Presaturation phase with no dipolar order in a quantum ferro-antiferromagnet

Abstract

Magnetization, magnetocaloric, calorimetric, neutron- and x-ray-diffraction, and inelastic-neutron-scattering measurements are performed on single crystals of BaCdVO(PO4)2. The low-temperature crystal structure is found to be of a lower symmetry than previously assumed. The result is a more complicated model spin Hamiltonian, which we infer from measurements of the spin wave dispersion spectrum. The main finding is a spin state which emerges in high magnetic fields after antiferromagnetic order is terminated at Hc1≃4.0T. It is a distinct thermodynamic phase with a well-defined phase boundary at Hc2≃6.5T and is clearly separate from the fully saturated phase, yet it shows no conventional (dipolar) magnetic long-range order. We argue that it is fully consistent with the expectations for a quantum bond-nematic state.