Engineering
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Visualizing Uniaxial-strain Manipulation of Antiferromagnetic Domains in Fe1+YTe Using a Spin-polarized Scanning Tunneling Microscope
Chapters
Summary March 24th, 2019
Using uniaxial strain combined with spin-polarized scanning tunneling microscopy, we visualize and manipulate the antiferromagnetic domain structure of Fe1+yTe, the parent compound of iron-based superconductors.
Transcript
This protocol is significant because it is a visualization of the integration of the uniaxial-strain device with scanning tunneling microscopy. It involves both applying strain and visualizing the manipulation of structural domains in SDM. The main advantage of this technique is it allows for increased amounts of strain since it's a mechanical device.
It's surface effects are able to be visualized using scanning tunneling microscopy. In high temperature superconductors by deliberately tuning and manipulating broken symmetry states, one can control and understand superconductivity. This process can be applied to any material provided it's a single crystal and it is SDM-compatible.
There's a lot of device testing to determine the set-up's response to varying conditions including how much strain can be achieved. It's a very challenging experiment where patience and absolute understanding of what you are doing is key. Visual demonstration of this process is critical, as it gives an insight into making and using the described device with SDM which is a complex process.
To begin, disassemble the u-shaped device and place it into acetone. Clean the device, the micrometer screws, the Belleville spring discs, and the base plate by sonicating them for 20 minutes. Then, transfer them into isopropanol and sonicate them for an additional 20 minutes.
Once clean, bake the device components in an oven for 15 to 20 minutes to get rid of any water residue and to degas. Then, using a sharp razor blade, cut the iron tellurium sample to one millimeter by two millimeters by 0.1 millimeters in size. Finally, assemble the parts together.
The opening inside the u is one millimeter and can be tuned smaller or large by a pair of micrometer screws located on the sides of the device. In two separate dishes, mix silver epoxy and non-conductive epoxy according to the instructions on the epoxy data sheets. Then, apply a thin layer of silver epoxy to create an electrical contact and mount the sample across the one millimeter gap so that its long axis is oriented along the b axis of the iron tellurium sample.
Place the sample holder and sample into a convection oven and bake them for 15 minutes to again cure the epoxy. Once the sample has cooled, cover its two sides with non-conductive epoxy so that the sample is firmly supported on the device. Then, place it back into the oven to cure the epoxy.
Using an optical microscope, examine the position of the sample from all angles to check for parallel alignment of the sides of the sample with the gap. With everything now prepared, begin to apply compressive strain by rotating the micrometer screw 50 degrees while observing the surface of the sample. There should be no cracks or bending of the sample after the pressure is applied.
Next, screw the device into the base plate. Once secured, apply a thin layer of silver epoxy from the base plate onto the u-shaped device to create electrical contact between the sample and the plate. Place the sample into the oven to cure the epoxy.
Once cooled, check the electrical contact using a multimeter. Then, use a thin layer of non-conducting epoxy to glue an aluminum post onto the sample so that it is perpendicular to the ab cleaving plane. The posts should be the same size as the sample.
When the post is correctly positioned, bake the assembled device until the epoxy is cured. First, transfer the device to the scanning tunneling microscope by placing it into the loading dock of the variable temperature, ultra-high vacuum scanning tunneling microscope. Using an arm manipulator, knock off the aluminum post in ultra-high vacuum at room temperature to expose a freshly cleaved surface.
Immediately transfer the device in situ with another set of manipulators to the scanning tunneling microscope chamber, enter the microscope head, which has been cooled down to nine degrees Calvin. Allow the sample to cool down to nine degrees overnight before carrying out the next steps and maintain this temperature during the experiments. Once the temperature equilibrium has been reached, prepare the platinum iridium tips prior to each experiment by field emission on a one one one copper surface that has been treated with several rounds of sputtering and annealing.
Using the voltage applied to the piezo electric materials in the microscope, move the sample stage to align with the tip. Then approach the sample. Once the tip is a few angstroms away from the sample and the tunneling current registers on the oscilloscope, it is ready for taking topographs.
Here is a 10 nanometer atomic resolution topographical image of an unstrained iron tellurium single crystal. The atomic structure seen corresponds to the tellurium atoms which are exposed after cleaving the sample. The 4DA transform of the topography shows four sharp peaks at the corners of the image along the a and b directions that correspond to the atomic Bragg peaks.
In contrast to the first image, this topographical image shows a topograph obtained with a magnetic tip. Unidirectional stripes with a periodicity of twice that of the lattice along the a axis are observed. The 4DA transform of this topograph shows in addition to the Bragg peaks, a new pair of satellite peaks, 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 iron atoms just below the surface. On some parts of the unstrained sample twin domain boundaries exist where the crystal structure with the long b axis and the accompanying AFM stripe order rotate 90 degrees. Here you can observe a 25 nanometers topograph of an AFM twin domain boundary.
The 4DA transform of this region shows two pairs of AFM order highlighted by green and yellow circles. Each magnetic domain contributes to only one pair of the peaks in the 4DA transform. For the strained sample, only one single domain can be seen as a result of the uniaxial pressure applied to the sample.
Here, a large-scale topography is shown spanning a total region of approximately 1.75 microns by 0.75 microns which is more than twice the total area spanned in the unstrained samples. The 4DA transform for each topograph shows only one pair of AFM peaks indicating only a single domain on this strained sample. The most important thing to consider while attempting this procedure is your ultimate goal.
Knowing why you're applying uniaxial strain should guide you as to the sample orientation and how much strain to apply. Following this procedure, a strain device could also be integrated with other techniques such as x-ray diffraction, resonant elastic x-ray scattering, and angular resolved photoemission spectroscopy. SDM's a powerful technique that enables one to visualize electrons in quantum materials.
And these are materials that are very sensitive to external perturbations such as strain, so the uniaxial strain integrated SDM technique will allow one to electronically tune these materials and visualize the response to strain with the ultimate goal of understanding superconductivity.
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