Superexchange interactions between Fe-3d and Ln-4f states: long range antiferromagnetism in perovskite derivatives Ba 3 LnFe 2 O 7.5
Résumé
Perovskite derivatives Ba3LnFe2O7.5, 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 TN ranging from 6.2 K for Y to 12.5 K for Dy. The evolution of TN 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.