Disorder-induced spin liquids

Tomas Northam De La Fuente

Research output: ThesisDoctoral Thesis

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Structural and magnetic studies of the disorder-induced classical spin liquid Ho2Ti2-xScxO7-x/2 and the candidate quantum spin liquid Pr2ScNbO7 were performed through a combination of neutron scattering techniques and computational modelling.
Disorder was investigated in Ho2Ti2-xScxO7-x/2 with x=0, x =0.1 and x=0.5 by doping with Sc ions. The difference in valency between Ti4+ and Sc3+ led to the need for charge compensating oxygen vacancies. This allowed the introduction of oxygen vacancies in a controlled manner throughout the structure uniformly. Total scattering and structural diffuse scattering measurements were performed in conjunction with RMC fits, Bragg refinements and ab initio density functional theory (DFT) calculations to determine the location of the vacancies. Not only was it found that the vacancies are located at O(2) sites but, in addition, other O(2) ions were displaced towards the centre of the tetrahedra formed by the B-sites.
Crystal electric field (CEF) measurements were performed to determine the effect of the vacancies on the single-ion magnetism. It was found that the ground state doublet of the Ho ions next to a vacancy split into two singlets separated by an energy of 0.12 meV. However, calculations performed at 0.5 K and with a 0.1 T applied magnetic field show that the Ho ions next to a vacancy developed a moment of 6.2 µB mostly along the <111> direction but with a 15.2º tilt towards the vacancy. Magnetic diffuse scattering measurements were performed to study the effects of disorder on the cooperative magnetism. While the resulting scattering is qualitatively similar to that of stoichiometric Ho2Ti2O7 [1,2], the pinch point width broadened as the levels of doping increased. It is possible that this broadening is caused by the tilting of the moments at the defective-Ho sites.
The structural arrangement of Sc and Nb ions in the highly substituted Pr2ScNbO7 system was studied by means of neutron scattering, DFT and RMC fits. It was found that the structure with the lowest energy contains a charge ice structure with equal numbers of Sc and Nb ions in each tetrahedron and with chains of alternating Sc and Nb. However, this structure is chemically frustrated, since the alternating chains cannot be accommodated in all directions. Total scattering calculations show that no long-range order of this lowest energy structure exists, and diffuse scattering calculations using a single unit cell of the lowest energy structure are in excellent agreement with the measured data. This suggests that the structure comprises small domains of the lowest energy structure with Sc/Nb disorder at the domain walls.
CEF analyses were performed on Pr2ScNbO7 to study the effect of the disorder on the single-ion magnetism. CEF fits were performed on the data, assuming a D3d symmetry at the Pr sites. This resulted in a doublet ground state, with the first excited state at 2.7 meV. The lowest energy DFT structure contains two different Pr sites. Point-charge-model
CEF calculations predict singlet ground states for both Pr sites, with the first excited state at 1.2 meV and 4.6 meV. Furthermore, both sites form spiral one-dimensional chains. Magnetic diffuse scattering measurements were performed to study the effects of the disorder on the cooperative magnetism, producing uncorrelated scattering. The one-dimensional arrangement of these ions could explain the measured uncorrelated scattering. These measurements were used to extract a Pr effective magnetic moment of 2.10(8) µB. Finally, low energy excitation studies show that, when subtracting the 5 K data from the lower temperature data, a peak near 0.7 meV energy transfer is present, very similar to the ones found in Pr2Zr2O7 [3] and Pr2Hf2O7 [4], which were claimed to be a signature of cooperative quantum fluctuations.
Original languageEnglish
Awarding Institution
  • Royal Holloway, University of London
  • Goff, Jon, Supervisor
  • Mangin-Thro, Lucile, Supervisor, External person
Award date1 May 2023
Publication statusUnpublished - 1 May 2023


  • Quantum spin liquid
  • Classical spin ice
  • Polarised neutron diffraction
  • Inelastic neutron scattering
  • Density functional theory (DFT)
  • Reverse Monte Carlo
  • Institut Laue Langevin

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