Page 29 - ILL Annual Report 2019

SCIENTIFIC HIGHLIGHTS
26-27
Figure 2
a) Graphic representation of the polar unit cell of Gd1.2Ba1.2Ca0.6Fe3O8. Arrows indicate the polarisation
in the different layers (Ba-O layers in green, Gd/Ca-O in orange, Fe-O in octahedral layers and Fe-O in tetrahedral layers), including the anionic and cationic contributions along the b-axis. ri is the shift-distance of the Fe atoms with respect to the A cations along the b-axis. Room temperature multiferroism in polycrystalline antiferromagnetic Tb1.2Ba1.2Ca0.6Fe3O8 is reported for the first time.
b) The magnetic unit cell of Tb1.2Ba1.2Ca0.6Fe3O8. Arrows indicate the spin direction.
Reflections corresponding to long-range magnetic order are visible in the diffraction patterns (Figure 1a), while the D20 data are used to determine TN to 690 K
(see Figure 1b)—one of the highest transition temperatures reported in Fe-perovskites [1]. These magnetic reflections can be indexed by a propagation vector [0 0 1⁄2], producing a magnetic unit cell
√2ap x √2bp x 6cp . The magnetic structure adopted is depicted in Figure 2b. It consists of a three-dimensional, G-type, antiferromagnetically ordered arrangement of the Fe spins with the spin directions lying along the b-axis. This magnetic transition is associated with an electric transition of electron transport at the same temperature.
In summary, the combination of magnetic behaviour
and polar structure makes this oxide, to the best of our knowledge, the first reported multiferroic polycrystalline material with an antiferromagnetic structure. In turn, the layered arrangement of the oxygen polyhedra around the Fe-atoms in a particular sequence, assisted by the A-cation ordering in this oxide, represents a novel approach to the design of multiferroic materials.
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