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 , 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 . 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 .
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.
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
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.
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.