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Condensed Matter Seminars
| Condensed Matter
Thursday, October 9, 2008
Physics Building, Room 313
Note special time.
[Host: Seunghun Lee]
ISSP, U of Tokyo
"Neutron scattering and magnetization studies of the kagome lattice antiferromagnet"
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. I
will talk about our 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 KFe3(OH)6(SO4)2, an ideal kagomé 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 kagomé 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 explain 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° 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° spin rotation at the transition.
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