Effect of rare earth site substitution on magnetic and transport properties of Ln2CoMnO6 (Ln = La, Sm and Gd) double perovskites

Highlights: • Systematic comparative study of different ionic radii (< RLn >) of Ln₂CoMnO₆ (Ln = La, Sm and Gd) double perovskites. • Enhancement of structural disorder with the decrease of < RLn > at Ln site. • Change of magnetic ordering non-monotonically with different substitution amount. • Observation of large MR (∼−26.35%) at 160 K for 60 kOe field in La₂CoMnO₆ which decreases with decrease of < RLn >. The influence of rare earth site substitution on structural, transport and magnetic characteristics of polycrystalline Ln₂CoMnO₆ (Ln = La, Sm and Gd) has been systematically investigated in details. The room temperature X-ray diffraction data confirm the monoclinic crystal structure of the samples, having P2₁/n space group. The magnetic study reveals that the ferromagnetic transition temperature (T_C1 ∼ 233.2 K) of La₂CoMnO₆ decreases very fast due to a decrease of ionic radii in Ln site and T_C2 (∼150 K) disappears in the Gd₂CoMnO₆ system due to the presence of only Co²⁺ and Mn⁴⁺ ion pair. The field dependent magnetization data at 5 K exhibit antiferromagnetic ordering induced in the Sm₂CoMnO₆ and Gd₂CoMnO₆ systems due to structural distortion and 4f-3d negative magnetic interactions. The temperature dependent resistivity data of La₂CoMnO₆ display semiconductor like behavior in a temperature range of 150–300 K and this behavior has been best described by the variable range hopping conduction mechanism. On the other hand, the resistivity of Sm and Gd based samples follows variable range hopping model in the temperature range of 200–300 K. Large magnetoresistance of ∼−26.35% has been observed at 160 K at a magnetic field of 60 kOe in La₂CoMnO₆ and it decreases to −8.28% for Sm₂CoMnO₆, and −1.01% for Gd₂CoMnO₆ as ionic radii decrease at the Ln site.