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Issue 25, 2020
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Cobalt-induced structural modulation in multiferroic Aurivillius-phase oxides

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Attaining robust magnetic long-range order in ferroelectric Aurivillius-phase oxides at room temperature has recently attracted considerable attention of materials scientists and engineers for the development of magnetoelectric-active materials in microelectronics and spintronics. Here, we report the structural evolution and its relation to the macroscopic magnetization of a series of samples of Aurivillius (Bi4.3Gd0.7)(Fe1−xCox)1+yTi3−2yNbyO15 (x = 0, 0.3, 0.5 and y = 0, 0.3) compounds prepared by a solid state reaction, aiming at shedding light on the Co substitution-induced ferromagnetism at room temperature and above. The Co-free composition showed a single-phase four-layered Aurivillius structure (space group A21am), while the Co substitution was found to give rise to a mixed-layer structure composed of four- and three-layered phases. Rietveld analysis of the synchrotron X-ray diffraction data showed that the reduction in the number of layers across the Aurivillius morphotropic transition boundary is accompanied by a structural phase transformation from A21am to B2cb. The disordered intergrowth of these phases was evidenced by high-resolution transmission electron microscopy and found to originate from a nanoscale structural modulation occurring at the interface between the two phases. A sextet suggesting long-range magnetic ordering in the doped samples was deduced from Mössbauer spectra. Magnetic-property measurements, indeed, confirmed a ferromagnetic state of these samples at elevated temperatures. The highest values of the remanent and saturation magnetization at room temperature were obtained for the compositions with x = 0.3, in which the occurrence and enhancement of the magnetization can be attributed to the ferromagnetic clustering of the FeO6 and CoO6 octahedra and, partly, also to spin canting effects and/or a double-exchange magnetic interaction between the mixed valence cobalt through oxygen. The cooperative freezing of randomly distributed Fe–O–Co clusters is suggested to be responsible for the spin glass-like behaviour observed at low temperatures. The occurrence and enhancement of the magnetization in the ferroelectric Co-doped Aurivillius ceramics are attributed, respectively, to the structural modulation and the population of the ferromagnetic clusters near the interface of the four- and three-layered phases.

Graphical abstract: Cobalt-induced structural modulation in multiferroic Aurivillius-phase oxides

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Supplementary files

Article information

20 Mar 2020
12 May 2020
First published
12 May 2020

J. Mater. Chem. C, 2020,8, 8466-8483
Article type

Cobalt-induced structural modulation in multiferroic Aurivillius-phase oxides

V. Koval, Y. Shi, I. Skorvanek, G. Viola, R. Bures, K. Saksl, P. Roupcova, M. Zhang, Ch. Jia and H. Yan, J. Mater. Chem. C, 2020, 8, 8466
DOI: 10.1039/D0TC01443E

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