| Scanning tunneling microscope (STM, for short), based on the principle of quantum tunneling in quantum mechanics, has showed its great power in the application of the field of surface and nano sciences because of its spatially atomic resolution. For example, people can use STM to characterize the electronic states of superconductors, manipulate the atoms under low temperature condition, do chemical researches on the samples in the solution and so on. It is noticeable that the size of samples that are studied utilizing STM is usually small; some are in micro-scale or even nano-scale, which brings great challenge for the applications of STM. Because before you attempt to attain a picture with STM, you have to accurately position the thin probe of STM over a small sample (micro-or nano-scale) first.Once put into extreme conditions, such as ultrahigh vacuum, low temperature, extreme low temperature and high magnetic field and so on, lots of materials exhibit intriguing and unique physical properties (for example, superconductivity, quantum Hall effect, de Hass-van Alphen effect, and quantum phase transition). Thus, it has become an important and imperative project for researchers worldwide to build up a STM that work well at low temperature and in high magnetic field. At present, some research groups have built up STM systems that can work at low temperature and in high magnetic field. But the working magnetic field is not too high (much is about10T or so), which greatly limits application research for samples.Magnetic force microscope (abbreviated as MFM), based on the dynamic property of microcantilever, is a powerful instrument for measuring distribution of magnetic domain on the surface of magnetic materials. It has been widely used in studying magnetic materials, and has now become a conventional tool for the characterization of physical property. However, the MFM used in high magnetic field is still rare, since many problems would occur when MFM is put into magnet, for example, the bore diameter of the core of magnet is too small to accommodate the microscopy; the magnet itself can bring much signal noise during work and so on.Taking into consideration the above conditions and questions, during the doctoral period and under my advisor’s supervision my work mainly contains the following several parts:First of all, I have carefully analyzed the issues incurred during positioning a tip over tiny samples using STM, elaborately designed and built up a STM system capable of focusing a tip to a tiny sample accurately just for one time. Using our STM, it is very convenient for us to measure a sample with dimension of microns, overcoming lots of drawbacks that met before in using small samples. Since the intriguing samples that can be investigated using STM are often found to be small. Many of them are at the micron scale. Examples include graphene, some important microcrystalline samples, electronic devices (such as magnetic tunnel junction, carbon nanotube ring transistor, etc.) and so on. Thus, our STM shows significant importance, especially for measuring these samples.At present, methods that are often used for measuring tiny samples worldwide are several:(1) Blindly approach the tip to the sample and rely on good luck or a large number of trials to find the desired small sample.(2) Using a STM with large area search ability where a piezoelectric motor can move the sample (or tip) in a large range so that the tip can reach and scan most of the sample area.(3) Using a STM-SEM or STM-TEM combo. No doubt, these methods have very obviously weak points:either too much pain is needed (even in vain) or the STM system is more complicated and have bad compatibility (cannot be put into magnet and so on).Compared to the above introduced methods, our new designed STM have several superiorities:(1) Realizing the positioning once for measuring tiny samples, saving much effort. The STM body takes separate scanning structure. The key part of the focusing structure is a stand-alone soft junction mechanical loop (SJML), in which a tip, small sample and a piezoelectric tube is contained. Two pairs of outer electrodes of the scanner performs scanning in the plane parallel to the sample surface, and the inner electrode of the scanner is used to perform a fine approaching between a tip and sample. The mechanical loop can be freely movable, after we focus the tip over a small sample surface (to facilitate the positioning, we use a thin Pt/Ir tip with the diameter of0.1mm), we fix the SJML to a sliding piece with silver paint, then put the slider to the top isosceles triangle plane formed by three sapphire balls of microscopy body.(2) Fully low voltage operation, thus decreasing noise interruption caused by conventional high approaching voltage. The coarse approaching of the slider piece takes the principle of combination of "forward probing and forward throwing". A gradually increasing voltage signal is added to the approaching piezoelectric tube to make the tube slowly bend forward, which is followed by the sliding piece due to static friction. If the output voltage of preamplifer is bigger than0.2V, the program of coarse approaching automatically stops and the slider also cease approaching. If the output voltage is less than0.2V, a swiftly decreasing signal is applied to the piezotube making the tube restore to original natural state, then a peak signal is quickly added to the tube. Due to the inertia, the sliding piece makes a step forward. After the coarse approaching is finished, we adjust the distance between the tip and sample by hand, and begin scanning when a tunneling current is stable.