  The field of double perovskites is now advancing to three magnetic elements
on the A, B and B$'$ sites. A series of iridium-based double perovskite
compounds, $Ln_2$CoIrO$_6$ ($Ln$ = Eu, Tb, Ho) with three magnetic elements
were synthesized as polycrystalline samples. The compounds crystalize in
monoclinic structures with the space group $P2_1/n$. Magnetic properties of
these hetero-tri-spin $3d$-$5d$-$4f$ systems were studied by magnetic
susceptibility and field dependent magnetization in both DC and pulsed magnetic
fields. All these compounds show ferrimagnetic transitions at temperatures
$T_C$ above 100 K, which are attributable to antiferromagnetic coupling between
Co$^{2+}$ and Ir$^{4+}$ spins. For Eu$_2$CoIrO$_6$, the magnetic properties are
similar to those of La$_2$CoIrO$_6$. The Eu$^{3+}$ spins show Van Vleck
paramagnetism that don't significantly interact with transition-metal cations.
By contrast, Tb$_2$CoIrO$_6$ and Ho$_2$CoIrO$_6$ reveal a second transition to
antiferromagnetic order below a lower temperature $T_N$. The
temperature-induced ferrimagnetic-to-antiferromagnetic phase transition might
be explained by a spin-reorientation transition. Moreover, a
magnetic-field-induced spin-flop transition with a small hysteresis was
observed below $T_N$ in these two compounds. The magnetic moment of all three
compounds do not saturate up to 60 T at low temperatures. Moderate
magnetocaloric effect was also observed in all three compounds. Our results
should motivate further investigation of the spin configuration on single
crystals of these iridium-based double perovskites.
