Revisiting the ground state of CoAl 2 O 4 : Comparison to the conventional antiferromagnet MnAl 2 O 4

The A-site spinel material CoAl2O4 is a physical realization of the frustrated diamond-lattice antiferromagnet, a model in which unique incommensurate or “spin-spiral-liquid” ground states are predicted. Our previous single-crystal neutron scattering study instead classified it as a “kinetically inhibited” antiferromagnet, where the long-ranged correlations of a collinear Néel ground state are blocked by the freezing of domain-wall motion below a first-order phase transition at T∗=6.5 K. This paper provides new data sets from a number of experiments, which support and expand this work in several important ways. We show that the phenomenology leading to the kinetically inhibited order is unaffected by sample measured and instrument resolution, while new low-temperature measurements reveal spin correlations are unchanging between T=2 K and 250 mK, consistent with a frozen state. Polarized diffuse neutron measurements show several interesting magnetic features, which can be entirely explained by the existence of short-ranged Néel order. Finally, and crucially, this paper presents some neutron scattering studies of single crystalline MnAl2O4, which acts as an unfrustrated analog to CoAl2O4 and shows all the hallmarks of a classical antiferromagnet with a continuous phase transition to Néel order at TN=39 K. 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 behavior is primarily an effect of greater next-nearest-neighbor exchange