Superexchange interactions between Fe-3d and Ln-4f states: long range antiferromagnetism in perovskite derivatives Ba3LnFe2O7.5

Abstract

Perovskite derivatives Ba₃LnFe₂O_7.5, with Ln = Sm, Eu, Gd, Dy, Ho, Er have been synthesized. These oxides isotypic to the already known yttrium phase crystallize in the P2₁/c space group. Their structure can be described as an assemblage of double “FeLn” chains built up of isolated FeO₄ tetrahedrons and YO₆ octahedrons, interconnected through dimeric tetrahedral Fe₂O₇ groups. Magnetization and specific heat measurements show that all the members of this series, including the Y-phase, exhibit long range antiferromagnetic ordering, with T_N ranging from 6.2 K for Y to 12.5 K for Dy. The evolution of T_N with respect to the rare earth size is complex and similar to that observed for the structurally related Ba₂LnFeO₅ perovskites, but shows significantly higher values. The antiferromagnetism of the Y-phases is ascribed to SSE Fe–O–O–Fe interactions along the chains in both oxides “Ba₃Y” and “Ba₂Y”, whereas in addition transversal SSE/SE Fe–O–O–Fe–O–Fe–O–O–Fe interactions between the chains are proposed for the “Ba₃Y” phase. For magnetic lanthanides, the antiferromagnetic behavior of both series implies strong 3d(Fe)–4f(Ln) interactions, i.e. SE Ln–O–Fe interactions along the chains in both series but in addition transversal SE Ln–O–Fe–O–Fe–O–Ln interactions between the chains in the “Ba₃Y” series. The particular role of the magnetic moment and magnetic anisotropy of the lanthanide is also considered.