Designing instruments: big puzzle on a tiny scale
At HFML - FELIX, we use magnets, lasers and many different instruments for our research. Most of them are custom-made, as you cannot buy them. Technicians and scientists work closely together on designing and building, as it is usually something new and uncommon. It is a joint discovery of what is possible.
It might take a while to get what you want. It took Lisa Rossi, PhD student at HFML, two years to design and build the scanning probe microscope that she needed. But now it is here. The only one in the world that can measure in high fields (above 30 Tesla) and in extreme cold (-270 degrees Celsius, more or less). Moreover, it is very compact, only 14 x 76 millimeter. Lisa: ‘We wanted to build this because we can have a much better look at what happens with matter in high fields. However, is has been a big puzzle. For instance: magnets make a lot of noise. A scanning probe microscope does not function well with noise. Therefore we had to think for a solution. In addition, the instrument should be able to oscillate a bit, so we needed room. But you still have to be able to put it inside a magnet. So every millimeter counts.’
Scanning probe microscopy
Scientist use scanning probe microscopy to create images of nanoscale surfaces and structures. A small tip, like a needle, scans the surface of a specimen. It gathers data and uses that data to generate an image. Therefore, you do not have a direct view of the surface like with an optical microscope, but you get an image that represents the structure of the surface. Scanning probe microscopes are very powerful and can have a very high resolution, even atomic scale. Until now, the maximum field for scanning probe microscopy has been limited to 20 T. This leaves many field-induced phase transitions of materials out of reach. Lisa: ‘I made this because scanning probe microscopy is one of the best techniques to analyse “hidden structures” in materials and the technique behind it is fascinating. A tip, connected to a lever that oscillates close to its resonance frequency, scans the surface of your sample. We try to keep or the frequency or the amplitude of the oscillation constant, but the interaction between the atomic forces of tip and sample cause the amplitude or the frequency of the cantilever's oscillation to change. We translate this effect in an image of the surface. There are of course different kind of forces that can play a role in these kind of measurements, and with some variation on the technique, we can “paint” all the different stories that the surface of the sample wants to tell.
(a) Diagram and (b) image of the HF-SPM head, with the main components indicated. See article for details.
Lisa Rossi working on the scanning probe microscope
An ultra-compact low temperature scanning probe microscope for magnetic fields above 30 T, L. Rossi, J. W. Gerritsen, L. Nelemans, A. A. Khajetoorians & B. Bryant, Review of Scientific Instruments 89,113706 (2018)