Interplay between 3d-3d and 3d-4f interactions at the origin of the magnetic ordering in the Ba2LnFeO5 oxides

A new family of oxides in which 3d-3d and 3d-4f interactions are of comparable strength has been synthesized and characterized both from structural and physical viewpoints. These compounds of formulation Ba(2)LnFeO(5) (Ln = Sm, Eu, Gd, Dy, Ho, Er, Yb) are isotypic to the perovskite derivative Ba2YFeO5. They exhibit an original structure consisting of isolated FeO4 tetrahedra linked via LnO(6) (or YO6) octahedra. Magnetic and calorimetric measurements show that all these compounds exhibit a unique, antiferromagnetic transition involving both the 3d and 4f ions. The antiferromagnetic properties of the Ln = Y phase (non-magnetic Y3+) and of the Ln = Eu (non-magnetic ground state multiplet of Eu3+) are ascribed to super-super exchange Fe-O-O-Fe interactions, leading to the lowest TN (5.5 K for Y and 4.6 K for Eu). The introduction of a magnetic lanthanide, i.e. Ln = Sm, Gd, Dy, Ho, Er, Yb, in the octahedral sites, leads to larger TN values (up to 9.8 K for Ln = Yb). It is found that several mechanisms must be taken into account to explain the complex evolution of the magnetic properties along the Ba(2)LnFeO(5) series. In particular, the super-exchange Ln-O-Fe, as well as the on-site Ln(3+) magnetocrystalline anisotropy, are suggested to play crucial roles. This Ba(2)LnFeO(5) series offers a rare opportunity to investigate experimentally a situation where the 3d-3d and 3d-4f interactions co-operate on an equal footing to trigger a unique long-range magnetic ordering in insulating oxides.