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Many of the most fascinating phenomena in condensed matter physics, such as superconductivity, density waves, magnetism and many others arise from interactions between electrons themselves and electrons and the crystal lattice. Despite the great advances in condensed matter physics in the last century not all of these phenomena have been fully understood. Sodium cobaltate, NaxCoO2, is an interesting candidate to study some of these open questions, as its phase diagram shows many different phases as a function of sodium concentration x and temperature. Among these are superconductivity, charge order, different magnetic orders and high thermopower regions. There are several reasons for this strong influence the sodium concentration x has on the electronic system. For one, the sodium atoms donate their 3s electrons to the cobalt-oxygen derived bands, which form the important electronic states near the Fermi level. Secondly, it is not only the pure sodium amount but also the pattern formed by the sodium ions, which influence the electronic properties by changing the Coulomb potential background for the CoO2 layers and providing exchange paths for magnetic coupling. The ion ordering itself depends on the sodium concentration, and the temperature, which makes it difficult to experimentally disentangle these two influences. This work concentrates on the high x region (x > 0.75) of the phase diagram, the different low temperature magnetic phases and how these phases are influenced by the sodium ion ordering. For the first time it could be shown that in NaxCoO2, x ∼ 0.80, two different sodium orderings, connected to a structural phase transition around 280 K, induce different magnetic transitions at 8 K, respectively 15 K. That means that in the same sample, i.e. at the exact same sodium concentration, different ground states can be induced by changing the sodium ordering. This opens a small experimental window to disentangle the influences of pure electron donation and the effects induced through the ion ordering. The experimental prove was established by using a heat capacity setup and a defined cooling protocol, which can suppress the 280 K structural transition by fast cooling the sample. Further heat capacity measurements were done to characterize the low temperature magnetic ground states. In addition the magnetic phases were probed on a local level by muon spin rotation, μSR, measurements, which revealed differences in magnetic structure of both ground states. A spin alignment for the magnetic ground states is proposed based on the heat capacity and μSR data. Resistivity measurements are presented, which also show a clear signature of the sodium mobility and rearrangement processes and suggest significant spin fluctuations above the magnetic transitions. Based on the μSR and resistivity data a spin density wave, which is often cited in the literature, is excluded as a magnetic ground state. An attempt

Switching magnetic phases by modifying the sodium vacancy ordering in NaxCoO₂