Multiferroic and magnetoelectric properties of BiFeO3/Bi4Ti3O12 bilayer composite films
Introduction
Multiferroic materials have drawn an increasing attention due to the strong magnetoelectric coupling effect between (anti)ferroelectric and (anti)ferromangnetic orders. However, because of mutual exclusiveness of ferroelectric and ferromagnetic orders, there are only few single-phase multiferroic materials that exist in nature [1], [2]. The scarcity of single phase multiferroics has motivated the design and development of composite materials combining ferroelectric and magnetic materials with transition temperature above room temperature and thus an artificial coupling can be engineered between the order parameters [3], without any constraint of coexistence of these orders in single phase. Thus multiferroic composites and heterostructures combining ferroelectric and ferromagnetic materials exhibit room-temperature magnetoelectric effect greatly exceeding those of single-phase magnetoelectric materials known to date. In fact, to meet the requirements of the rapidly developing micro- and nano-electro-mechanical systems (MEMS&NEMS) devices the ability to create high quality multiferroic magnetoelectric films stands as the more significant landmark toward technological application in micro-devices. Multiferroic composite films possess the unique advantages such as the modulation of their thickness, composition, connectivity, and orientation at nanoscale. However, for magnetoelectric composite films, giant magnetoelectric effect is limited to some shortcomings such as the interface diffusion coming from chemical reactions between the constituents, the loss of the mechanical stress mediating between the ferroelectric and ferromagnetic phases, and low resistivity of the ferromagnetic phase or eddy currents induced in the conducting phase by the applied ac voltage [4]. These difficulties limit the further application for magnetoelectric composite films. The magnetoelectric coupling in these composite films relies on strain which induce crystal deformations in either the ferroelectric phase or the magnetic phase through magnetostriction or the converse piezoelectric effect. In order to achieve large magnetoelectric effect, it is urgent to select strong ferroelectric and ferromagnetic materials with excellent coupling between them. Up to date, much work has been carried out to prepare the composite films by combining perovskite ferroelectric oxides [e.g., PbZr0.52Ti0.48O3 (PZT)] with ferromagnetic materials e.g., (CoFe2O4, ReFe) [5], [6], [7], which present obvious magnetoelectric coupling effect. However, these kinds of lead based magnetoelectric composite films are neither favorable to require of environmental protection nor easy to obtain strong magnetoelectric coupling due to the crystal mismatch between ferroelectric and ferromagnetic phases. So it is attractive to find lead-free based magnetoelectric composite films with similar structure, which can obtain outstanding magnetoelectric properties. BiFeO3 (BFO) is multiferroic materials with both ferromagnetic and ferroelectric orders occurring above room temperature: high ferroelectric Curie temperature and Neel temperature, which has attracted interesting because of their potential magnetoelectric coupling behaviors [8], [9]. However, its first-order and second-order ME coefficients were reported 10–20 mV/cm·Oe [10], which does not satisfy the actual demand on magnetoelectric performance. Thus, BFO based magnetoelectric composite films with similar structure (perovskite structure) are promised to obtain strong magnetoelectric effect. As a kind of environmental friendly lead-free ferroelectric film, Bi4Ti3O12 (BTO) films can be hopefully applied in magnetoelectric composite as ferroelectric phases due to their excellent piezoelectric, ferroelectric and leakage properties [11], [12]. More importantly, BFO and BTO have similar structures with perovskite structure, which can form enhanced magnetoelectric coupling due to the excellent crystal lattice matching. Therefore, BFO/BTO bilayer composite films are expected to obtain strong magnetoelectric effect.
Based on those above, this work prepared BFO/BTO bilayer composite films by chemical solution deposition, and systematically investigated the microstructure, domain structure, ferroelectric, piezoelectric, dielectric, leakage, magnetic, magnetoelectric properties of BFO/BTO composite films. The origins of the strong magnetoelectric effect are discussed in detail.
Section snippets
Experiment
BTO/BTFO films were deposited on Pt (100)/Ti/SiO2/Si substrates using chemical solution deposition. The details on the preparation of BTO precursor solutions could be found elsewhere [14]. The raw materials of BFO films used for the precursor solutions were bismuth nitrate pentahydrate [Bi(NO3)3·5H2O] and iron nitrate nonahydrate [Fe(NO3)3·9H2O]. Bismuth nitrate pentahydrate and iron nitrate nonahydrate were added to the ethylene glycol solvent in proportions of 1.1:1, with an excess 10% Bi to
Results and discussion
Fig. 1(a) shows the XRD patterns of BFO/BTO bilayer composite films deposited on Pt(100)/Ti/SiO2/Si substrates. It can be seen that XRD patterns of BTO/BTFO composite films can be disassembled to two sets of well-defined peaks, one of which belongs to BTO phase and the other belongs to BFO phase from their individual XRD patterns available in the JCPDS data card No. 38-1257 and 20-0169, respectively. With all of preferable process parameters, the overall crystallinity of BFO/BTO composite films
Conclusions
In summary, BFO/BTO composite films were prepared by chemical solution deposition. BTO/BTFO composite films display excellent ferroelectric, leakage and piezoelectric, dielectric and magnetic properties, which also display clear surface morphology and domain structure. Further a strong magnetoelectric effect is observed in BFO/BTO composite films, which comparable with other composite films. Thus results offer valuable information for the potential applications of BTO/BTFO multiferroic
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 11264026, 11564028), and Inner Mongolia Science Foundation for Distinguished Young Scholars (Grant No. 2014JQ01).
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