Evolution of structural, magnetic and transport properties of 3d-5d based double perovskites Nd_2–xSr_xCoIrO₆

The evolution of the structural, magnetic and transport properties of the intermediate compounds Nd2-xSr_<i>x</i>CoIrO₆with<i>x</i>= 0.2, 0.4, 0.6, 1 and 1.5 have been studied to establish important roles of sizes and oxidation states of cations on various phases. The replacement of Nd³⁺by Sr²⁺primarily influences the oxidation states of Co (Co²⁺→Co³⁺) and Ir (Ir⁴⁺→Ir⁵⁺) ions to maintain the charge neutrality in the entire system. The Sr dopants give rise to an increasing Co/Ir antisite disorder (ASD) to accommodate the variation of charge state and ionic radius of Co and Ir. The nature of magnetic interaction induced by Sr changes from being a ferrimagnetic (FIM) to a more dominant antiferromagnetic. The suppression of the second magnetic transition below 30 K in samples for<i>x</i>>0.2 is entirely due to dilution of the Nd-Nd magnetic interaction. The combined effects of ASD and mixed oxidation state of Co and Ir ions generate various types of magnetic exchange pathways and create competitive magnetic interactions to stabilize a particular magnetic ground state. In the middle compound NdSrCoIrO₆, a Griffith like phase in the temperature region 65-150 K and exchange bias field of 658 Oe at 2.3 K under a cooling field of 50 kOe has been observed. The compounds show an insulating kind of behaviour, and with hole doping the value of room temperature resistivity drastically decreases. The nature of conduction is found to follow three dimensional Mott's variable range hopping process.