Strain mediated magnetoelectric coupling induced in (x) Bi_0.5Na_0.5TiO₃-(1−x) MgFe₂O₄ composites

Abstract

Particulate composites of ferroelectric and ferrite phases having general formula (x)Bi_0.5Na_0.5TiO₃-(1−x)MgFe₂O₄ (x=0, 0.5, 0.6, 0.7, 0.8 and 1.0), equivalently denoted as (x)BNT-(1−x)MgFO were synthesized by solid state reaction method. From X-ray diffraction analysis, the structural transformation from tetragonal to rhombohedral corresponding to BNT phase was observed in composites with x=0.5 to x=0.8. Shifting of (012) BNT peak towards left (x=0.5, 0.8) and right side (x=0.6, 0.7) observed due to occupation of Bi³⁺ ions at interstitial and substitutional sites into host MgFO lattice respectively. BNT produced strain in MgFO lattice. Scanning electron micrographs show closely packed microstructure with grain size variations from 0.62 to 2.90 µm. The value of magnetoelectric (ME) coupling coefficient increases from 2.42 (x=0.5) to 4.79 (x=0.8) mV/cm. Oe due to strain produced in MgFO and BNT lattices.

Introduction

Multiferroic materials due to their scientific and technological interest have drawn much attention in last few years. These materials exhibit simultaneous presence of ferroelectricity and ferromagnetism in same phase. The most fascinating characteristic of these materials is the presence of magnetoelectric (ME) effect, which allows the coupling between ferroelectricity and ferromagnetism, implying that electric polarization can be produced by applying either an electric field or magnetic field [1], [2]. These can be used in sensors and actuators which exploit their ferroelectric and ferromagnetic properties independently, and also these are potential candidates for devices such as magnetic field sensors, multiple state memories which exploit ME coupling [3]. Multiferroic materials can be acquired in two forms: single phase and composites. The single phase multiferroics exhibit feeble ME coupling, which limits their applications in devices. The composite materials are the better alternative to these materials as high ME coupling based on the concept of product property, can be realized in these materials by taking a suitable combination of good quality piezoelectric and magnetostrictive phases [4], [5].

BNT has high Curie temperature ~330 °C [6] and shows an anomalous dielectric transition from antiferroelectric to the ferroelectric state around 200 °C. BNT is considered to be a prominent candidate for ultrasonic generators and ferroelectric random access memory (FRAM) [7]. BNT is a perovskite ABO₃ type ferroelectric material. It exhibits a rhombohedral R3c polar structure at room temperature [8] and high piezoelectric constant with good electromechanical coupling factor [9].

Due to the magnetic phase in the composites, the problem of high leakage current arises in these systems, hence we choose a ferrite containing magnesium (MgFe₂O₄) having high specific resistance [10]. MgFe₂O₄ (MgFO) is an important magnetic oxide with spinel structure. Magnesium ferrite has found wide applications in microwave devices due to its lower magnetic and dielectric losses [11]. In the past years, various magnetoelectric composites such as Ni_0.65Zn_0.35Fe₂O₄-PbZr_0.53Ti_0.47O₃ [12], CuFe₂O₄-PbZr_0.53Ti_0.47O₃ [13], BaTiO₃-MgFe₂O₄ [10], [14], [15], (x)Co_0.5Zn_0.5Fe₂O₄–(1−x)PLZT [16]. As lead (Pb)-based compounds exhibit high piezoelectric response as a ferroelectric phase, therefore in most of the composites these are taken as ferroelectric phase. But the Pb-based ferroelectric has detrimental nature with respect to living beings as well as environment, hence the lead-free ferroelectric materials have become worldwide research topic [17]. In this paper, the synthesis and characterization of (x)Bi_0.5Na_0.5TiO₃-(1–x)MgFe₂O₄ composites, containing Magnesium ferrite (MgFe₂O₄) as the ferrite component and Bismuth sodium titanate (Bi_0.5Na_0.5TiO₃) as the ferroelectric component has been carried out. Hence, on detailed literature survey it has been found that this system has not been reported yet. In this present work, we have investigated the structural, microstructural, dielectric, magnetic, magneto-electric and optical properties of composite of MgFO and BNT synthesized by solid state reaction (SSR) method.