Synthesis and room-temperature multiferroic properties of lead-free Bi4Ti3O12/NiFe2O4 nanocomposite films

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Abstract

Lead-free Bi4Ti3O12/NiFe2O4 nanocomposite films have been prepared via sol-gel spin-coating route. The successful phase formation of orthorhombic Bi4Ti3O12 and cubic NiFe2O4 has been confirmed using x-ray diffraction without any detectable extra phase. The scanning electron microscopy with EDS measurement reveals homogeneous and dense growth of both phases accompanied by clear interface between the two layers. The loop of magnetization versus magnetic field indicates the ferromagnetic properties at room temperature with saturation magnetization of ~257.48 emu/cm3, coercive field of ~121 Oe and remnant magnetization of ~57.03 emu/cm3. Room-temperature ferroelectric behavior is also obtained along with the good fatigue endurance properties and low leakage current density. The frequency dependence of dielectric constant displays the monotonous decreasing tendency with low dielectric loss. Moreover, evident magnetodielectric behavior is achieved in the nanocomposite films at room temperature. The coexistence of ferromagnetic and ferroelectric behavior including the appearance of magnetodielectric effect proves the room-temperature multiferroic properties in Bi4Ti3O12/NiFe2O4 nanocomposite films, which makes the films meaningful as the lead-free multiferroic systems for device applications at room temperature.

Introduction

In the past few decades, multiferroic materials which present two or more ferroic order parameters have been widely investigated because of the fundamental science significance, technology relevance and the numerous potential applications in high density storage, multiple memory state elements and multifunctional devices such as sensors, transducers, actuators, etc [[1], [2], [3], [4]]. Among them, magnetoelectric performance with coexistence and coupling of ferroelectric and ferromagnetic orders has attracted tremendous research interest. However, it has been confirmed that multiferroic behavior in single-phase materials is commonly rather weak and appears at low temperature. In addition, the fundamental physics is contradictory in the origin of ferroelectricity and magnetism [[5], [6], [7]]. The above actualities suggest that single-phase multiferroics are quite scarce. Therefore, composite multiferroic materials combining ferromagnetic and ferroelectric phases are the better choice to obtain excellent magnetoelectric effect at room temperature. With the outstanding piezoelectric properties, lead-based ferroelectrics such as PbZr1-xTixO3 have been widely used to improve the magnetoelectric coupling coefficient [8,9]. Whereas, with the deterioration of environment, the toxicity of lead-based materials makes it an urgent problem to find environmentally friendly alternatives.

As a kind of lead-free materials, bismuth layer structured ferroelectric Bi4Ti3O12 (BTO) has gained an increasing concern on account of its high Curie temperature, large ferroelectric polarization, low aging rate and outstanding temperature stability [10]. The general formula of bismuth layer structured ferroelectrics is (Bi2O2)2+(An-1BnO3n+1)2- with an intergrowth structure of (An-1BnO3n+1)2- units and (Bi2O2)2+ slabs along the c-axis. BTO (n = 3) with orthorhombic structure displays the Bi3+ ions at A sites and Ti4+ ions at B sites [11]. Ferrite NiFe2O4 (NFO) has been intensively investigated with higher magnetic ordering temperature than room temperature. Accompanied by an inverse spinel structure, the location of Ni2+ ions in the crystal structure is closely related to the magnetic properties. Thus, it presents various applications in magnetic storage systems, gas-sensor, magnetic resonance imaging, magnetic fluids, catalysts, photomagnetic materials and microwave devices [12,13].

The composite multiferroic materials with two phases can be prepared in bulk ceramic or thin film forms. The ceramic form generally exhibits some inherent problems, for instance, the impurities, poor phase connectivity and high leakage current, which weaken the multiferroic behavior of composites. As the counterpart of bulk materials, the form of thin film can effectively improve the physical properties due to the tight connectivity between composition phases, better crystal orientation and interfacial coupling, low leakage current and controllable layer thickness.

Through investigating abundant researches, the room-temperature multiferroic properties of composite films with BTO and NFO have not been studied so far. Hence, based on the above discussion, we try to synthesize the lead-free Bi4Ti3O12/NiFe2O4 (BTO/NFO) composite films. The multiferroic properties are characterized by various measurements. Further, the physical performances are also analyzed in depth.

Section snippets

Materials

Raw materials required for the experiment are Bi(NO3)3·5H2O, C16H36O4Ti, Ni(CH3COO)2·4H2O, Fe(NO3)3·9H2O, acetylacetone, 2-methoxyethanol and acetic acid.

Detailed experimental process

BTO/NFO composite films were prepared by the sol-gel procedure and spin coating technique. The films were deposited on Pt/Ti/SiO2/Si substrates. The whole process could be divided into two steps: the synthesis of precursors and the growth on the substrates. The schematic diagram and composition structure of the composite films were shown in

Results and discussion

Fig. 1(d) shows the XRD pattern of BTO/NFO composite films, which consists of cubic NFO and orthorhombic BTO phases without any detectable impurities. The XRD pattern displays the coexistence of both ferroelectric BTO and ferromagnetic NFO phases in the composite films. The diffraction peaks of composite films are indexed using the standard JCPDS card no. 00-054-0964 for NFO phase with space group Fd-3m and JCPDS card no. 00-012-0213 for BTO phase with space group Fmmm, respectively. The broad

Conclusions

In conclusions, the BTO/NFO nanocomposite films have been successfully prepared via sol-gel procedure and spin coating technique. The studies of crystal structure and micro-morphology using XRD and FESEM with EDS confirm the good quality formation of BTO/NFO composite films, which are embodied as the homogeneous structure without any diffusion between the two functional layers. The room-temperature multiferroic properties are obtained due to the coexistence of ferromagnetism and

Declaration of competing interest

The manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work has not been published previously and it will not be submitted elsewhere before a decision is made by this journal.

Acknowledgements

This work was supported by Natural Science Foundation of Guangxi Province (No. 2018JJB160049), Middle-aged and Young Teachers' Basic Ability Promotion Project of Guangxi (No. 2019KY0096), National Natural Science Foundation of China (Nos. 11164003 and 11664003), Guangxi Science and Technology Base and Talents Program (Nos. 2019AC20116 and 2019AC20117).

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    L.G. Wang and G.B. Yu equally contributed to this work.

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