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 5d(1) electron configuration. Resistivity, magnetization, and heat capacity measurements using single crystals show that the compound is a Mott insulator with a magnetic transition at T-m = 18K, which is accompanied by a weak ferromagnetic moment with [110] anisotropy. Another transition is observed at T-q = 33K in the 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 similar to 0.68 mu(B) at high temperatures above T-q and the total electronic entropy close to Rln 4. These features indicate that Ba2MgReO6 is a spin-orbit coupled Mott insulator possessing a J(eff) = 3/2 quartet state, which exhibits quadrupolar and dipolar orders at T-q and T-m, respectively.