  Insulating ferromagnets with high TC 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 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 TC = 62 K and
short range FM order in higher temperature range. A huge coercivity is found as
high as HC~11.96 kOe at 10K, 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.
