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Abstract

Novel magnetic materials are constantly emerging. This thesis reports an Electron Spin Resonance (ESR) study on three novel materials of great scientific interest: monoclinic SeCuO3 compound representing a network of tetramers, large assembly of ultrathin graphitic nanoparticles obtained by heavy sonication of graphite powder, and newly synthesized organic system (EDT-TTF-CONH2)6Re6Se8(CN)6 which spans a kagom ́e topology above 200 K. Respecting the order of the selected materials, the following phenomena have been observed. The inter tetramer antiferromagnetic coupling in SeCuO3 was found responsible for the development of a fluctuating long-range antiferromagnetic order at T = 8 K with an unusual temperature dependence of the effective g-tensors. These observations were explained by site selective quantum correlations. In consistency with the picture of magnetic correlations at 1D graphene edges, the temperature dependence of the ESR spectra in the graphitic sample signified the onset of ferromagnetic correlations below 25 K, but the true long-range ferromagnetic ordering was missing and the magnetic state was interpreted as a superparamagnetic like phase. Through ESR measurements at low and high frequencies, the temperature and pressure dependence of the magnetic response of (EDT-TTF-CONH2)6Re6Se8(CN)6 were investigated. The major finding was the cohabitation of correlations and metallicity in a dynamically disordered kagom ́e lattice at high temperatures. Below 200 K the system looses the kagom ́e geometry and an antiferromagnetic spin exchange stabilizes the system. Conclusively, the thesis also demonstrates the powerfulness of ESR spectroscopy to link and combine different physical phenomena in emergent magnetic materials that is of great fundamental and of possibly practical significance.

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