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Ultra-high-resolution low-temperature magnetic force microscopy

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2019
Badami Behjat, Arash
Scanning probe microscopy is a conventional technic which has opened new methods to investigate surface properties. Imaging atoms or manipulating them as well as measuring surface structures with high resolution and accuracy are fantastic features which are utilized in surface science to study the characterization of materials. Atomic Force Microscopy (AFM) a standard popular method can measure the interaction forces between the sharp tip and the sample surface. This method allows us imaging atoms individually with the atomic resolution or the structure of molecules. AFM could measure various types of forces like van der Waals, electrostatic, friction or magnetic forces under the different environment condition, high vacuum or Ultra high vacuum, ambient, and aqua, as well as high magnetic field at room or cryogenic temperature. Affordable price, easy sample preparation, and operation, specifically high resolution down to the nanometer are the advantages of this method. Moreover, ability to image almost any type of samples such surface of the ceramic material, or the dispersion of the metallic nanoparticles, or so soft material, such very flexible polymers, the human cell, and DNA is the most considerable advantages of AFM. Magnetic Force Microscopy (MFM) is another standard method for surface investigation of magnetic properties, which is the aim of this thesis. MFM is one of the significant roles in material science that magnetic resolution of 10 nm and less, provides critical information in the different area of science such as spintronics, spin glass system, magnetic nanoparticles, superconductivity, high-density magnetic recording media, magnetic phase transition, etc. The capability of AFM/MFM in working at a low-temperature range of 300 kelvin to hundreds of millikelvin increases the versatility of the microscope. Principally, low-temperature Atomic/Magnetic Force Microscope (LT-AFM/MFM) is working with the measuring the cantilever deflection; This means the fiber interferometer which directs the laser light to the cantilever tip by a fiber cable can measure the deflection with calculating the percentage of laser light coming back from the tip. The significant role here is the operation of the microscope in extremely low temperature for further material evaluation and also the reliability of working that in various temperature properly. The critical issue is microscope alignment, which the cantilever and fiber respectively could collapse in the cooling status. Therefore, the design of the microscope head should be wholly centric, and a particular mechanism aligns considering the different thermal contraction of the material. Besides, most cryogenic systems are limited in the sample space; consequently, this limitation should be applied in the whole design and system alignment. In my master thesis, I have improved a Low-Temperature Fabry-Perot Atomic Force Microscope / Magnetic Force Microscope (LT-AFM/MFM). This instrument was developed earlier in our group and used standard tips and a Fabry-Perot fiber interferometer for measuring the cantilever deflection. The earlier version of this LT-AFM had some reliability issues with the fiber nanopositioner at low temperatures. Principally, in a Fabry-Perot AFM/MFM it is required to reduce the distance between cantilever and fiber by moving the latter. This issue is also an essential means to improve the resolution because thereby, the signal intensity of the reflected light can be increased. In the previous setup, the piezo nanopositioner was not able to move the ferrule that holds to fiber, due to the limited space in the vertical direction in the cryostat. Therefore, I modified the design, resulting in improved reliability of the fiber nanopositioner. This was achieved by (1) centering the AFM tip at the piezo tube; (2) improving the surface quality of the groves in which the ferrule is sliding; (3) increasing the inertial mass of the fiber holder. The new concentric design enabled the piezoelectric nanopositioner not only to move the fiber forward and backward in the vertical direction but also worked reliably and precisely at extremely low temperatures down to 300 milliKelvins. The movability of the fiber to optimize its position with respect to the cantilever is essential for the performance of the microscope. Thereby the slope is increased, i.e., the change of the signal at the photodiode with respect to a change of cantilever position, of the reflected laser signal, and hence the vertical resolution of the AFM. With this new design, a slope of 148 (mV/Ȧ) was reached using a laser power of 3.5 mW, whereas before the slope only amounted to 120 (mV/Ȧ). Increasing this slope also means improved vertical resolution in AFM and MFM as well. In this research, we have improved the previously developed AFM/MFM Fabry-Perot low-temperature microscope with an outer dimension less than 25.4 mm which is completely commercialized for various types of cryogen-free cryostats from different manufacturers or liquid He bathes. All measurements conducted in the newly installed cryogen-free cryostat with the capability to reach 1.3 K from Cryomagnetic instrument; This ultra-low temperature is a vital issue in material science and also physics investigation on many phenomenon. The working potential of the microscope, in both AFM and MFM mode at various temperature, besides ultra-low noise level around 25 fm/Hz1/2 in 300 K and 12 fm/Hz1/2 in 1 K gives high-resolution images.