Multiferroism Induced by Spontaneous Structural Ordering in Antiferromagnetic Iron Perovskites

Room-temperature multiferroism in polycrystalline antiferromagnetic Fe perovskites is reported for the first time. In the perovskite-type oxides RE_1.2Ba_1.2Ca_0.6Fe₃O₈ (RE = Gd, Tb), the interplay of layered ordering of Gd­(Tb), Ba, and Ca atoms with the ordering of FeO₄-tetrahedra (T) and FeO₆-octahedra (O) results in a polar crystal structure. The layered structure consists of the stacking sequence of RE/Ca-RE/Ca-Ba-RE/Ca layers in combination with the TOOT sequence in a unit cell. A polar moment of 33.0 μC/cm² for the Gd-oxide (23.2 μC/cm² for the Tb one) is determined from the displacements of the cations, mainly Fe, and oxygen atoms along the b-axis. These oxides present antiferromagnetic ordering doubling the c-axis, and the magnetic structure in the Tb-compound remains up to 690 K, which is one of the highest transition temperatures reported in Fe perovskites.