Double perovskites containing heavy transition-metal ions are an important family of compounds for the study of the interplay between electron correlation and spin-orbit coupling. Here, by combining magnetic susceptibility, heat capacity, and neutron scattering measurements, we investigate the dipolar and quadrupolar correlations in two prototypal rhenium-based double perovskite compounds ${\mathrm{Ba}}_2{\mathrm{YReO}}_6$ and ${\mathrm{Ba}}_2{\mathrm{ScReO}}_6$. A type-I dipolar antiferromagnetic ground state with a propagation vector $\mathbf{q}=(0,0,1)$ is observed in both compounds. At temperatures above the magnetic transitions, a quadrupolar ordered phase is identified. Weak spin excitations, which are gapped at low temperatures and softened in the correlated paramagnetic phase, are explained using a minimal model that considers both the dipolar and quadrupolar interactions. At larger wave vectors, we observe dominant phonon excitations that are well described by density functional calculations.

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• Exchange interaction
• Magnetic order
• Multipole order
• Order parameters
• Spin-orbit coupling
• Perovskites
• Strongly correlated systems
• DC susceptibility measurements
• Density functional theory
• Inelastic neutron scattering
• Mean field theory
• Neutron diffraction
• Neutron scattering
• Random phase approximation
• Specific heat measurements