SHORT RANGE ORDER AND CONTINUUM EXCITATIONS IN FRUSTRATED MAGNETS

Abstract

Magnetic materials generally develop magnetic long range order at low temperatures. This standard low temperature instability can however be suppressed in frustrated magnets, a set of magnetic materials where the selection of a particular low energy ground state is ``frustrated'' due to competing interactions and geometrical constraints. Such a magnetically disordered ground state, called ``spin liquid'', represents a novel state of matter that promises exotic properties such as fractionalized excitations.\cite{Balents2010}

In this thesis, the magnetic properties of three candidate materials $\rm Pr_2Zr_2O_7$, $\rm NiGa_2S_4$, and $\rm SrHo_2O_4$, all of which elude conventional magnetic long range order, are studied extensively. Neutron scattering is the experimental tool of choice in that it enables direct probing of spatial and temporal spin correlations, which arguably carry the most salient manifestations of a spin liquid state.

$\rm Pr_2Zr_2O_7$ is conjectured to host a quantum spin ice state, a possible realization of $U(1)$ quantum spin liquid.\cite{PhysRevB.69.064404} A recipe was developed for synthesis of high quality stoichiometric single crystalline $\rm Pr_2Zr_2O_7$. Through elastic neutron scattering, quasi-static low temperature spin correlations are found that resemble those of classical spin ice, while inelastic neutron scattering shows a dominant role for quantum spin fluctuations. Analysis of the data indicates these result from a combination of inter-spin interactions and inhomogeneous splitting of the single ion magnetic ground state doublets, which are not protected by time reversal symmetry and can be lifted by sufficiently anisotropic electric field environment. These findings highlight an intricate interplay between spin and lattice degrees of freedom that could be important in related materials. Indeed we shall see that the very fact that the energy scale for ordering is suppressed compared to the bare interactions leads to an inherent sensitivity to symmetry breaking impurities.

$\rm NiGa_2S_4$ is a rare realization of undistorted spin-1 triangular lattice. Surprisingly only a short range ordered magnetic state is achieved at low temperature. Inelastic neutron scattering uncovers a broad yet dispersive magnetic spectrum that cannot be explained by conventional spin wave theory. The seemingly inconsistency between the continuum neutron spectrum and low temperature specific heat measurements which suggest the existence of coherent quasiparicles\cite{Nakatsuji2005} indicates the excitations could be of novel nature.

Elastic neutron scattering measurements on $\rm SrHo_2O_4$ showed this material can be viewed as an assembly of two types of $J_1-J_2$ Ising chains that have N\'eel ($\uparrow\downarrow\uparrow\downarrow$) and double-N\'eel ($\uparrow\uparrow\downarrow\downarrow$) ground states respectively. At low temperature the N\'eel chains develop three dimensional long range order, which causes the double-N\'eel chains to freeze into a disordered incommensurate state. A ``disorder by order'' mechanism was proposed to account for this unusual spin disorder.