Octahedral tilting and emergence of ferrimagnetism in cobalt-ruthenium based double perovskites

Rare earth based cobalt-ruthenium double perovskites A$_2$CoRuO$_6$ (A = La, Pr, Nd and Sm) were synthesized and investigated for their structural and magnetic properties. All the compounds crystallize in the monoclinic $P2_1/n$ structure with the indication of antisite disorder between Co and Ru sites. While, La compound is already reported to have an antiferromagnetic state below 27 K, the Pr, Nd and Sm systems are found to be ferrimagnetic below $T_c$ = 46, 55 and 78 K respectively. Field dependent magnetization data indicate prominent hysteresis loop below $T_c$ in the samples containing magnetic rare-earth ions, however magnetization does not saturate even at the highest applied fields. Our structural analysis indicates strong distortion in the Co-O-Ru bond angle, as La$^{3+}$ is replaced by smaller rare-earth ions such as Pr$^{3+}$, Nd$^{3+}$ and Sm$^{3+}$. The observed ferrimagnetism is possibly associated with the enhanced antiferromagnetic superexchange interaction in the Co-O-Ru pathway due to bond bending. The Pr, Nd and Sm samples also show small magnetocaloric effect with Nd sample showing highest value of magnitude $\sim$ 3 Jkg$^{-1}$K$^{-1}$ at 50 kOe. The change in entropy below 20 K is found to be positive in the Sm sample as compared to the negative value in the Nd counterpart.