Antiferro octupolar order in the 5⁢𝑑¹ double perovskite Sr₂⁢MgReO₆ and its spectroscopic signatures

“Hidden”-order phases with high-rank multipolar order parameters have been recently detected in several cubic double perovskites of 5⁢𝑑 transition metals. Here, by constructing and solving an ab initio low-energy Hamiltonian, we show that an antiferroic order of magnetic octupoles also forms in the tetragonal 5⁢𝑑¹ double perovskite Sr₂⁢MgReO₆ . The low-temperature order in this material is determined by a tetragonal crystal field dominating over exchange interactions. This results in a well isolated crystal-field doublet ground state hosting octupolar low-energy degrees of freedom. Very weak dipole moments entangled with the primary octupole order parameters are induced by admixture of the excited 𝑗⁢1/2 spin-orbit multiplet. We show that the octupolar order leads to characteristic quasigapless magnetic excitation spectra, as well as to the intensity of superstructural neutron diffraction reflexes peaking at large scattering momenta.