Monoclinic distortion and magnetic coupling in the double perovskite Sr_2-xCa_xYRuO₆

This work investigates in the insulating double perovskites Sr_2-xCa_xYRuO₆. We address the angular dependence of the strength of the magnetic coupling due to the deviation from planarity of the basal layers of the monoclinic structure, characterized by the in-plane angle α<180°, in order to probe the impact of the structural distortions in the magnetic properties of the compounds. High resolution x-ray powder diffraction, susceptibility, and specific heat measurements were performed. The deviation from planarity significantly increases (α=144° for x=1) while the bond distances vary in a complex way as a consequence of the strong monoclinic distortion. We found that the magnetic transition temperature, T_M, shows a linear dependence on cos [(π-α)/2]. This result is discussed in terms of t_2g(π)-e_g(σ) mixing of the magnetic orbitals of the Ru⁵⁺ ions and unbalanced competitive super-exchange interactions. The deleterious effect of Ca doping for the magnetic coupling is confirmed by the reduction in the short-range antiferromagnetic correlations characteristic of the parent compound at T>>T_M and the enhancement of magnetic frustration for T<T_M.