APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011;
Dallas, Texas
Abstract: A18.00011 : Neutron Scattering Study of Frustration and Magnetic Order in Spinels
10:24 AM–11:00 AM
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The `A-site spinels' are materials with magnetic cations
constrained to lie on the diamond sublattice of the spinel
structure. These systems have been of increasing interest, as
novel theoretical and experimental results have emphasized the
central role of frustration arising from competing interactions.
In particular, the sub-family of diamond lattice antiferromagnets
with spin-only degrees-of-freedom is predicted to exhibit novel
`spiral-spin-liquid' and order-by-disorder physics (Bergman et
al., Nature Physics (2007)). Real systems MnSc2S4 (Krimmel et
al., PRB (2006)) and CoAl2O4 (Krimmel et al., Physica B (2006))
have been studied with powder neutron diffraction, and the
results are argued to be consistent with this theory. However, as
has been expressed on several occasions, studies on
single-crystals are needed for a definitive answer. Here I will
discuss neutron scattering results on single-crystalline CoAl2O4,
which have given a much more complete picture of magnetic
correlations in this material. Both elastic and inelastic
measurements have been made using triple-axis and cold chopper
spectrometry. With decreasing temperature, we observe intense
diffuse scattering centred about locations in reciprocal space
associated with collinear antiferromagnetism. At T*~6.5K, we
further observe an unexpected change in the diffuse scattering
lineshape, coupled with the emergence of well-defined spin-wave
excitations. This temperature has been associated with an
anomalous spin glass transition in the past, but we argue instead
that the available data implies a first-order phase transition to
an ordered state, possibly via the order-by-disorder mechanism.
The ground state is degenerate, and kinetically frozen walls
separating different domains give rise to the broadened
scattering at magnetic wavevectors. This scenario may be present
in many other frustrated systems. If time permits, I will also
talk about new results on crystals of related systems MnAl2O4 and
FeAl2O4.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2011.MAR.A18.11