Multiferroism Induced by Spontaneous Structural Ordering in Antiferromagnetic Iron Perovskites

Abstract

Room-temperature multiferroism in polycrystalline antiferromagnetic Fe perovskites is reported for the first time. In the perovskite-type oxides RE 1.2 Ba 1.2 Ca 0.6 Fe 3 O 8 (RE = Gd, Tb), the interplay of layered ordering of Gd(Tb), Ba, and Ca atoms with the ordering of FeO 4 -tetrahedra (T) and FeO 6 -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/cm 2 for the Gd-oxide (23.2 μC/cm 2 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.

3D to 2D Magnetic Ordering of Fe3+ Oxides Induced by Their Layered Perovskite Structure

“…Substitution of RE by Ca in the systems modifies the layered ordering of these cations and the oxygen content in such a way that the RE 3+ , Ba 2+ , and Ca 2+ ordering is established in blocks of 3 a p periodicity in the RE 1.2 Ba 1.2 Ca 0.6 Fe 3 O 8 oxides; in blocks of 5 a p periodicity in the REBa 2 Ca 2 Fe 5 O 13 oxides; and by intercalation of blocks of 3 a p and 5 a p periodicity in the intermediate RE 2.2 Ba 3.2 Ca 2.6 Fe 8 O 21 compounds. 17 , 18 The different RE 3+ , Ba 2+ , and Ca 2+ ordering drives the formation of the different Fe-coordination polyhedra (tetrahedron, octahedron, and square pyramid) and their different layered ordering leading to the current structures. 17 , 18 …”

“… 17 , 18 The different RE 3+ , Ba 2+ , and Ca 2+ ordering drives the formation of the different Fe-coordination polyhedra (tetrahedron, octahedron, and square pyramid) and their different layered ordering leading to the current structures. 17 , 18 …”

“…These RE 1.2 Ba 1.2 Ca 0.6 Fe 3 O 8 oxides are multiferroic, exhibiting antiferromagnetic properties and a polar structure with a moment of 33, 0 μC/cm 2 in the case of the Gd compound and 23, 2 μC/cm 2 in the Tb one. 17 Interestingly, there are three different coordination polyhedra of the Fe atoms in the crystal structure of REBa 2 Ca 2 Fe 5 O 13 . This uncommon structural feature has only previously been found to be unstable in air in the Nd 2 Ba 2 Ca 2 Fe 6 O 15 oxide 22 and in nanodomains of some compounds of the RE 2– x Ba 3+ x Fe 5– x Co x O 15-δ (TR = Y, Pr, Nd, Sm y Gd) systems.…”

“…In fact, two different magnetic moments associated with Fe 3+ in octahedral and tetrahedral coordination are clearly distinguished in the recently reported multiferroic Fe oxides with the general formula RE 1.2 Ba 1.2 Ca 0.6 Fe 3 O 8 (RE = Gd, Tb). 17 …”

“…We have studied the influence of the ordering of the A cations in the type and ordering of the Fe-coordination polyhedra in three compounds of the series with the general formula RE 1.2 Ba 1.2 Ca 0.6 Fe 3 O 8 , RE 2.2 Ba 3.2 Ca 2.6 Fe 8 O 21 , and REBa 2 Ca 2 Fe 5 O 13 (RE = Gd, Tb), which correspond to the m and n values 1, 0; 1, 1; and 0, 1, respectively. 17 , 18 …”