Faculté des sciences de base SB, Section de physique, SB - Section de physique SB-SPH

Towards magnetic engineering at metal surfaces

Messina, Paolo ; Kern, Klaus (Dir.)

Thèse sciences Ecole polytechnique fédérale de Lausanne EPFL : 2003 ; no 2754.

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    The aim of this thesis is twofold: on the one hand to demonstrate a new method of building magnetic nanostructures at surfaces and on the other hand to present the construction of a new 1K UHV Low Temperature STM aimed at coupling the spatial resolution of the STM microscope with the capability to detect magnetism. The focus of the first chapter is the construction of the entire system incorporating the STM microscope. Design and the functionalities of the instrument are emphasized. The system is an Eigler type UHV LT STM. This class of low T STM relies on a pendulum mechanism which mechanically decouples the STM from the entire UHV system. As a result the mechanical stability of the microscope is particularly improved. Besides, this design will allow eventually to develop a temperature control allowing the instrument to work between 1 and 80 K. Unlike the first generation of Eigler type STM, the temperature can be lowered below 4.2 K by the use of a Joule-Thompson expansion closed cycle. He3 is let to expand into the cavity allocating the pendulum. Upon circulation of the gas around the pendulum the microscope is cooled down to a temperature as low as 0.5 K. The STM microscope itself is adapted from a design originally proposed by Pan and then developed by Pietzch. This design is compact, stable, provides a good thermal stability and allows for in situ 180° sample rotation. Different proposals for the detection of magnetism with the STM are described at the end of the chapter. The second chapter is inherently focused on the construction of magnetic nanostructures at surfaces under UHV conditions. A VT-STM is used to characterize organo-metallic assemblies formed on Cu(100). A magnetic atom, Fe, and an organic molecule, TMA, are deposited on the surface. A chemical reaction between the two adsorbates takes place. The TMA molecules offers three carboxyl moieties for lateral linkage to the Fe atoms. In this respect the coordinative interaction between the carboxyl moiety and the Fe atom can be suitably used to build a range of different nano structures. By tuning the reciprocal amount of ligand [L] and metal [M] i.e. [M]/[L], the sequence of adsorbates deposition, the substrate temperature, a number of different structures can be synthesized. At low values of [M]/[L] mononuclear Fe[TMA]4 complexes evolve. These complexes are chiral. For increasing values [M]/[L] multiple linkage of TMA molecules is observed. By inverting the order of adsorbates deposition polynuclear complexes are observed at flat terraces. Upon annealing the formation of 1D and 2D structures arises. The 2D assemblies comprise chiral arrangements of 1D structures. The 2D assemblies may result from an hierarchical effect of chemical interactions within and among 1D structures, i.e. coordinative bonding and hydrogen bonding. Unlike crystal engineering the adsorbate-substrate interaction still plays a relevant role in this scenario. A thorough discussion of the possible applications of this technique to reproduce results of magneto chemistry is also presented.