Dielectric, piezoelectric and magnetic behavior of CoFe2O4/BNT–BT0.08 monolayer thin films composites
Graphical abstract
Introduction
The composites are important materials considering they exhibit improved specific properties for each component and/or new properties or functionalities compared to the constituent phases. For example, in a composite of a metal and an insulator, one could control the electrical resistivity by controlling the volume fraction and distribution of the two phases [1]. Also, an example for the composites with new properties is the electric field-to-magnetic field conversion (or vice versa) that could be realized in a magnetostrictive-piezoelectric composite [2], [3], [4], [5], [6], [7], [8]. There are many reports on composites with electrical, ferroelectric and ferromagnetic behaviors, for example: multiferroic-ferrimagnetic (BiFeO3-CoFe2O4) composites [9], ferromagnetic-piezoelectric oxide (nickel ferrite (NiFe)- Pb0.92La0.08Zr0.52Ti0.48O3 (PLZT)) composites [10], ferroelectric-ferrimagnetic (BaTiO3-CoFe2O4) composites [11] and (La0.7Sr0.3MnO3-PbZr0.2Ti0.8O3 heterostructures [12]), and ferrimagnetic-piezoelectric CoFe2O4–PZT [13]). These composites were fabricated in the form of bulk, thin films and tubes, by using various methods. For bulk composites with core–shell structure various techniques were used, such as: the coprecipitation method [14], sol–gel method [15], sonochemical technique [16], conventional solid-state sintering method [17], two-step hydrothermal method [18], etc. Bilayer composite films were prepared using magnetron sputtering [19], combining method of sol–gel and electrophoretic deposition [11], chemical solution deposition [20], sol–gel deposition [21] and, pulsed laser deposition (PLD) [13]. CoFe2O4/BNT-BT0.08 core–shell nanotubes were prepared by using a preformed removable polycarbonate membrane template-assisted sol–gel process [22], [23], [24] and electrospinning [25]. The present reports on the ferrimagnetic-ferroelectric monolayer composites thin films CoFe2O4-BNT-BT0.08, with molar ratios of 0.5:1, 1:1 and 1.5:1, (samples S1, S2 and S3), respectively. Ferrites and perovskites are two important classes of metal oxides, as ferrites have high resistivity (i.e. low dielectric constant), while perovskites have diverse applications such as cathode material in solid oxide fuel cells, catalytic applications, and can also be magnetic in nature depending upon the presence of iron contents. It was already demonstrated that by combining the magnetic spinel CoFe2O4 phase with the piezoelectric perovskite (Bi0.5Na0.5)0.92Ba0.08TiO3, abbreviated as BNT-BT0.08 [21] or (Bi0.5Na0.5)0.65Ba0.35TiO3, (BT–BNT0.35) [17] it results in composite materials with magnetic, piezoelectric and magnetoelectric properties.
Furthermore, we have already reported on the ferrite (CoFe2O4)-ferroelectric (BNT-BT0.08) composites with various architectures, such as: core–shell nanoparticles [26], bilayer ferrite–piezoelectric composites of CoFe2O4 and BNT-BT0.08 [21] and, core–shell nanotubes [22], prepared by sol–gel route. Thus, compared to the previous report on bilayer ferrite–piezoelectric composites of CoFe2O4 and BNT-BT0.08 [21], this work has as a novelty, the preparation procedure to obtain a monolayer film (samples S1, S2 and S3) that consists in the use of a sol precursor mixture of CoFe2O4 and BNT-BT0.08. As a result, their structural, microstructural, dielectric, piezoelectric and magnetic properties are investigated and discussed here, while a comprehensive comparison between the properties of these monolayer composites thin films and those of bilayer composites thin films is also displayed.
Section snippets
Experimental section
Composites thin films of BNT–BT0.08 and CoFe2O4, with different molar ratios: 0.5:1 (sample S1), 1:1 (sample S2) and 1.5:1 (sample S3) were prepared by sol–gel method. Bismuth (III) acetate ((CH3COO)3Bi, 99.99%), sodium acetate (CH3COONa, 99.995%), barium acetate ((CH3COO)2Ba, 99%), titanium (IV) isopropoxide (Ti{OCH(CH3)2}4) in isopropanol, acetic acid, acetylacetone and dimethylformamide were used as starting reagents in order to prepare BNT–BT0.08 precursor sol. Cobalt acetate (Co(CH3CO2)2·4H
Structural and morphological aspects
Fig. 1 shows the XRD patterns of the CoFe2O4/BNT–BT0.08 = 0.5:1; 1:1; 1.5:1 composite thin films, annealed at 700 °C, 1 h in air.
As can be seen in Fig. 1, there are peaks of the cubic phase of CoFe2O4 (JCPDS card No.05-7078 [27]), peaks of the rhombohedral phase of Na0.5Bi0.5TiO3 (JCPDS card No. 70-9850 [28]) and, peaks of the Si-Pt substrate. The amplitude of the peaks attributed to the cubic CoFe2O4 phase increases with the increase of the CoFe2O4/BNT–BT0.08 ratio while, the intensity of the
Conclusions
In conclusion, the monolayer CoFe2O4/BNT-BT0.08 composites ferrimagnetic–piezoelectric thin films, with molar ratios of 0.5:1, 1:1 and 1.5:1, were synthesized by sol–gel synthesis and deposited by spin coating methods. The XRD analysis evidenced the existence of the two phases: cubic CoFe2O4 and rhombohedral BNT–BT0.08; the peaks intensity increases for the magnetic phase and decreases for the piezoelectric phase from the sample S1 to S3. Through SEM and EDS, it has been shown that the chemical
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge the funding through Core Program PN18–11 from Romanian Ministry of Research and Innovation.
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