Curley, Samuel Patrick Michael (2021) Exotic ground states and excitations in quantum magnets. PhD thesis, University of Warwick.

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Abstract

Within this thesis I will outline experimental and theoretical investigations on five quantum magnets, which are primarily composed of lattices of transition-metal ions joined via molecular linkers. The first study compares the two isostructural quasi-one-dimensional ferromagnetic chain compounds M(NCS)2(thiourea)2 where M = Ni(II), Co(II) and thiourea = SC(NH2)2. Experimental measurements reveal a large difference in the strength of the intrachain exchange coupling in each material, with JN ͂ 100 K and JCo ͌ 4 K, despite the highly similar exchange pathways in the two compounds. Charge density calculations reveal that the changes in the electronic structure, caused by substitution of the spin-species, are the underlying cause for the large difference in the two exchange energies. The work highlights the important role of the spin-species in effectively mediating magnetic exchange
interactions.

Secondly, multiple measurement techniques were used to determine the magnetic ground-state of the coordination polymer NiI2(3,5-lutidine)4, where 3,5-lutidine = C7H9N. Owing to a minimal Ising-like anisotropy D = -1:2(1) K compared to the intrachain coupling J = 17:5(2) K and vanishingly small interchain coupling, the system was found to reside deep within the Haldane phase of the zero-field phase diagram for quasi-one-dimensional S = 1 antiferromagnets. Magnetometry data combined with inelastic neutron scattering studies show that the anisotropic critical fields for field parallel µ0Hk
c̎ = 5:3(1) T and perpendicular µ 0H?c ̎ = 4:3(1) T to the chain axis were best described by a quantum-field-theoretical ‘fermion’ model, previously posited to only be applicable to easy-plane Haldane systems. Furthermore, in-field inelastic neutron scattering studies reveal a novel excitation in the system above 2 T, attributable to spin-half end-chains predicted to exist in Haldane systems. High-pressure magnetometry studies on this compound also demonstrate it is exceedingly amenable to pressure being used as an external tuning parameter to alter the strength of the magnetic exchange interactions in the system.

Lastly, the magnetic ground-state of the asymmetric S = 1=2 antiferromagnetic dimer CuVOF4(H2O)6 ̇H2O was characterised using magnetometetry, electron-spin resonance and density functional theory calculations. The work highlights that the Jahn-Teller distortion of the Cu(II) octahedral environment is not only responsible for the unique structure, but also the low-dimensional magnetism in this compound. A study of the pressure-evolution of the magnetic and structural properties of CuVOF4(H2O)6 ̇H2O was performed and compared to a parallel pressure study on the symmetric dimer compound [Cu(pyrazine)0:5(glycine)]ClO4 (where pyrazine = C4H4N2 and glycine = C2H5NO2). The behaviour of each compound under applied hydrostatic pressure was found to radically different, with the quantum-disordered ground-state of CuVOF4(H2O)6 ̇H2O being made more robust with increasing pressure whilst, in contrast, pressure serves to close the singlet-triplet energy gap in [Cu(pyrazine)0:5(glycine)]ClO4 as the system is driven through a pressure-induced quantum critical point around 16.5(3) kbar.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics
Library of Congress Subject Headings (LCSH):	Magnetic materials, Quantum theory, Magnetism, Ferromagnetic materials, Transition metal ions
Official Date:	August 2021
Dates:	Date Event August 2021 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Goddard, Paul (Paul Andrew)
Extent:	xxvi, 139 leaves : illustrations (chiefly colour), charts
Language:	eng

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