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Visualizing Uniaxial-strain Manipulation of Antiferromagnetic Domains in Fe1+YTe Using a Spin-polarized Scanning Tunneling Microscope

The quest to understand correlated electronic systems has pushed the frontiers of experimental measurements toward the development of new experimental techniques and methodologies. Here we use a novel home-built uniaxial-strain device integrated into our variable temperature scanning tunneling microscope that enables us to controllably manipulate in-plane uniaxial strain in samples and probe their electronic response at the atomic scale. Using scanning tunneling microscopy with spin-polarization techniques, we visualize antiferromagnetic domains and their atomic structure in Fe1+yTe samples, the parent compound of iron-based superconductors, and demonstrate how these domains respond to applied uniaxial strain. We observe the bidirectional antiferromagnetic domains in the unstrained sample, with an average domain size of 50 to 150 nm, to transition into a single unidirectional domain under applied uniaxial strain. The findings presented here open a new direction to utilize a valuable tuning parameter in scanning tunneling microscopy, as well as other spectroscopic techniques, both for tuning the electronic properties as for inducing symmetry breaking in quantum material systems.

cleave the strained samples in ultrahigh vacuum (UHV). In the last few years, an alternative direction has been to epoxy a thin sample on piezo stacks 9,10,15,31 or on plates with different coefficients of thermal expansion 11,32 . Yet in both cases, the magnitude of the applied strain is quite limited.
Here we demonstrate the use of a novel mechanical uniaxial-strain device that allows researchers to strain a sample (compressive strain) without constraints and simultaneously visualize its surface structure using STM (see Figure 1). As an example, we use single crystals of Fe1+yTe, where y = 0.10, the parent compound of the iron chalcogenide superconductors (y is the excess iron concentration). Below TN = ~60 K, Fe1+yTe transitions from a high-temperature paramagnetic state into a low-temperature antiferromagnetic state with a bicollinear stripe magnetic order 26,33,34 (see Figure 3A,B). The magnetic transition is further accompanied by a structural transition from tetragonal to monoclinic 26,35 . The in-plane AFM order forms detwinned domains with the spin structure pointing along the long b-direction of the orthorhombic structure 34 . By visualizing the AFM order with spin-polarized STM, we probe the bidirectional domain structure in unstrained Fe1+yTe samples and observe their transition into a single large domain under applied strain (see the schematic in Figure 3C-E). These experiments show the successful surface tuning of the single crystals using the uniaxial-strain device presented here, the cleaving of the sample, and the simultaneous imaging of its surface structure with the scanning tunneling microscope. Figure 1 shows the schematic drawings and pictures of the mechanical strain device. The U-shaped body is made of 416 grade stainless steel, which is stiff and has a low coefficient of thermal expansion ∼ 9.9 / − ℃ as compared to ∼ 17.3 / − ℃ for 304 grade stainless steel). The opening inside the U is 1mm and can be tuned smaller or large by a pair of micrometer screws located on the sides of the device. The uniaxial pressure is applied by the micrometer screw (1-72 corresponding to 72 rotations per inch). The sample, of size (1mm x 2mm x ~0.1mm) is mounted, with its long axis oriented along the b-axis of the sample, on top of the device using H20E conductive (silver) epoxy across the 1 mm gap. The two ends of the sample are then covered by H74F non-conductive epoxy for further reinforcement. Since uniaxial pressure is the tuning parameter in the experiment described here, it is imperative that the thermal stress generated from cooling down is not transferred directly to the sample. For this, we employ a series of Belleville spring disks. Using the working load of the Belleville spring disks of 67 N, and the deflection at working load of 50 µm, we calculate the spring constant for each disk as k = 1.3 x 10 6 N/m, which yields a total spring constant of k = 1.625 x 10 5 N/m for 4 pairs of springs in series. This ensures the thermal stress on the sample through cooling from room temperature to 4 K to be less than 0.05% for an applied strain of 1% and therefore negligible. In the experiment, we rotate the micrometer screw by 50° which corresponds to Δx = 50 μm. The force applied on the sample through the springs can be calculated to be F = kΔx = 8 N. The pressure is therefore p = F/A = 8 N/(0.1 x 10 -6 m 2 ) = 0.08 GPa. For a Young's modulus of 70 Gpa for FeTe 36 , the applied uniaxial pressure corresponds to 0.1% strain.
A major challenge in integrating the strain devices with the STM is the application of strain without breaking or introducing cracks in the sample. Test experiments on several samples of Bi-2212, Sr3Ru2O7, and FeTe have shown that, depending on the sample thickness, the samples withstand strains of up to ~0.8-1.0 % corresponding to ~1GPa of applied pressure. No indications of cracks on the sample surface are observable below this value as seen visually by an optical microscope. Recent work following the same principles, has successfully demonstrated the application of ±1% strain on Sr2RuO4 22 Aluminum cleaving post is attached to strained sample perpendicular to the a-b cleaving plane using H74F non-conducting epoxy. The stain device with the sample and the post is then transferred, through the loading dock of the variable-temperature ultra-high vacuum STM, to the analysis chamber (see Figure 2a). All operations required to move the samples into and inside the STM are carried out using sets of arm manipulators. Using an arm manipulator, the aluminum post is knocked off in ultra-high vacuum at room temperature exposing a freshly cleaved surface. The device (with the strained sample) is then immediately transferred in situ with another set of manipulators to the STM chamber and into the microscope head (see Figure 2b), which had been cooled down to 9 K. All experiments are carried out at 9 K. The STM is maintained at low temperatures by liquid nitrogen and liquid helium, and the sample cools down for at least 12 h before being approached. This allows the sample and microscope temperature to reach thermal equilibrium. To isolate electric and acoustic noise, the STM is placed in an acoustic and radio frequency shielded room. The microscope head is further suspended from springs for optimized instrumental stability. The sample stage can be translated by several millimeters that enable access to different parts of the 1 mm strained samples.
STM topographs were taken in constant current mode with a setpoint bias of -12 meV applied to the sample and a setpoint current of -1.5 nA collected on the tip. Pt-Ir tips were used in all experiments. Tips were prepared prior to each experiment by field emission on a Cu (111) surface that had been treated with several rounds of sputtering and annealing. To achieve spinpolarized STM, the scanning tunneling microscope tip has to be coated with magnetic atoms, which can be quite challenging. In this case of studying Fe1+yTe, the sample itself provides a simple means of achieving this. The excess irons (y in Fe1+yTe) are weakly bound on the cleaved surface. Scanning the tip at a low bias and with a high enough current exceeding a few nanoamperes brings the tip in close proximity to these Fe atoms and a few of those atoms can be picked up by the tip 37 . The other method that yields a spin-polarized tip is by the rapid decrease of the sampletip separation until contact is made (on the location of excess iron concentration) as measured by a saturation current. During the process, the excess irons bond onto the tip. The successful preparation of a spin-polarized tip is revealed by the magnetic contrast in the topography, whose periodicity is twice that of the lattice constant of top tellurium atoms. This additional modulation is the antiferromagnetic order in the sample. Figure 4A shows a 10 nm atomic-resolution topographical image on an unstrained Fe1+yTe single crystal with a nonmagnetic scanning tunneling microscope tip. The atomic structure seen corresponds to the Te atoms, which are exposed after cleaving the sample (see Figure 3A). The Fourier transform (FT) of the topography shows four sharp peaks at the corners of the image along the a-and b-directions, labeled qTe a and qTe b , that correspond to the atomic Bragg peaks. The central broad peak in the FT corresponds to long-wavelength inhomogeneity, which is not relevant for the current study. Figure 4C shows another topograph of the same size as in Figure  4A, obtained with a magnetic tip. Unidirectional stripes with a periodicity of twice that of the lattice along the a-axis are observed. The FT of the topograph seen in Figure 4D shows, in addition to the Bragg peaks, a new pair of satellite peaks at QAFM1, corresponding to half the Bragg peak momenta and, therefore, twice the real space wavelength. The new structure corresponds to the AFM stripe order of the Fe atoms just below the surface.
