Interplay of 3d- and 5d-sublattice magnetism in the double perovskite substitution series La2Zn1−xCoxIrO6

We report on the interplay of $3d$- and $5d$-sublattice magnetism in polycrystalline samples of the double perovskite substitution series ${\mathrm{La}}_{2}{\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{IrO}}_{6}$. Powder x-ray diffraction reveals no major structural changes within the series. In magnetization measurements, a gradual shift of the transition temperature from ${\mathrm{T}}_{N} \ensuremath{\approx}91$ K for the Co parent compound to ${\mathrm{T}}_{N} \ensuremath{\approx}8.7$ K for the Zn parent compound is observed. The data on the Zn-rich members of the substitution series indicate that this is accompanied by changing roles of the $3d$ sublattice of ${\mathrm{Co}}^{2+}$ and the strongly spin-orbit coupled $5d$-sublattice of ${\mathrm{Ir}}^{4+}$ with its ${\mathrm{j}}_{\mathrm{eff}}=1/2$ ground state, as a function of the Co/Zn ratio. Temperature-dependent specific-heat studies revealed a reduced magnetic entropy, pointing towards a large spin-orbit coupling and orbital contribution in the system.