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An investigation into the magnetic properties of low-dimensional 3d magnets

Messina, G; (2016) An investigation into the magnetic properties of low-dimensional 3d magnets. Doctoral thesis , UCL (University College London).

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This thesis describes investigations of the effects of low dimensionality on two different antiferromagnetic systems. First, coherent spin-density-waves (SDWs) were studied in thin films of epitaxial chromium grown on MgO. This system exhibits antiferromagnetic ground states where the density of the spin of the conduction electrons is spatially modulated with an incommensurate period. The SDW features a wave vector Q and is accompanied by a charge-density-wave (CDW) and a sinusoidal lattice distortion, or strain-wave (SW), whose wave vectors are related by the rule QSW = QCDW = 2QSDW . The CDW and the SW produce satellite peaks around Cr ionic reflections. It is known that the SDW antinodes coincide with the maxima elongation of the lattice constant and by fitting the X-Ray diffraction data collected from our samples we extracted the phase of the SW which implies that the SDW has a node at the interface with non-magnetic MgO and an antinode at the free surface (where a ferromagnetic oxide layer is expected to grow). This confirms the idea that the SW and CDW are consequences of magnetic order. Second, a new technique was designed and implemented to grow single crystals of BaNi2V2O8. This material is a quasi-two-dimensional (2D) antiferromagnet on a honeycomb lattice, made of edge-sharing NiO6 octahedral layers separated by V5+O4 and Ba2+ layers. The only ion carrying a magnetic moment is Ni2+, which in octahedral coordination has an e2 g configuration making BaNi2V2O8 a spin-1 system. Neutron inelastic scattering studies of the magnetic excitations of this compound were conducted, as a detailed investigation is still missing due to the lack of large single crystals. The correlation lengths above TN= 48 K extracted from the analysis of the critical properties are compatible with the Berezinskii-Kosterliz-Thouless behaviour, indicating the presence of vortex-type excitations.

Type: Thesis (Doctoral)
Title: An investigation into the magnetic properties of low-dimensional 3d magnets
Event: UCL (University College London)
Language: English
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/1508560
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