On this unstrained sample, it is not difficult to observe twin domain boundaries where the crystal structure with the long b-axis and the accompanying AFM stripe order rotate 90°. Figure 4E shows a 25 nm spin-polarized topograph of an AFM twin domain boundary. The FT of the image now shows two pairs of AFM order (highlighted by green and yellow circles). Each magnetic domain contributes to only one pair of the QAFM peaks in the FT. To visualize this clearly, we Fourier-filtered each pair of AFM peaks and inversed FT back to real space. The results are shown in Figure 4G,H highlighting the two unidirectional stripe domains.
Thus, we studied the domain structure and boundaries on the surface on a large scale. Figure 5A, Figure 6A, and Figure 7A display large-scale topographs on three different unstrained samples spanning a total region of slightly over 0.75 µm x 0.75 µm. Several smaller zoomed-in topographs are also shown to highlight the stripe structure. The topographs are taken with a high spatial resolution (1024 x 1024 pixels per 0.25 µm 2 ) to allow the Fourier filtering and inverse Fourier transform analysis on the large scale. The corresponding domain structures and boundaries are displayed in Figure 5B, Figure 6C, and Figure 7H. Overall, several alternating stripe domains are observed covering the overall equal areas, as expected for these unstrained samples. It is important to note that on this large scale the surface is overall atomically flat, yet a few different structural irregularities, such as line defects ( Figure 5A) and atomic steps ( Figure  7A), can be observed. The stripe domains are not affected by these irregularities.
From here, we moved on to the strained sample. Figure 8 shows a large-scale topograph, spanning a total region of ~1.75 µm x 0.75 µm, which is more than twice the total area spanned in the unstrained samples shown in Figure 5, Figure 6, and Figure 7. In stark contrast, the FT for each topograph shows only one pair of AFM peaks indicating only a single domain on this strained sample. This can further be visualized by the Fourier-filtered iFT analysis confirming the single stripe domain over the entire area. Once again, the unidirectional stripe order is not to be affected by the different surface irregularities in this strained sample.
The success of this technique lies in the careful execution of the correct alignment of the sample across the 1 mm gap and application of the strain on the sample without breaking or bending it. Another important consideration is the cleaving process, which allows the exposure of a clean flat surface. This is a random process and works best for materials that cleave easily. A last consideration is having a sharp tip that yields atomic resolution and can pick up some excess iron atoms to achieve magnetic contrast.
In conclusion, the experiments and analysis described here successfully demonstrate the incorporation of our strain device with STM, providing a new tuning parameter that can be invaluable in the study of competing orders in correlated electron systems. The advantage of the current device is the wide range of positive and negative strain that can be applied to the sample. This demonstration may impact other spectroscopic experiments such as ARPES.  figure panels A and B. A combination of H20E and H74F epoxies are applied to the sample and cured at 100 °C. Once the epoxy on the sample is cured, a post of about the same surface area as that of the sample is epoxied onto the sample's surface using H74F. (B) The actual setup of the strain device, with a top view, front view, and a zoom-in of the sample. The device is screwed to a sample holder that slides into the microscope head. A contact is created by using conductive epoxy from the device to the sample plate. The transfer of pressure is enabled using a screw and a series of Belleville spring disks. The last panel of B shows the strain device set up, ready to be moved into the UHV analysis chamber. (C) An alternative method is to have a sample inside the gap of the strain device. In the two middle panels of C, a second unstrained sample is epoxied on the device for reference. The microscope is placed in an acoustic chamber, which is shielded from radio-frequency (RF) noise. (B) The microscope head with a bare sample holder. The Pt/Ir tip is visible. The sample stage can be moved by a set of piezo actuators so that the sample is right above the tip. (C) The microscope head is placed inside two radiation shields.