Disorder-induced spin liquids

Structural and magnetic studies of the disorder-induced classical spin liquid Ho₂Ti_2-xSc_xO_7-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 Ho₂Ti_2-xSc_xO_7-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 µ_B 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 D_3d 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) µ_B. 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.