Antiferro octupolar order in the 5d1 double perovskite Sr2MgReO6 and its spectroscopic signatures

"Hidden"-order phases with high-rank multipolar order parameters have been recently detected in several cubic double perovskites of 5d 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 5d1 double perovskite Sr2MgReO6. 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 j1/2 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.