  In this work, we report on the synthesis and magnetic properties of a series
of double perovskites Ln$_2$ZnIrO$_6$ with Ln = Nd, Sm, Eu & Gd. These
compounds present new examples of the rare case of double perovskites (general
formula A$_2$BB'O$_6$) with a magnetic 4f -ion on the A-site in combination
with the strongly spin-orbit coupled 5d-transition metal ion Ir$^{4+}$ on the
B-sublattice. We discuss the impact of different rare earths on the macroscopic
magnetic properties. Gd$_2$ZnIrO$_6$ and Eu$_2$ZnIrO$_6$ show weak canted
antiferromagnetic order below T$_N$ = 23 K and T$_N$ = 12 K, respectively.
Sm$_2$ZnIrO$_6$ orders antiferromagnetically at T$_N$ = 13 K. Nd$_2$ZnIrO$_6$
exhibits complex magnetic properties with strong field dependence ranging from
a two-step behavior at H = 0.01 T to an antiferromagnetic ground state at
intermediate external fields and a spin-flop phase at H$\geq$4 T, which
suggests complex interplay between Nd$^{3+}$ and Ir$^{4+}$ . To further shed
light on this magnetic interaction, the magnetic structure of Nd$_2$ZnIrO$_6$'s
ground state is examined via neutron powder diffraction.
