Abstract
The A-site spinel material, CoAl2O4, constitutes a physical realization of the frustrated diamondlattice
antiferromagnet, a model in which competition between nearest (J1) and next-nearest neighbor
(J2) exchange interactions are predicted to drive unique incommensurate or ‘spin-spiral liquid’
(SSL) ground state behavior. Our previous single-crystal neutron scattering study instead classi-
fied this material as a ‘kinetically-inhibited’ antiferromagnet, where the system has a collinear N´eel
ground state but long-range correlations are blocked by the freezing of domain wall motion during
the coarsening step of a first-order phase transition at T
∗ = 6.5 K1
. The current paper expands
on our original results in several important ways. New elastic and inelastic neutron measurements
are presented that show our initial conclusions are affected by neither the sample measured nor the
instrument resolution, while measurements to temperatures as low as T = 250 mK limit the possible
role being played by low-lying thermal excitations. Polarized diffuse neutron measurements confirm
reports of short-range antiferromagnetic correlations and diffuse streaks of scattering, but also show
that these correlations are largely isotropic and exhibit no spin chirality. Simple modeling explains
major diffuse features as a signature of overlapping critical correlations between neighboring Brillouin
zones. Finally, and critically, this paper presents detailed elastic and inelastic measurements
of magnetic correlations in a single-crystal of MnAl2O4, which acts as an unfrustrated analogue to
CoAl2O4. Our results show that the unfrustrated material undergoes a classical continuous phase
transition to a collinear ordered state at TN = 39 K, complete with collective spinwave excitations
and Lorentzian-like critical correlations which diverge at the transition. Direct comparison between
the two compounds indicates that CoAl2O4 is unique, not in the nature of high-temperature diffuse
correlations, but rather in the nature of the frozen state below T
∗
. The higher level of cation
inversion in the MnAl2O4 sample indicates that this novel behavior is primarily an effect of greater
J2
J1
