The effects of introducing quantum fluctuations into spin system with finite entropy in the zero temperature limit are explored using neutron scattering experiments in the single layer and the bi-layer triangular lattice antiferromagnets K2Co(SeO3)2 and K2Co2(SeO3)3. The magnetism of both these is based on effective spin-1/2 3d7 Co2+ ions with easy-axis antiferromagnetic super-exchange interactions mediated by the selenite polyanion [SeO3]-2.

By probing the distinct static and dynamic spin correlations in K2Co(SeO3)2 versus field and temperature we provide evidence for two super-solid phases of bosons with repulsive nearest neighbour interactions on the triangular lattice. There is a field driven transition to a collinear 1/3 magnetization plateau phase wherein the magnetic excitations take the form of coherent spin waves from which we determine the Hamiltonian. A second super-solid phase is found through pulsed field magnetization measurements near the upper critical field.

The bi-layer system K2Co2(SeO3)3 has five magnetization plateau phases. In zero field we find a near equidistant spectrum of non-dispersive modes from which we
obtain the dominant exchange interactions. Quasi-long-range √3 × √3 order develops at low temperatures though with a gapless excitation spectrum indicative of rotational symmetry breaking. We associate the abrupt decoupling of the Co nuclear spin system from the electronic spin system in the 1/3 magnetization plateau phase with the opening of a gap in the magnetic excitation spectrum, which is also apparent from inelastic neutron scattering.