Neutron scattering studies of the spin correlations in the frustrated kagome lattice antiferromagnet

Kittiwit Matan, MIT

Abstract: The collective behavior of interacting magnetic moments can be strongly influenced by the topology of the underlying lattice. In geometrically frustrated spin systems, interesting spin dynamics and chiral correlations may develop that are related to the spin arrangement on triangular plaquettes. We report studies of the spin- wave excitations and spin chirality on a two-dimensional geometrically frustrated lattice. Our new chemical synthesis methods allow us to produce large single crystal samples of KFe_3(OH)_6(SO_4) _2, an ideal kagome lattice antiferromagnet. The spin-wave excitations have been measured using high-resolution inelastic neutron scattering. We directly observe a flat mode which corresponds to a lifted 'zero energy mode', verifying a fundamental prediction for the kagome lattice. A simple Heisenberg spin Hamiltonian provides an excellent fit to our spin-wave data. The antisymmetric Dzyloshinskii-Moriya interaction is the primary source of anisotropy and explains the low-temperature magnetization and spin structure. In addition, combined thermodynamic and neutron scattering measurements reveal that the phase transition to the ordered ground-state is unusual. At low temperatures, application of a magnetic field induces a transition between states with different non-trivial spin-textures. The transition indicated by the sudden increase in the magnetization arises as the spins on alternating layers, which are previously oppositely canted due to the ferromagnetic interplane coupling, rotate 180 degrees to align the canting moment along the c-axis. These observations are consistent with the ordering induced by the Dzyloshinskii-Moriya interaction. Elastic neutron scattering measurements in high field verify the 180-degree spin rotation at the transition.