Giant magnetic field dependence of the coupling between spin chains in BaCo2V2O8

We use nuclear magnetic resonance to map the complete low-temperature phase diagram of the antiferromagnetic Ising-like spin-chain system BaCo2V2O8 as a function of the magnetic field applied along the chains. In contrast to the predicted crossover from the longitudinal incommensurate to the transverse antiferromagnetic ordered phase, we find a sequence of three magnetically ordered phases between the critical fields 3.8 and 22.8 T. Their origin is traced to the giant magnetic field dependence of the total effective coupling between spin chains, found to vary by a factor of 24. We explain this interesting phenomenon as emerging from a combination of nontrivially coupled spin chains and incommensurate spin fluctuations in the chains treated as Tomonaga-Luttinger liquids.