Corresponding author

Peter Wahl
Email:wahl@...

University of St Andrews, St Andrews, UK

Homepage of Peter Wahl in St Andrews

References

1.
Bao, W.; Qiu, Y.; Huang, Q.; Green, M.A.; Zajdel, P.; Fitzsimmons, M.R.; Zhernenkov, M.; Chang, S.; Fang, M.; Qian, B.; Vehstedt, E.K.; Yang, J.; Pham, H.M.; Spinu, L.; Mao. Z.Q.
Tunable (δπ,δπ)-type antiferromagnetic order in α-Fe(Te, Se) superconductors
2.
Wiesendanger, R.
Spin mapping at the nanoscale and atomic scale
3.
Enayat, M.; Sun, Z.; Singh, U.R.; Aluru, R.; Schmaus, S.; Yaresko, A.; Liu, Y.; Lin, C.; Tsurkan, V.; Loidl, A.; Deisenhofer, J.; Wahl, P.
Real-space imaging of the atomic-scale magnetic structure of Fe1+yTe

In collaboration with:

V. Tsurkan, A. Loidl, J. Deisenhofer (Universität Augsburg)

Research Group "Tunneling Spectroscopy of Strongly Correlated Electron Materials"

Atomic-scale imaging of magnetic order in strongly correlated electron systems

Authors

R. Aluru, M. Enayat, Z. Sun, U. R. Singh, S. Schmaus, A. Yaresko, Y. Liu, C. T. Lin, and P. Wahl

Departments

Research Group "Tunneling Spectroscopy of Strongly Correlated Electron Materials"

Many unconventional superconductors – including the well-known copper-oxide superconductors – have a phase diagram in which magnetism and superconductivity occur close to each other. This suggests an intimate relationship between the two which might hold the clue to the mechanism of superconductivity in these materials. Spin-polarized Scanning Tunneling Microscopy (SP-STM) allows characterizing superconductivity and magnetism in the same measurement at the atomic scale. We have demonstrated SP-STM on the non-superconducting parent compound of the iron chalcogenide superconductors.

Introduction

Iron tellurium is the nonsuperconducting parent compound of the iron chalcogenide superconductors, in which superconductivity is induced by the substitution of Te with Se. Among the parent compounds of the iron-based superconductors, the magnetic structure of FeTe is unique, as it is rotated by 45° with respect to the magnetic structure found in the iron-pnictide materials and is also not connected to a nesting vector between different sheets of the Fermi surface. The magnetic structure of Fe1+yTe exhibits at low excess iron concentrations y a bicolinear antiferromagnetic order [1], with lines of iron atoms which are aligned ferromagnetically. The transition to magnetic ordering is accompanied by a structural phase transition to a monoclinicallz distorted phase. Spin-polarized Scanning Tunneling Microscopy a technique well established for the study of magnetic nanostructures at surfaces offering insights into magnetic order with high spatial resolution [2]. Spin-polarized STM yields atomically resolved images of the magnetic order in a material as it is sensitive to the magnetization of the sample. It relies on preparation of a magnetic tip, typically by coating a metallic tip in ultra high vacuum (UHV) with a magnetic material.The tip preparation is both experimentally and from the instrument perspective challenging, as it requires an insitu tip exchange mechanism as well as a reproducible and established recipe for tip preparation. These experimental challenges have hindered application of spin-polarized STM to strongly correlated electron systems so far. Here, we report real space imaging of the magnetic structure in Fe1+yTe, the non-superconducting parent compound of the iron chalcogenides, by low temperature spin-polarized scanning tunneling microscopy [3].

STM Experiments

The experiments were performed in two home-built low temperature STMs, one operating at temperatures down to 10 mK and one down to 1.5 K, both operating in cryogenic vacuum and in magnetic fields up to 14 T. Samples are prepared by in-situ cleaving at low temperatures. Non-magnetic STM tips are cut from PtIr and Vanadium, which have subsequently been prepared by field emission on a Au(111) single crystal.

