Spin dynamics in the magnetically ordered state of the RbNiCl3 Haldane chain

Research on one-dimensional antiferromagnets with low-spin numbers at their magnetic ions has gained significant interest following Haldane’s conjecture. This theory stands that integer Heisenberg spin chains exhibit a gap in their excitation spectrum, while half-integer spin chains have gapless excitations. This work focuses on a spin-1 antiferromagnet chain, specifically examining RbNiCl3. This material features a hexagonal lattice crystal structure, with nickel ions forming antiferromagnetic chains along c-axis. The strength of interactions along the chain is about 30 times greater than between the chains, resulting in a system with a strong one-dimensional character. In such antiferromagnetic chains, the ground state is a spin singlet, and long-range order is not fully achieved, even at T=0 K, due to strong quantum fluctuations. However, the system is not purely one-dimensional and three-dimensional order may be achieved at sufficient low temperatures. This study focuses on investigating this ordered phase to gather information about the magnetic interactions within the system. To study the dynamics of these systems, inelastic neutron scattering is the most effective method, as neutrons interact with the magnetic moments of unpaired electrons in the crystal, and their energy range matches the energy required to induce magnetic excitations. Previous studies on RbNiCl3 using a triple-axis spectrometer have determined the magnitude of exchange interactions. However, with recent advancements in inelastic neutron scattering techniques, there is now an opportunity for a deeper understanding of the system. In this study, we used the multiplexing spectrometer CAMEA at the SINQ neutron source, located at the Paul Scherrer Institute in Switzerland. The data collected from this instrument were processed and analysed using the software MJOLNIR. Simulations were then performed using SpinW software, employing linear spin wave theory, to compare the experimental results with theoretical predictions. The goal is to build the most reliable model possible by determining all magnetic interactions and their respective strengths. While there are theoretical limitations concerning low-spin systems, this work represents an initial step toward understanding such systems by comparing discrepancies between theoretical predictions and the observed experimental behaviour.