with respect to $B$ at an angle $\mathrm{\Theta }=0^{\circ }$ and $\mathrm{\Theta }={70}^{\circ }$ relative to the surface normal. The spectra in (a) and (b) are shifted by an offset of approximately 0.6 arbitrary units with respect to each other.

Figure 6

(a),(b) STM topographs of pristine ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ ($U=0.25$ V; $I=0.1$ nA) and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ ($U=-0.5$ V; $I=0.3$ nA); (c);(d) corresponding STS measurements performed on these surfaces. The spectra for ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ were acquired at $U=0.2$ V; $I=0.1$ nA and $U=0.5$ V; $I=0.3$ nA, respectively.

Figure 7

(a) STM topography of the ${\mathrm{Bi}}_{1.9}{\mathrm{Mn}}_{0.1}{\mathrm{Te}}_3$ surface at $U=200$ mV and $I=0.5$ nA and (b) corresponding STS measurements performed on this surface. The $dI/dU$ spectra for the off dopant area were collected at a distance of about 3 nm from impurities. (c) STM topography of the ${\mathrm{Bi}}_{1.98}{\mathrm{Fe}}_{0.02}{\mathrm{Se}}_3$ surface at $U=500$ mV and $I=0.3$ nA. (d) The line profile revealing different shapes of Fe dopants in (c). (e) $dI/dU$ curves for different Fe dopants. The spectra in (b) and (e) were acquired at $U=-1$ V, $I=0.4$ nA and $U=400$ mV, $I=0.3$ nA, respectively.

Figure 8

Magnetic characterization of ${\mathrm{Bi}}_{1.9}{\mathrm{Mn}}_{0.1}{\mathrm{Te}}_3$ single crystals with an external magnetic field applied parallel to the $c$ crystal axis ($\mathrm{\Phi }=0^{\circ }$), predominantly along the $a-b$ plane ($\mathrm{\Phi }={70}^{\circ }$), and within the $a-b$ plane ($\mathrm{\Phi }={90}^{\circ }$). (a) Temperature dependence of bulk magnetic properties revealing an ordering temperature of $T\simeq 12$ K as shown by inverse susceptibility (main panel), ZFC/FC curves at 100 Oe (inset). (b) Bulk magnetization measured at $T=1.8$ K and Arrott plot (inset); (c) Mn $L_{2,3}$ XAS and XMCD spectra measured at $H=50$ kOe and $T=5$ K at two orientations of the incoming beam (parallel to the external magnetic field) with respect to the surface normal. The XAS spectra are shifted by an offset of approximately 5 arbitrary units with respect to each other.

Figure 9

Magnetic characterization of ${\mathrm{Bi}}_{1.98}{\mathrm{Fe}}_{0.02}{\mathrm{Se}}_3$ single crystals with an external magnetic field applied parallel to $c$ crystal axis ($\mathrm{\Phi }=0^{\circ }$), predominantly along $a-b$ plane ($\mathrm{\Phi }={70}^{\circ }$), and within $a-b$ plane ($\mathrm{\Phi }={90}^{\circ }$). (a) Field dependence of magnetization at $T=1.8$ K. (b) Fe ${\mathrm{L}}_{2,3}$ XAS and XMCD spectra measured at $H=50$ kOe and $T\simeq 5$ K. The XAS spectra are shifted by an offset of approximately four arbitrary units with respect to each other.