Magnetic properties of double perovskite Ln2CoIrO6 (Ln = Eu, Tb, Ho): Hetero-tri-spin 3d-5d-4f 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 was 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 Co2+ and Ir4+ spins. For Eu2CoIrO6, the magnetic properties are similar to those of La2CoIrO6. The Eu3+ spins show Van Vleck paramagnetism, and do not significantly interact with transition-metal cations. In contrast, Tb2CoIrO6 and Ho2CoIrO6 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-like transition with a small hysteresis was observed below T-N in these two compounds. The magnetic moments of all three compounds do not saturate up to 60 T at low temperatures. A 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.