Magnetic properties of double perovskite L n 2 CoIrO 6 L n Eu Tb Ho Hetero tri spin 3 d 5 d 4 f systems
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.