Magnetization plateaux in the anisotropic triangular quantum antiferromagnet Cs₂CoBr₄

We report the highly unusual magnetic phase diagram of a novel frustrated antiferromagnet Cs₂CoBr₄. Strong planar single ion anisotropies are competing in the neighboring cobalt S=3/2 chains that are, in turn, coupled in geometrically frustrated zig-zag fashion. The resulting effective Hamiltonan is pseudospin-1/2 and features both "triangular" geometric frustration and bond-dependent anisotropies in the spirit of Kitaev model. By means of calorimetric studies we find an intricate and highly anisotropic phase diagram below 1 K. In the particular high-symmetry direction we find up to five magnetic phases before the pseudospins-1/2 become nearly polarized at 6 T. Further ultralow-T investigation with the Faraday balance magnetometer demonstrates that two of these five phases are plateaux, with M=0 and M≈1/3 in terms of the pseudospin magnetization. We discuss how this behavior stems from the proposed Hamiltonian and argue that Cs₂CoBr₄ represents a novel type of frustration for a 3d transition metal ion-based magnet.

[1] Povarov et al., arXiv:2004.09893 (2020)