The effect of antisite disorder on magnetic and exchange bias properties of Gd-substituted Y$_2$CoMnO$_6$ double perovskite

Combining experimental investigations and first-principles DFT calculations, we report physical and magnetic properties of Gd-substituted Y$_2$CoMnO$_6$ double perovskite, which are strongly influenced by antisite-disorder-driven spin configurations. On Gd doping, Co and Mn ions are present in mixed-valence (Co$^{3+}$, Co$^{2+}$, Mn$^{3+}$ and Mn$^{4+}$) states. Multiple magnetic transitions have been observed: i) paramagnetic to ferromagnetic transition is found to occur at \textit{T}$_C$=95.5 K, ii) antiferromagnetic transition at \textit{T}$_N$=47 K is driven by $3d-4f$ polarisation and antisite disorder present in the sample, iii) change in magnetization below \textit{T}$\leq$20 K, primarily originating from Gd ordering, as revealed from our DFT calculations. AC susceptibility measurement confirms the absence of any spin-glass or cluster-glass phases in this material. A significantly large exchange bias effect (\textit{H}$_{EB}$=1.07 kOe) is found to occur below 47 K due to interfaces of FM and AFM clusters created by antisite-disorder.