STUDY OF MAGNETIC AND ELECTRONIC PROPERTIES OF LOW DIMENSIONALITY SYSTEMS USING AUGER PHOTOELECTRON COINCIDENCE SPECTROSCOPY

The objective of this thesis is to exploit the high degree of selectivity of Auger photoelectron coincidence spectroscopy (APECS) to get singular information, not otherwise obtainable by means of more conventional spectroscopies, in fields which are presently of wide interest in material sciences, like magnetic systems with low dimensionality or hybrid materials where organic molecules are assembled on conductor/ semiconductors substrates. In core-valence-valence Auger decays, two holes are created in the valence band. In conventional Auger spectroscopy, only one of the two electrons leaving the valence band is measured (i.e. the Auger electron) and consequently many details of the other electron filling the core hole are lost. In APECS, the coincident detection of the Auger electron and its parent photoelectron allows one to put constraints on the electron which leaves the valence band and fills the core hole, thereby getting additional information of the two-hole Auger finale state. In Angle Resolved Auger photoelectron coincidence spectroscopy (AR-APECS), by exploiting some constrains put on the m sublevel of the emitted electrons, a spin selectivity on the two hole final state is provided, which is moreover sensitive to the electron correlation. We have used AR-APECS to study metallic ferromagnetic (FM) and metal oxide antiferromagnetic (AFM) systems. AR-APECS measurements for FM Ni/Cu(001) show the sensitivity to the band spin splitting as well as to the correlation effect in the Ni (M23M45M45) Auger line shape. A strong correlation is ascribed to interactions within majority sub band only. We have also carried out thickness dependent study for FM Ni/Cu(001) which suggest that, such a correlation in the two hole Auger final state follow a quantum confinement behavior. In case of AFM NiO/Ag(001) system, the AR-APECS spectra acquired for NiO in its AFM phase show a strong geometric dichroism due to the spin selectivity acting on atomic-like multiplet terms (singlet, triplet and quintet). Such a dichroism completely disappears in the paramagnetic phase. This opens up the possibility of monitoring magnetic transitions from a localized perspective, without having to rely on long range order crystal periodicity or bulk thermodynamic measurements. On the other hand APECS offers the capability to probe the valence band in a chemical selective way, by exploiting the chemical core level shift (which is typical characteristic of the photoemission technique XPS) of the parent photoelectron detected in coincidence with the Auger electron. By using APECS we have studied, with an atomistic approach, the charge transfer phenomenon involving the Copper Phthalocyanine (CuPc) macromolecule at the interface with the Aluminum surface. Our results indicate that the charge transfer from Al substrate to the CuPc molecule is not uniformly distributed all over the molecule, but the benzene ring of the CuPc molecule show the higher extent of charge transfer than the pyrrole ring.