TY - BOOK

T1 - Neutron spectroscopy studies of geometrically frustrated antiferromagnets

AU - Janas, Sofie

PY - 2020

Y1 - 2020

N2 - In this PhD thesis two geometrically frustrated classical Heisenberg antiferromagnets on the paradigmatic triangular and kagomé lattices are studied. The two compounds, hexagonal yttrium manganite (h-YMnO3) and chromium jarosite (KCr3(OD)6(SO4)2), are investigated using magnetometry and neutron spectroscopy, and their behaviour is set in relation to the concept of classical spin liquids.h-YMnO3 is a spin-2 frustrated and multiferroic antiferromagnet on thestacked triangular lattice. The antiferromagnetic phase transition at TN = 72 Kwas investigated with neutron spectroscopy on a single-crystal and shown to have an anomalous critical exponent of β = 0:207(3). This critical exponent does not match any known universality class, and we suggest the existence of a separate universality class for frustrated pseudo-two-dimensional triangular lattice Heisenberg antiferromagnets.Furthermore, significant correlated disorder was observed in h-YMnO3 bothfar above and below TN as diffuse, directional, and quasielastic scattering. Thiswas measured using neutron spectroscopy, including the newly-commissionedmultiplexing triple-axis spectrometer CAMEA at the Paul Scherrer Institute.The diffuse dynamic scattering is two-dimensional in the triangular plane with acorrelation length that seems to diverge as the temperature approaches zero. Wemodel this scattering with critical two-dimensional spin correlations existing ina vastly extended critical region due to the frustration. We argue that this is amanifestation of classical spin liquid behaviour, and suggest critical scattering asa broader framework for understanding frustrated antiferromagnets.Cr-jarosite, KCr3(OD)6(SO4)2, is a spin-3=2 kagomé antiferromagnet. Paststudies has been plagued by significant non-stoichiometry. In this thesis, themost comprehensive structural characterisation of polycrystalline Cr-jarosite ispresented. The hydrothermal-redox synthesis is shown to have problems withsignificant K+ substition for D3O+ in Cr-jarosite, which causes stacking faults inthe c direction. The most stoichiometric sample with Cr occupancy of 98.2(2)%and K occupancy of 84 - 89% is concluded to order antiferromagnetically atTN = 3:8(1) K in a q = 0 structure with a small ferromagnetic canting due to theDzyloshinkii-Moriya interaction.Subsequently, the dynamics of polycrystalline, deuterated Cr-jarosite was studied using time-of-fight neutron spectroscopy. This revealed spin waves below TN with diffuse broadening due to finite life-times. The spin waves were modelled using linear spin wave theory. Above TN, distinct diffuse excitations were observed that indicated short-ranged, two-dimensional magnetic scattering subsisting until 35 K. The excitations were modelled using a heuristic model for powder-averaged two-dimensional features in three-dimensional (hkl) space. This cluster-like approach revealed an increasing correlation length for decreasing temperature, very similar to the extended critical scattering observed in h-YMnO3.

AB - In this PhD thesis two geometrically frustrated classical Heisenberg antiferromagnets on the paradigmatic triangular and kagomé lattices are studied. The two compounds, hexagonal yttrium manganite (h-YMnO3) and chromium jarosite (KCr3(OD)6(SO4)2), are investigated using magnetometry and neutron spectroscopy, and their behaviour is set in relation to the concept of classical spin liquids.h-YMnO3 is a spin-2 frustrated and multiferroic antiferromagnet on thestacked triangular lattice. The antiferromagnetic phase transition at TN = 72 Kwas investigated with neutron spectroscopy on a single-crystal and shown to have an anomalous critical exponent of β = 0:207(3). This critical exponent does not match any known universality class, and we suggest the existence of a separate universality class for frustrated pseudo-two-dimensional triangular lattice Heisenberg antiferromagnets.Furthermore, significant correlated disorder was observed in h-YMnO3 bothfar above and below TN as diffuse, directional, and quasielastic scattering. Thiswas measured using neutron spectroscopy, including the newly-commissionedmultiplexing triple-axis spectrometer CAMEA at the Paul Scherrer Institute.The diffuse dynamic scattering is two-dimensional in the triangular plane with acorrelation length that seems to diverge as the temperature approaches zero. Wemodel this scattering with critical two-dimensional spin correlations existing ina vastly extended critical region due to the frustration. We argue that this is amanifestation of classical spin liquid behaviour, and suggest critical scattering asa broader framework for understanding frustrated antiferromagnets.Cr-jarosite, KCr3(OD)6(SO4)2, is a spin-3=2 kagomé antiferromagnet. Paststudies has been plagued by significant non-stoichiometry. In this thesis, themost comprehensive structural characterisation of polycrystalline Cr-jarosite ispresented. The hydrothermal-redox synthesis is shown to have problems withsignificant K+ substition for D3O+ in Cr-jarosite, which causes stacking faults inthe c direction. The most stoichiometric sample with Cr occupancy of 98.2(2)%and K occupancy of 84 - 89% is concluded to order antiferromagnetically atTN = 3:8(1) K in a q = 0 structure with a small ferromagnetic canting due to theDzyloshinkii-Moriya interaction.Subsequently, the dynamics of polycrystalline, deuterated Cr-jarosite was studied using time-of-fight neutron spectroscopy. This revealed spin waves below TN with diffuse broadening due to finite life-times. The spin waves were modelled using linear spin wave theory. Above TN, distinct diffuse excitations were observed that indicated short-ranged, two-dimensional magnetic scattering subsisting until 35 K. The excitations were modelled using a heuristic model for powder-averaged two-dimensional features in three-dimensional (hkl) space. This cluster-like approach revealed an increasing correlation length for decreasing temperature, very similar to the extended critical scattering observed in h-YMnO3.

M3 - Ph.D. thesis

BT - Neutron spectroscopy studies of geometrically frustrated antiferromagnets

PB - Niels Bohr Institute, Faculty of Science, University of Copenhagen

ER -