Spin orders and lattice distortions of geometrically frustrated 6H-perovskites Ba3B′Ru2O9(B′ = La3+, Nd3+, and Y3+)

Abstract

The magnetic and crystal structures of the title materials have been investigated using high-resolution neutron and synchrotron x-ray powder diffraction. Ferromagnetic Ru dimers are observed in the spin-ordered ground states of all three materials, evidencing an intradimer double exchange interaction. Antiferromagnetic order between dimers breaks the degeneracy of the triangular lattice but no distortion of the ${B}^{\ensuremath{'}}=\mathrm{La}$ and Y structures is observed. The magnetic ordering transition temperature increases with the radius of the ${B}^{\ensuremath{'}}$ cation. An anomalous 120 K structural transition in Ba${}_{3}$NdRu${}_{2}$O${}_{9}$ is attributed to Jahn-Teller distortion, evidenced by large-amplitude ${\ensuremath{\Gamma}}_{6}$ deformations of the NdO${}_{6}$ octahedra. Further lattice anomalies accompany magnetic ordering transitions at 24 and 18 K in Ba${}_{3}$NdRu${}_{2}$O${}_{9}$. Nd spins order ferromagnetically and a substantial magnetic anisotropy leads to large coercivity and $\frac{1}{3}$-magnetization steps in the magnetic hysteresis loop.