Preparation of magnetic tips

Rather than employing an elaborate tip coating recipe, the approach we have taken is to pick-up individual iron atoms as well as clusters from the surface of iron tellurium. The weakly bound iron tellurium planes enable cleavage between the iron tellurium layers, resulting in a tellurium-terminated surface. Iron tellurium (Fe1+yTe) contains naturally a certain amount (y) of excess iron, which resides in between the layers of iron tellurium (FeTe). The excess iron atoms are only weakly bound to the surface, and can be picked-up from the surface on bringing the STM tip sufficiently close. Magnetic tips have been obtained in two different ways: (1) by scanning at large tunneling current or small bias voltage, collecting excess iron atoms from the surface and therefore rendering the tip magnetic, (2) by indentation of the tip into the sample surface, thereby picking up clusters of FeTe. Using these tip preparation techniques we were able to prepare both ferromagnetic and antiferromagnetic tips. Successful preparation of a magnetic tip is confirmed by the observation of an additional modulation in topographic STM images.

Magnetic Structure

<strong>Fig. 1:</strong> Magnetic Order in FeTe and its detection in STM. (a) topography z(x) of a Fe<sub>1+y</sub>Te sample with <em>y</em> = 0.08, acquired with a tip which shows no magnetic contrast, excess iron atoms show up as protrusions (<em>V</em><sub>b</sub> = 60 mV, <em>I</em><sub>t</sub> = 200 pA, <em>T</em> = 3.8K). Next to the topography, its Fourier transform z̃(q) is displayed, showing the peaks associated with the Te lattice at q<sup>a</sup><sub>Te</sub> = (&plusmn;1, 0) and q<sup>b</sup><sub>Te</sub> = (0,&plusmn;1). One pair of peaks, marked q<sub>CDW</sub> (= q<sup>a</sup><sub>Te</sub>) shows up with stronger intensity than the other; (B) topography acquired in the same place as A with a tip which shows magnetic contrast (<em>V</em><sub>b</sub> = 60 mV, <em>I</em><sub>t</sub> = 200 pA, <em>T</em> = 3.8 K) with its Fourier transform, showing beside the peaks due to the Te lattice as in (a) additional ones associated with magnetic order at q<sub>AFM</sub> = (&plusmn;1/2, 0). (c) Topographic image of a twin boundary. The stripes in the two domains are perpendicular to each other (<em>T</em> = 30 mK, <em>V</em><sub>b</sub> = 150 mV, <em>I</em><sub>t</sub> = 30 pA).<strong><br /></strong> Zoom Image
Fig. 1: Magnetic Order in FeTe and its detection in STM. (a) topography z(x) of a Fe1+yTe sample with y = 0.08, acquired with a tip which shows no magnetic contrast, excess iron atoms show up as protrusions (Vb = 60 mV, It = 200 pA, T = 3.8K). Next to the topography, its Fourier transform z̃(q) is displayed, showing the peaks associated with the Te lattice at qaTe = (±1, 0) and qbTe = (0,±1). One pair of peaks, marked qCDW (= qaTe) shows up with stronger intensity than the other; (B) topography acquired in the same place as A with a tip which shows magnetic contrast (Vb = 60 mV, It = 200 pA, T = 3.8 K) with its Fourier transform, showing beside the peaks due to the Te lattice as in (a) additional ones associated with magnetic order at qAFM = (±1/2, 0). (c) Topographic image of a twin boundary. The stripes in the two domains are perpendicular to each other (T = 30 mK, Vb = 150 mV, It = 30 pA).
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Topographic images of the sample surface obtained before and after picking up excess iron atoms are shown in Fig. 1, (a) and (b). The Fourier transform of the topography in Fig. 1(a) clearly shows the peaks associated with the tellurium lattice at the surface with the noticeable different intensities of the two nonequivalent spots at qa and qb. With an STM tip which has accumulated sufficient amounts of excess iron, the Fourier transform of topographic STM images exhibits an additional contrast at the wave vector of the magnetic order qSDW (Fig. 1(b)). Antiferromangetic or spin density wave (SDW) order is expected to be accompanied by a charge density wave (CDW) with twice the wave vector of the magnetic order is consistent with our data. Comparison with calculations show that the Te atoms acquire a finite spin-polarization. So even if the STM would be only sensitive to the Te states at the surface, imaging with a magnetic tip can be expected to yield a magnetic contrast. The stripes caused by the antiferromagnetic order were observed over large surface areas and found to switch direction only at domain boundaries of the monoclinic distortion (Fig. 1(c)).


