Exploring quantum magnetism in various spin models: an experimental study

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Abdeldaim, Aly H. ORCID: https://orcid.org/0000-0001-8417-7441 (2023). Exploring quantum magnetism in various spin models: an experimental study. University of Birmingham. Ph.D.

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Abstract

Correlated many body electron systems provide a rich source of collective quantum phenomena. These depend on the interplay of the spin and orbital degrees of freedom, the hierarchy of the interaction energy scales, alongside the host lattice geometry. This thesis presents an experimental study demonstrating the magnetic properties of various material realizations of spin models that highlight the complex magnetic behavior arising in such strongly correlated systems.

In the first results chapter, a series of S = 1/2 Mo5+-based materials, AMoOP2O7, where A = Li, Na, K, and Cs, is investigated. Despite a lattice geometry that hosts pairs of Mo5+-containing chains, a combination of magnetometry, specific heat, and inelastic neutron scattering measurements reveals dominant one-dimensional interactions with no frustrating interchain couplings in A = Na, K, Cs. This conclusion is supported by the lack of long-range magnetic order in KMoOP2O7, as examined through powder neutron diffraction, and by ab-initio calculations which reveal the role of the distortions in the octahedral Mo5+ geometry in stabilizing an active magnetic orbital favoring interactions along the chain direction. Meanwhile, LiMoOP2O7 was found to adopt an alternative magnetic sublattice comprised of three-legged spin ladders containing octahedrally coordinated Mo5+ ions. Evidence for the onset of long-range magnetic order is seen across magnetic susceptibility and specific heat measurements and confirmed through a neutron powder diffraction study. Characterization using inelastic neutron scattering, combined with an ab-initio-based simulation of the experimental spectra, confirm this non-frustrated three-legged spin ladder model. However, further optimization of the model parameters remains necessary for an accurate description of the spin Hamiltonian.

Next, the crystal structure and magnetic properties of a novel jeff = 1/2 Ru3+-based system, RuP3SiO11, are investigated. The trigonal R3c crystal structure of this material, which forms a honeycomb magnetic sublattice comprised of Ru3+ ions within an octahedral coordination formed by PO4 groups, is confirmed using synchrotron X-ray diffraction. Magnetometry and specific heat measurements suggest long-range magnetic order which is revealed to adopt a collinear Neel order through neutron powder diffraction. The relevance of this material to the Kitaev model is then investigated using a combination of inelastic neutron scattering measurements and ab-initio models that place RuP3SiO11 within a previously unaccessed region of the extended Kitaev phase diagram. A confirmation of the relevant exchange parameters, however, remains outstanding as a full optimization of the suggested spin model is yet to be completed. The magnetic field and temperature dependence phase diagram is also examined and suggests a critical magnetic field of Hc = 3.8 T.

The last results chapter is concerned with the magnetic properties of the alpha and beta psuedo-polymorphs of the S = 1/2 T3+-based coordination framework, KTi(C2O4)2.xH2O. Using a combination of single-crystal X-ray and neutron powder diffraction studies, alpha-KTi(C2O4)2.xH2O was found to adopt a tetragonal I4/mcm space group with a crystal structure containing a square planar network of Ti3+ ions in a square antiprismatic crystal field. Analysis of magnetometry and specific heat data reveal dominant antiferromagnetic interactions along the sides of the squares and minimal frustration across the diagonal. Through a neutron powder diffraction study, a Neel ordered magnetic structure was found to describe the ordered state. These results place alpha-KTi(C2O4)2.xH2O within the unfrustrated region of the phase diagram of the S = 1/2 Heisenberg square antiferromagnet model. In contrast, beta-KTi(C2O4)2.2H2O, forms a diamond-like magnetic sublattice of Ti3+ ions within the the hexagonal P6_222 space group. Fitting the S = 1/2 Heisenberg diamond lattice antiferromagnet model to the magnetic suscpetibility and specific heat yields exchange parameters that are an order of magnitude larger than in alpha. Ab-initio calculations reveal that it is the interplay of the active magnetic orbital and the superexchange pathway that results in this discrepancy. Finally, an antiferromagnetic structure is characterized by analyzing neutron powder diffraction data. By examining these results, the alpha and beta psuedo-polymorphs are identified as material realizations of the S = 1/2 Heisenberg square and diamond lattice antiferromagnet models, respectively.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):