Successive Symmetry Breaking in a Jeff = 3/2 Quartet in the Spin-Orbit Coupled Insulator Ba2MgReO6

We report on the cubic double perovskite Ba2MgReO6 containing Re6+ ions with the 5d1 electron configuration. Resistivity, magnetization, and heat capacity measurements using single crystals show that the compound is a Mott insulator with a magnetic transition at Tm = 18 K, which is accompanied by a weak ferromagnetic moment with [110] anisotropy. Another transition is observed at Tq = 33 K in heat capacity, where the inverse of magnetic susceptibility changes its slope, indicating a substantial change in the electronic state. The significance of spin-orbit coupling is revealed by the reduced effective magnetic moment of ~0.68{\mu}B at high temperature above Tq and the total electronic entropy close to Rln4. These features indicate that Ba2MgReO6 is a spin-orbit coupled Mott insulator possessing a Jeff = 3/2 quartet state, which exhibits quadrupolar and dipolar orders at Tq and Tm, respectively.