  We perform a comparative magnetic study on two series of rare-earth (RE)
based double perovskite iridates RE2BIrO6 (RE=Pr,Nd,Sm-Gd;B=Zn,Mg), which show
Mott insulating state with tunable charge energy gap from ~330 meV to ~560 meV
by changing RE cations. For nonmagnetic RE=Eu cations, Eu2MgIrO6 shows
antiferromagnetic (AFM) order and field-induced spin-flop transitions below
N\'eel temperature (TN) in comparison with the ferromagnetic (FM)-like
behaviors of Eu2ZnIrO6 at low temperatures. For magnetic-moment-containing RE
ions, Gd2BIrO6 show contrasting magnetic behaviors with FM-like transition
(B=Zn) and AFM order (B=Mg), respectively. While, for RE=Pr, Nd and Sm ions,
all members show AFM ground state and field-induced spin-flop transitions below
TN irrespective of B=Zn or Mg cations. Moreover, two successive field-induced
metamagnetic transitions are observed for RE2ZnIrO6 (RE=Pr,Nd) in high field up
to 56 T, the resultant field temperature (H-T) phase diagrams are constructed.
The diverse magnetic behaviors in RE2BIrO6 reveal that the 4f-Ir exchange
interactions between the RE and Ir sublattices can mediate their magnetism.
