PhD defense by Sofie Janas

Title: Neutron spectroscopy studies of geometrically frustrated antiferromagnets

Frustrated magnetism, in which competing exchange interactions suppress magnetic order, is ubiquitous within condensed matter physics. One of the most heavily discussed topics is the quantum spin liquid (QSL), which is defined as a fluid-like state, where the spins are highly correlated and continue to fluctuate down to temperatures of absolute zero, although experimental verification is still contentious. Many frustrated compounds show features reminiscent of QSL_s, but order magnetically at finite temperatures, which excludes them as QSL candidates. Above this ordering, these compounds are referred to as classical spin liquids, cooperative paramagnets, or merely as having spin-liquid-like phases, although no clear definition of these terms is readily available. The clarification of the concept of classical spin liquids is the central topic throughout in this thesis. This is studied through the investigation of frustrated classical antiferromagnets on the two paradigmatic geometries: the stacked triangular lattice in hexagonal yttrium-manganite (h-YMnO₃), and the stacked kagomé lattice in chromium jarosite (KCr₃(OH)₆(SO₄)₂).

h-YMnO₃ is a spin-2 frustrated and multiferroic antiferromagnet on the stacked triangular lattice. We investigated the antiferromagnetic phase transition and showed it to have an anomalous critical exponent. This 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, we observed significant correlated disorder in h-YMnO₃ both far above and below T_N as diffuse, directional, and quasielastic scattering as measured using neutron spectroscopy. We model this scattering with critical two-dimensional spin correlations existing in a vastly extended critical region due to the frustration. We argue that this is a manifestation of classical spin liquid behaviour, and suggest critical scattering as a broader framework for understanding frustrated antiferromagnets. 

Cr-jarosite, KCr₃(OH)₆(SO₄)₂, is a spin-3/2 kagomé antiferromagnet. Past studies has been plagued by significant non-stoichiometry. In this thesis, we present the most comprehensive structural characterisation of polycrystalline Cr-jarosite using newer synthesis methods. However, these are shown to still have problems with significant K+ defects.

We proceeded to study the dynamics of polycrystalline, deuterated Cr-jarosite using time-of-flight neutron spectroscopy. This revealed spin waves below T_N with diffuse broadening due to finite life-times. Above T_N , distinct diffuse excitations are present that indicate short-ranged, two-dimensional magnetic scattering. The excitations were modelled using a cluster-like approach, which revealed an increasing correlation length for decreasing temperature, very similar to the extended critical scattering observed in h-YMnO₃.

By zoom:  https://ucph-ku.zoom.us/j/67830564482?pwd=dW5pZC9iZGl5cTNtenpuM0lZNlhyZz09