Revisiting the ground state of CoAl$_2$O$_4$: comparison to the conventional antiferromagnet MnAl$_2$O$_4$

TL;DR

This study revisits the ground state of CoAl₂O₄, revealing it is a kinetically-inhibited antiferromagnet with a frozen state below 6.5 K, and compares it to the unfrustrated MnAl₂O₄ which undergoes a classical phase transition.

Contribution

The paper provides new neutron scattering data confirming the kinetically-inhibited nature of CoAl₂O₄ and compares its magnetic behavior to MnAl₂O₄, highlighting the role of exchange interactions and cation inversion.

Findings

CoAl₂O₄ exhibits a frozen antiferromagnetic state below 6.5 K.

MnAl₂O₄ shows a classical phase transition at 39 K with spinwave excitations.

Differences in behavior are linked to cation inversion and exchange interactions.

Abstract

The A-site spinel material, CoAl2O4, is a physical realization of the frustrated diamond-lattice antiferromagnet, a model in which is predicted to contain unique incommensurate or `spin-spiral liquid' ground states. Our previous single-crystal neutron scattering study instead classified it as a `kinetically-inhibited' antiferromagnet, where the long ranged correlations of a collinear Neel ground state are blocked by the freezing of domain wall motion below a first-order phase transition at T* = 6.5 K. 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 major diffuse features are explained as signatures 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. The unfrustrated material is shown to have a classical continuous phase transition to Neel order at T_N = 39 K, 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 next-nearest-neighbor exchange.