The Magnetism of Charge-Stripe Ordered Nickelates.

Hanson, Jack Lee (2020) The Magnetism of Charge-Stripe Ordered Nickelates. Masters thesis, University of Central Lancashire.

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The work presented in this thesis uses neutron scattering and magnetization techniques to investigate the magnetic ordering processes and magnetic excitations of charge-stripe ordered La2-xSrxNiO4+δ.

When investigating the magnetic ordering, a x = 0.37 sample has been studied using neutron diffraction and magnetization measurements. The results from these investigations have been compared to earlier work on a samples with doping levels of x = 1/3, where commensurate charge-stripe order is the most stable, and x = 0.275, which is incommensurately charge-stripe ordered. In studying the temperature evolution of the magnetic Bragg peak widths of both of these samples, an anomalous broadening was observed on cooling. This broadening indicates that there is a reduction of magnetic correlation length. The magnetic order is anticipated to stabilise at these lower temperatures, where the thermal energy is removed from the system, reducing the amount of lattice vibrations that perturb the magnetic order. From neutron diffraction and magnetization measurements a new possible transition has been identified, this transition is linked to the changing periodicity of the charge order.

The correlation lengths and percentage broadening of the magnetic Bragg peak widths in both the x = 0.275 and x = 0.37 samples, has also been compared both in-Ni-O-plane and out-of-Ni-O-plane on cooling. In doing so, a thermal hysteresis effect was observed in the x = 0.37 sample, leading to further disorder of the magnetic order.
Whilst the in-Ni-O-plane broadening suggests a clear reduction of in-Ni-O-plane magnetic correlation length, clearly observed in the x = 0.37 sample. The similarities and differences in the magnetic ordering processes of both samples will be discussed throughout this thesis.

Inelastic neutron scattering was used to study the magnetic excitations of charge-stripe ordered La2-xSrxNiO4+δ, to see if the magnetic excitations respond to the changing periodicity of the charge-stripe order as we would expect theoretically. In doing so, it has been observed that only magnetic excitations induced at the lowest energy transfers appear to kink to track the changing periodicity. However, magnetic excitations with transfers of energy 3 meV or greater, appear unaffected by this changing magnetic order and as so, no longer kink.

From this work, it is clear that to fully understand the charge-stripe order in La2-xSrxNiO4+δ a range of doping levels must be studied. These being < 1/3, = 1/3 and > 1/3.

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