Weak ferromagnetic insulator with huge coercivity in monoclinic double perovskite La₂CuIrO₆

Insulating ferromagnets with high T _C are required for many new magnetic devices. More complexity arises when strongly correlated 3d ions coexist with strongly spin-orbit coupled 5d ones in a double perovskite. Here, we perform the structural, magnetic, and density functional theory (DFT) study of such double perovskite La₂CuIrO₆. A new P2₁/n polymorph is found according to the comprehensive analysis of x-ray, Raman scattering and phonon spectrum. The magnetization reveals a weak ferromagnetic (FM) transition at T _C  =  62 K and short range FM order in higher temperature range. A huge coercivity is found as high as H _C ~ 11.96 kOe at 10 K, which, in combination with the negative trapped field, results in the magnetization reversal in the zero field cooling measurement. The first principle calculations confirm the observed FM state and suggest La₂CuIrO₆ of this polymorph is a Mott insulating ferromagnet assisted by the spin-orbit coupling.