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4. INVESTIGATION OF METAL-INSULATOR TRANSITION methods of surface cleaning leave a significant mark or destroy the surface in a way that it would no longer be suitable for AFM measurements. The easiest method would be mechanical cleaning by rubbing away surface material with a tool that has higher hardness. However the tool’s roughness would definitely in- crease the roughness of the film’s surface which would cause serious difficulties in AFM measurements such as alteration of film surface morphology, creation of gaps and discontinuities/voids or possible tip breaking due to increased surface roughness. Higher quality of surface, sufficient for force microscopy imaging is obtained with ion beam polishing or ion beam etching. A beam of argon, neon or xenon ions in vacuum is directed onto a surface, when the energy of the ions is sufficient the surface deposits are etched with the impinging beam, uncovering the lower layers of sample. The AFM method is appropriate when studying the mor- phology and structural properties of the surface, or electrical/magnetic properties of metallic surfaces. For non-metallic surfaces a known phenomena is reduction of cation oxidation state by the ion beam. This may influence the variation of electrical properties and is not recommended when performing local conductivity measurements. On the other hand, it is obvious that resistivity measurements require elec- trical contact between the used electrodes to ensure a good unobstructed charge flow. However a certain amount of impurities may block percolation of electrical current through the grains of investigated phase and inhibit possibility of long range charge transport. This is the case in investigated thin film materials. Per- formed four-point probe resistivity measurements did not yield much valuable information. The measurements were performed in the UHV conditions and the observed temperature dependence of resistivity was often irreproducible. Similar challenges are present in atomic force microscopy in local conductivity mode, however they can be overcome by the specific character of the technique. Electrical contact between the scanning tip of the microscope and outer electrode should be insured to investigate metal-insulator transition properly. AFM tips have either cone-like or pyramidal shape, this allows for a tip to dive into the material. The tip can penetrate through the layer of adsorbates and probe the material below. This way the electrical contact is maintained not only with the sole surface but also with a few of the sub-surface layers of material, and is increased due to increased area of contact. This way there is a possibility that the interior of material is mostly of the proper structure and a long range paths of the material exist. The idea of the experiments was to investigate whether this is true and study the sample behaviour in such limited conditions. 88PDF Image | Investigation of metal-insulator transition in magnetron sputtered samarium nickelate thin films
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