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Title: Magnetic properties of frustrated Shastry-Sutherland magnets : rare earth tetraborides
Author: Brunt, Daniel C.
ISNI:       0000 0004 7223 9432
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2017
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Geometrically frustrated magnetism arises when the competing interactions are incompatible with the geometry of the lattice. This leads to a large ground state degeneracy and the system typically has difficulty establishing a unique ground state, often giving rise complex intermediate magnetic phases and other unusual phenomena. Recently the rare-earth tetraborides, RB4, family of compounds has garnered attention as it is a rare experimental realisation of the frustrated Shastry-Sutherland lattice. Here the R3+ ions form a network of squares and triangles which is topologically equivalent to the Shastry-Sutherland lattice. The competing interactions leads to a rich variety of magnetic phase in the RB4 family in both zero and applied magnetic field. An investigation into two members of the RB4 family, HoB4 and NdB4 is presented in this thesis. Both compounds show successive magnetic phase transitions, where HoB4 orders with an incommensurate magnetic state at 7.1 K and orders with a non-collinear antiferromagnetic structure at 5.7 K, as well as magnetisation plateaux with fractional values of the saturation magnetisation of 1/6,1/3 and 5/3. NdB4 orders at 17 K with a non-collinear antiferromagnetic structure followed by two distinct incommensurate structures at 7 K and 4.9 K. NdB4 shows a single magnetisation plateau at 1/5 the saturation magnetisation. Bulk property measurements of magnetisation, magnetic susceptibility, resistivity and heat capacity were used to maps out and construct the complex phase diagrams of HoB4 and NdB4 with measurements down to 0.5 K and magnetic fields up to 500 kOe. HoB4 shows six distinct magnetic phases, while NdB4 has five The stabilised magnetic phases were determined in zero field using single crystal neutron diffraction measurements utilising both polarised and un-polarised measurements. While the un-polarised neutron measurements were extended to investigate the magnetic structures of the field-induced magnetic structure in both HoB4 and NdB4. Finally inelastic neutron scattering measurements on polycrystalline samples were used to map out the crystal field scheme in HoB4 and NdB4. Single crystal samples were used to investigated the low temperature magnetic excitations in HoB4. The initial analysis of the inelastic neutron studies is presented. Overall, both HoB4 and NdB4 exhibit a variety of magnetic phenomena, and their magnetic properties contribute to the understanding of the physics of the RB4 family of compounds.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics