Consequences of Frustration for Order in A-site Spinel Antiferromagnets

G. J. MacDougall (Oak Ridge National Laboratory)

Magnetic frustration refers to the inability of a material with strong exchange interactions to achieve order due to competing interactions or topological constraints. Such systems exhibit highly degenerate ground states and strong fluctuations, and are of intense interest due to the novel and useful physics which often emerges. The spinels, with chemical formula AB2X4, have been particularly fruitful for researchers interested in such phenomena. In this talk, I will discuss some of my neutron scattering experiments on A-site spinel systems, where magnetic cations reside on the diamond sublattice of the spinel structure. Specifically, I will compare and contrast the results of the closely-related spinels, MnAl2O4 and CoAl2O4. The former is only weakly frustrated and is thought to order magnetically at TN~40K. The latter is more strongly frustrated, and previous studies on powders have reported an anomalous spin-glass transition at a temperature, T* < 9 K.

To follow up on these initial results on powders, our collaboration has grown large single crystals of MnAl2O4 and CoAl2O4 using the float-zone technique and performed both elastic and inelastic neutron scattering measurements using triple-axis and cold-chopper spectrometers at ORNL. In MnAl2O4, we report the existence of both sharp magnetic Bragg peaks and critical scattering, characteristic of the ordering of Heisenberg spins in three-dimensions to a Néel ordered ground state. Analysis of the spin-wave spectrum allows for an estimation of the relative strength of nearest and next-nearest neighbor exchange interactions. In CoAl2O4, we observe the emergence of intense diffuse scattering at low temperatures, centered about specific locations in reciprocal space associated with the same Néel order. At T*~6.5 K, we further observe a characteristic change in scattering lineshape, coupled with the emergence of well-defined spin-wave excitations. We argue that this behavior is evidence of a first-order phase transition at T=T*, but with true long-range order inhibited by the kinetic freezing of domain walls. I will talk about implications of these observations for CoAl2O4 and other frustrated systems containing discontinuous transitions.

^a Proceedings of the National Academy of Science USA 108, 15693-15698 (2011).

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