Magnetic Field Dependence

<strong>Fig. 2: </strong>Field dependence of Magnetic Contrast. (a) Topography measured at 3.8 K at a magnetic field of 5 T in a direction close to the surface normal, (b) topography taken in the same place in a magnetic field of &ndash;5 T (<em>V</em><sub>b</sub> = 80 mV, <em>I</em><sub>t</sub> = 100 pA). (c) average of the two images in (a) and (b) showing the topography as it would be obtained with a non-magnetic tip. (d) map of spin polarization obtained from the difference of the two images in (a) and (b) as it would be obtained with a magnetic tip.<strong><br /></strong> Zoom Image
Fig. 2: Field dependence of Magnetic Contrast. (a) Topography measured at 3.8 K at a magnetic field of 5 T in a direction close to the surface normal, (b) topography taken in the same place in a magnetic field of –5 T (Vb = 80 mV, It = 100 pA). (c) average of the two images in (a) and (b) showing the topography as it would be obtained with a non-magnetic tip. (d) map of spin polarization obtained from the difference of the two images in (a) and (b) as it would be obtained with a magnetic tip.
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To rule out other origins of this additional modulation, such as surface reconstruction or a charge modulation, we have performed magnetic field dependent measurements. In moderate magnetic fields, the magnetization of the tip is expected to be changed, whereas the magnetization of the sample will stay the same. Fig. 2(a) and (b) show two STM images, taken in the same location of the sample with the same tip at magnetic fields of B = +5 T and B = –5 T. It can be seen that between the two images, the phase of the vertical stripes has shifted by 180°, i.e. maxima of the modulation in Fig. 2(a) occur at the positions of minima in Fig. 2(b) and vice-versa. The applied field switches the magnetization of the apex of the tip only, the antiferromagnetic coupling between the spins on the iron atoms and an appreciable magnetocrystalline anisotropy prevents the spins in the sample from aligning with the external magnetic field. The magnetic field dependence of the imaging contrast confirms that the stripe modulation is due to spin-polarized tunneling, at the same time it allows to extract quantitative information about the spin-polarization. Figure 2(c) and (d) show the average and difference of the two images shown in (a) and (b), they yield the non-magnetic topographic contrast (c) as well as the purely magnetic contrast (d). Comparison of Fig. 2(c) and (d) shows that the largest magnetic contrast emerges in between the rows of tellurium atoms, indicating that direct tunneling from the tip into the strongly spinpolarized iron d-states is important.

Perspective

<strong>Fig. 3: </strong>Magnetic structure at higher excess iron concentration (y &gt; 0.12). (a) topography obtained from a sample with high excess iron concentration (y = 0.15), showing stripe modulation in two directions superimposed (<em>V</em><sub>b</sub> = 100 mV, <em>I</em><sub>t</sub> = 100 pA, <em>T</em> = 1.8 K). A large part of excess iron has been picked up by the tip, leaving an almost clean Te-terminated surface. Inset: Fourier transform of the topography showing peaks associated with magnetic contrast around (&plusmn;1/2, 0) and (0,&plusmn;1/2). (b) Filtered image showing the components associated with magnetic contrast in different colours to visualize the bidirectional stripe order.<strong><br /></strong> Zoom Image
Fig. 3: Magnetic structure at higher excess iron concentration (y > 0.12). (a) topography obtained from a sample with high excess iron concentration (y = 0.15), showing stripe modulation in two directions superimposed (Vb = 100 mV, It = 100 pA, T = 1.8 K). A large part of excess iron has been picked up by the tip, leaving an almost clean Te-terminated surface. Inset: Fourier transform of the topography showing peaks associated with magnetic contrast around (±1/2, 0) and (0,±1/2). (b) Filtered image showing the components associated with magnetic contrast in different colours to visualize the bidirectional stripe order.
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At higher excess iron concentrations y > 12%, the magnetic order becomes slightly incommensurate and forms a more complex structure revealing stripe like patterns in both directions (Fig. 3(a), (b)). The two dimensional patterns in these samples could be caused by a transition toward a plaquette order or by a superposition between the two unidirectional modulations. This clearly indicates that the magnetic structure in FeTe becomes more complex at higher excess iron concentrations. Our results demonstrate the applicability of spin-polarized STM to strongly correlated electron systems without the need of sophisticated ultra-high vacuum tip preparation methods. Our work can further be extended to obtain real-space images of stripe order in cuprates and search for magnetic order accompanying the spatially modulated electronic states found in the pseudogap phase. This opens up the possibility to study the relation between magnetic order and superconductivity in heavy fermion materials and high temperature superconductors. By characterizing both, magnetic structure and superconductivity, in the same measurement at the atomic scale, their microscopic relationship can be established.

 
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