Low temperature scanning probe microscopy

A local probe to help visualize the electron density in nanometer scale samples is a powerful tool in understanding the mechanisms of electron transport, especially in hybrid systems incorporating different material types such as normal metals, ferromagnets and superconductor. A scanning tunneling microscope (STM) is the most appropriate tool for this purpose. However, performing scanning tunneling measurements on real devices fabricated on chips is difficult, as most of the chip area is insulating. In order to overcome this problem, one needs to first find the areas of interest with an atomic force microscope (which does not require a conducting surface), then switch over to scanning tunneling microscopy to perform the spectroscopy. In order to do this, we are currently developing a combination AFM/STM based on a tuning-fork transducer.

We have a commercial Thermomicroscopes Atomic Force Microscope (AFM) available in the lab that is used by group members to view fabricated nanoscale samples at room temperature. This microscope uses a laser beam to measure the deflection of the cantilever as it is scanned over a sample. Our scanning probe microscopy project is developing an AFM/STM that runs at millikelvin temperatures inside our new Oxford MX100 dilution refrigerator.	AFM Image of a Ni dot array taken with a tuning fork transducer
	As a conventional AFM, which acquires images by bouncing a laser off a scanning cantilever, dissipates too much heat and is too cumbersome, our AFM design is based on quartz tuning forks like those found in most watches. These tuning forks have a very sharp resonance (Q = 10000) at around 32 kHz. Excited at this frequency, the tuning fork is lowered toward the sample surface until a tip attached to one of the fork's tines begins to interact with the sample surface. These force interactions slightly shift the resonance peak of the tuning fork. By scanning the tip and fork across the sample surface and tracking the resonance shifts an image of the sample topography can be produced. The power dissipation with this tuning fork transducer is much less than with other techniques (such as laser deflection), making it suitable for operation at millikelving temperatures. With adjustments to the type of tip, one can also do Magnetic Force Microscopy (MFM) and Scanning Tunneling Microscopy (STM) on samples.
	In addition to developing the electronics to control the tuning fork transducer, we have developed a technique to mount a commercial cantilver tip onto the end of a tuning fork (see image above), as well as carbon nanotube tips, and we are developing techniques for growing the tips directly on the tuning fork tines.
With this instrument, we have been able to perform AFM, MFM and EFM at temperatures down to 4.2 K in a magnetic field.  The image on the right shows a MFM image of an array of elliptical permalloy particles fabricated by e-beam lithography, taken at 4.2 K in a magnetic field.
The advantage of a low temperature scanning probe microscope is that it enables one to see transitions that occur at lower temperatures.  The image on the right shows topographic and MFM images of an array of epitaxial strontium ruthenate (SRO) pillars at room temperature and 77 K.  Since the ferromagnetic transition of SRO occurs only at ~160 K, only the low temperature MFM scan shows a magnetic image. The hardware, electronics and software for the existing scanning probe microscope with a tuning fork transducer was entirely developed in house. This gives us the flexibility to modify it as per requirements. The microscope is currently being modified to work at variable temperatures (from 4.2 K to 300 K). It will be made operating down to milikelvin temperatures soon. This will help us to probe physical properties of complex systems at the nanometric scale for a wide temperature and magnetic field range.