Based on the Bethe ansatz approach and inelastic neutron scattering experiments, we reveal the evolution of confinement of many-body Bethe strings in ordered regions of the quasi-one-dimensional antiferromagnet ${\mathrm{YbAlO}}_3$. In the antiferromagnetic phase, the spin dynamics is dominated by confined length-1 Bethe strings, whose dominancy in the high-energy branch of the excitation spectrum yields to confined length-2 Bethe strings when the material is tuned to the spin-density-wave phase. In the thermal-induced disordered region, the confinement effect disappears, and the system restores the conventional quantum integrable physics of the one-dimensional Heisenberg model. Our results establish a unified picture based on a Bethe string for the spin dynamics in different magnetic phases of ${\mathrm{YbAlO}}_3$, and thus provide profound insight into many-body quantum magnetism.

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• Magnetic order
• Magnetic phase transitions
• Quasiparticles & collective excitations
• Spin dynamics
• 1-dimensional spin chains
• Bethe ansatz
• Inelastic neutron scattering
• Quantum spin chains