Magnetic force microscope study on high-anisotropy UMn2Ge2 and construction of a spin polarized scanning tunneling microscope
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The first part of this dissertation introduces the theoretical background for the Magnetic Force Microscope (MFM), the Spin Polarized Scanning Tunneling Microscope (SP-STM), and the theoretical basis for magnetic domains.The second part addresses issues on the design and construction of a low temperature magnetic force microscope (LT-MFM) and its operation. The third part focuses on the LT-MFM experimental investigation on ternary UMn₂Ge₂ crystals. The forth part describes the construction and implementation of a low temperature SP-STM system. Scanning probe microscopy (SPM), beginning with the invention of the STM, was first developed to study the electronic properties of different materials, such as imaging high T[subscript c] superconductors. But soon, as the SPM family expanded, some of them developed into powerful techniques to characterize magnetic features. This category includes MFM and SP-STM. The former was widely used for imaging surface magnetic properties from hundreds of micrometers down to the nanometer scale, ideal for imaging magnetic domains. With our homemade LT-MFM system, we studied UMn₂Ge₂ single crystals, in which both the Uranium and Manganese atoms are magnetic. Flower-like magnetic domain pattern were found at room temperature, and they persisted all the way down to low temperature. Around 150K, Uranium atom ordering was revealed in the form of magnetic domain wall jumps, by partially saturating the sample and warming it up in zero field. In addition, the underlying mechanism of the flower pattern was explained using the domain branching scheme. On the other hand, a Low temperature SP-STM (LT-SP-STM) was designed and constructed, paving the way for spin mapping at the atomic scale thus characterizing magnetic materials with ultra-resolution.