Evolution of Magnetism in Single-Crystal Ca2Ru1-xIrxO4 (0< x <0.65)

We report structural, magnetic, transport and thermal properties of single-crystal Ca2Ru1-xIrxO4 (0 < x< 0.65). Ca2RuO4 is a structurally-driven Mott insulator with a metal-insulator transition at TMI = 357 K, which is well separated from antiferromagnetic order at TN = 110 K. Substitution of 5d element, Ir, for Ru enhances spin-orbit coupling (SOC) and locking between the structural distortions and magnetic moment canting. In particular, Ir doping intensifies the distortion or rotation of Ru/IrO6 octahedra and induces weak ferromagnetic behavior along the c-axis. Moreover, the magnetic ordering temperature TN increases from 110 K at x = 0 to 215 K with enhanced magnetic anisotropy at x = 0.65. The effect of Ir doping sharply contrasts with that of 3d-element doping such as Cr, Mn and Fe, which suppresses TN and induces unusual negative volume thermal expansion. The stark difference between 3d- and 5d-element doping underlines a strong magnetoelastic coupling inherent in the Ir-rich oxides.