Detection of multipolar orders in the spin-orbit-coupled 5d Mott insulator Ba2 MgRe O6

Abstract

Synchrotron x-ray-diffraction measurements on single crystals of the double perovskite Ba2MgReO6 with 5d1 electronic configurations reveal the successive phase transitions from a disordered state to a quadrupolar order at Tq = 33 K, and then, to a dipolar ordered state at Tm = 18 K. The magnetic reflections observed below Tm are indexed to a propagation vector with k = [0 0 1], indicating a canted antiferromagnetic order. The very small deformation of ReO6 octahedra observed below Tq demonstrates that the quadrupolar order is composed of antiferroically arranged Qx2–y2 and ferroically arranged Q3z2–r2 moments. These observations are consistent with the mean-field theory for spin-orbit-entangled electrons. Our findings are a significant step towards understanding the intriguing physics of multipoles realized by spin-orbit-entangled 5d electrons.