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Though microscopes have been in use for centuries, there is still much that we cannot see at the smallest length scales. Current microscopies range from the simple optical microscopes used in high school science classes, to x-ray microscopes that can image through visibly-opaque objects, to electron microscopes that use electrons instead of light to capture images of vaccines and viruses. However, there is a great need to see beyond the static structure of an object—to be able capture how a nano- or biosystem functions in real time, or to visualize magnetic fields on nanometer scales. A team of researchers from the STROBE Center have been working together to overcome these challenges. STROBE is an NSF Science and Technology Center that is building the microscopes of tomorrow. A large multidisciplinary team from the Miao and Osher groups from UCLA, the Kapteyn-Murnane group at CU Boulder, Ezio Iacocca from CU Colorado Springs, David Shapiro and collaborators at Lawrence Berkley National Laboratory, and the Badding and Crespi groups from Pennsylvania State University. They developed and implemented a new method to use x-ray beams to capture the 3D magnetic texture in a material with very high 10-nanometer spatial resolution for the first time (published in Nature Nanotechnology, see reference below).

Hedgehogs and Anti-Hedgehogs

The team investigated a nanostructured magnetic sample, consisting of tiny spheres of nickel, only ~30nm across, connected together by slender few-nm “necks” of nickel, that together form a structure called a magnetic metalattice. This complex nanostructured magnet is expected to produce swirling magnetic fields with topological spin textures that are far more complex than in a uniform magnet. These are called 3D topological magnetic monopoles – or hedgehogs, due to their spiny shape in magnetic rotation – if the magnetic field points outward. Conversely, they can be thought of anti-hedgehogs if the magnetic field points inward.  However, until recently, there was no experimental method to measure the 3D spin texture at the deep nanoscale. Using advanced algorithms to recover the image, and a microscope at the x-ray synchrotron light source at the Lawrence Berkley National Laboratory, the researchers overcame these challenges.

Imaging spin textures is extremely important, as it can help physicists to better understand magnetism at a fundamental level, and to design more energy-efficient data storage, memory, and nanodevices.  Using electron microscopy, one can capture beautiful 2D images of a static spin-texture, but it is challenging to capture a full 3D image. In the past, other scientists were able to capture a 3D image at a spatial resolution of about 100 nanometers, but they had to make assumptions about the sample to extract the 3D image. With this new technique, researchers do not have to make any assumptions.

Armed with this new visualization technique, the team of researchers is excited to study spin textures further. STROBE is developing tabletop setups and helping with national facilities that can capture the static and dynamic spin texture in materials. All algorithms developed for this data analysis will be open-sourced soon. In this experiment, as with others, they found that collaboration is key for moving scientific progress forward.

Electromigration is one of the most critical problems limiting the future scaling of integrated circuits (ICs).  As more transistors are packed into ever smaller volumes, the total length of interconnecting wire is increasing as the wire simultaneously becomes narrower and more vulnerable to failure. Surprisingly, while chip fabs are presently churning out microprocessors at the “5-nm process node” with correspondingly tiny interconnects, before recent work by a STROBE team no measurements of electromigration pressures had been reported at length scales smaller than 10 μm.

Using electron-beam lithography, the  team fabricates tiny aluminum nanowires on electron-transparent silicon nitride membranes. They then apply an electrical current density of 10⁸ A/cm²,  which is enough to drive electromigration, and map the nanowire’s density with parts-per-thousand precision using electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM).  Two separable effects appear.  First, thermal expansion decreases the wire’s density as Joule heating raises its temperature. Using aluminum’s known coefficient of thermal expansion, one can convert the density map into a temperature map with the same spatial resolution.  Second, via the electron “wind force” the electrical current simultaneously compresses one end of the wire while it tensions the other. Using aluminum’s known bulk modulus,  one can convert the density map into a pressure map.  The temperature effect is independent of the direction of the current, while the pressure effect changes sign with the current’s direction, which allows the two effects to be separated.  Thus the thermodynamic state variables T and P can be mapped with high spatial resolution everywhere inside the wire. The ability to simultaneously see the prompt thermal and electron-wind-force effects in nanoscale interconnects opens entirely new avenues for studying electromigration, a key technological problem in the microprocessors powering modern computing devices.