Evolution of magnetism in single-crystal Ca₂ Ru_1-xI r_xO₄ (0≤x≤0.65)

We report structural, magnetic, transport, and thermal properties of single-crystal Ca₂ Ru_1-xI r_xO₄ (0≤x≤0.65). Ca₂RuO₄ is a structurally driven Mott insulator with a metal-insulator transition at TMI=357K, which is well separated from antiferromagnetic order at T_N=110K. Substitution of a 5d element, Ir, for Ru enhances spin-orbit coupling and locking between the structural distortions and magnetic moment canting. Ir doping intensifies the distortion or rotation of Ru/IrO₆ octahedra and induces weak ferromagnetic behavior along the c axis. In particular, Ir doping suppresses TN but concurrently causes an additional magnetic ordering T_N2 at a higher temperature up to 210 K for x=0.65. The effect of Ir doping sharply contrasts with that of 3d-element doping such as Cr, Mn, and Fe, which suppresses T_N 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.