The weakening of Gd-sublattice magnetic couplings in GdBa2Fe3O8.12

Using zero-field X AC (T) and C p (T) (0.1 K < T < 2.0 K), the magnetic properties of the Gd-sublattice in the semiconducting GdBa 2 Fe 3 O 8.12 was investigated. As reported earlier, though the Fe-sublattice is strongly superexchange coupled, the Gd-sublattice is paramagnetic down to 1.2 K and orders only at T N which is an order-of-magnitude lower than that of the structurally related GdBa 2 Cu 3 O 7-δ (T N = 2.24 K). Each of X AC (T) and C p (T) presents a widely broadened peak and the magnetic entropy up to 2 K attains only 30% of the value expected for a full Gd 3 + spin degeneracy. These features can be understood as follows: in contrast to the triperovskite Cu-based compound, disordering due to a random intermixing of the Ba 2+ and Gd 3+ ions induces a further weakening of the already weak Gd 3+ -couplings. Finally, while the RKKY-type mechanism can be straightforwardly ruled out, the possibility of dipole-dipole or weak anisotropic superexchange couplings among the Gd-moments will be evaluated.