Magnetic field-induced tetragonal domain alignment and structural distortion in the spin-orbit-coupled insulator Ba 2 MgReO 6

We investigate how magnetic fields affect the quadrupolar domains in the $5d$ transition-metal oxide ${\mathrm{Ba}}₂{\mathrm{MgReO}}₆$ (BMRO), which crystallizes in the double-perovskite structure. At ${T}_{\mathrm{q}}=33$ K, BMRO undergoes a quadrupolar-order transition, resulting in a cubic-to-tetragonal structural change, followed by a magnetic transition to a canted antiferromagnetic phase at ${T}_{\mathrm{m}}=18$ K. These transitions cause the formation of three types of tetragonal domains below ${T}_{\mathrm{q}}$ and four types of magnetic domains for each tetragonal domain below ${T}_{\mathrm{m}}$. One tetragonal domain is preferred when the magnetic field is applied along the cubic [1 1 0] direction, and its volume fraction exceeds 80% at 10 K. The magnetic-field direction has a significant impact on domain selection. It is revealed that a magnetic field induces not only magnetic but also structural domain switching via unique spin--lattice coupling in the spin--orbit-entangled quadrupolar-order state in BMRO.