(3) Excellent performance of compatibility, the structure can be conveniently transplanted to high magnet for measurement. Since the SJML is separate from the approaching structure, it can be moved freely to extreme condition, such as low temperature and high magnet.Our practical measurement results indicate that the STM has a very high stable positioning resolution, performance and reproducibility. High quality atomic resolution image of HOPG is still obtained after one month for the first measurement. Besides, the feature of tunneling current spectrum also indicates excellent stability of our STM. Related STM structure and measurement data have been highly recommend by reviewers, and have been published in SCI article of the second region-Review of Scientific Instruments.In addition, with the assistance of J. H. Wang, we have first discovered that current can easily modulate the atomic image of graphene. The comparison between atomic images measured in the condition of adding current to graphene and no current indicates an intriguing distinction.Second, Q. F. Li who has graduated from our lab last year, the associate researcher Y. B. Hou in our lab and I have accomplished the examination of the research project about the measurement of STM part of three-in-one SMA combo system (includes STM, MFM, and atomic force microscope) at low temperature and high magnetic field (up to18/20T). We have got12fA current resolution of the preamplifier of magnet insert and clear atomic image of HOPG in18T. The former is higher than20fA (the best record) that has been measured by Q. F. Li before and published in a paper of RSI, and much higher than49fA that is the best data measured by other groups in the world hitherto.In terms of SMA combo system, we can study one position of a sample in the mean while using three different microscopes that perform three different functions. Our low-T high magnetic STM has three superiorities listed in the following:(1) Owning the best current resolution (12fA), making it possible for us to measure insulators in low temperature and high magnet.(2)20T, in which the clear atomic image of HOPG is the highest field in field of SPM, and is the same as the record of a research group in Japan. However, in comparison with the record, our STM have following strong points. First, our measurement condition is low temperature, and magnetic inset is immersed in liquid helium. We can measure samples in low temperature and high magnetic field. In comparison, the image of Japanese group is at room temperature. Second, the motor of our STM take the technique of splitting the inner electrode of piezoelectric tube into two parts, which has greatly decreases the approaching voltage (-6V) and therefore reduces the coupling interruption to the tunneling current during approaching process.(3) We have designed and built up a series of strategies and tools for vibration and noise isolation, greatly decreasing outer interruption (the vibration due to the ununiform of magnetic gradient and the natural evaporation of liquid helium etc.). We hung the whole superconducting magnet with several springs, and wrapped up the magnet with sound-absorbing foam. The SMA-STM microscope is manufactured in terms of nonmagnetic metals, and can be freely hanged to the SMA insert using springs with good convenience for operation and compatibility.As for the testing of current resolution power of the preamplifier of SMA-STM, we take a large resistor with100G Ohm to simulate real tunneling junction resistor, and modulating different tunneling currents by changing input input voltages. By Gauss curve-fitting method, we finally know that the current resolution power is20fA. The experts of examination of the project have given very positive remarks:"......The successful construction of SMA-STM system, not only fills the gap in the research about STM under high-resolution and high magnetic field conditions in China, but also realizes completely low voltage and the highest current resolution worldwide............it will make a great promotion effect in advancing the development of materials science and condensed matter physics and so on......".In addition, I have also tested one newly-designed STM that owns the in-situ cutting-sample ability and can exactly reach helium temperature for the sample platform. I have obtained excellent tunneling current spectrum and good atomic resolution image, which has made profound foundation for further cutting-sample measurement by putting STM into Oxford magnet (18/20T).Third, using the MFM of SMA combo microscopy system that has been tested successfully by Y. Z. Shi who has graduated from our lab last year, I have made measurements for La(5/8-x)PrxCa3/8MnO3thin film at variable temperature and high magnetic field. We have intuitively confirmed that electronic phase separation does exist in La(5/8-x)PrxCa3/8MnO3. What is more important, ferromagnetic and insulating phase coexist at a certain temperature region, and the ferromagnetic zone becomes bigger and bigger with gradually decreasing temperature.The working principle of MFM takes the style of amplitude modulation feedback circuit. We apply an AC voltage to the piezoelectric plate on which a MFM probe is pasted to vibrate the probe at a certain frequency. The piezoresistive value of the probe makes corresponding changes that is amplified by a bridge circuit once the probe approaches to the regime of magnetic interaction. The vibration amplitude of the probe is maintained constantly via PID feedback circuit during scanning, and the modulation signal of PID circuit is employed to image. |
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