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Etude par RMN et MuSR des propriétés électroniques et magnétiques des cobaltates de sodium

Abstract : Fitting into the framework of strongly correlated materials, Na_xCoO2 cobaltates exhibit rich properties in terms of magnetism, superconductivity and thermoelectricity. They feature a vast doping range (0<=x<=1) together with a quasi-2D structure, with a triangular lattice allowing for geometrically frustrated interactions. Doping of CoO2 layers can be inhomogeneous, thus the possibility of charge and spin ordering of the cobalts, in which the structural order of the Na dopants could play a role. Using NMR, MuSR and SQUID, this thesis investigates the nature of magnetic correlations and the cobalt charge and spin states responsible for them, as well as the origin of the sometimes observed ordered magnetism. We show that for the extremal doping x=1 the system is non-magnetic as expected, with only the low-spin S=0 Co3+ state. On doping with holes (0.67<=x<=0.87), an apparent excess of magnetic correlations is evidenced. Starting from x~0.6, spin susceptibility is suddenly far less dependent on temperature, and shows a maximum compatible with a pseudogap at low-temperature. Simultaneously, spin fluctuations develop a non-ferromagnetic nature. Co3+ sites are progressively replaced with 3.5+ sites, which are the only sites at x=0.5, independently of temperature. Both for x=0.5 and x~0.8, the features of the ordered magnetism suggest an itinerant nature, possibly linked to Fermi surface reconstruction.
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Contributor : Guillaume Lang Connect in order to contact the contributor
Submitted on : Monday, November 24, 2008 - 7:25:48 PM
Last modification on : Saturday, June 25, 2022 - 9:57:29 PM
Long-term archiving on: : Thursday, October 11, 2012 - 12:05:10 PM


  • HAL Id : tel-00341278, version 1



Guillaume Lang. Etude par RMN et MuSR des propriétés électroniques et magnétiques des cobaltates de sodium. Matière Condensée [cond-mat]. Université Paris Sud - Paris XI, 2007. Français. ⟨tel-00341278⟩



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