Ferroelectric and ferromagnetic properties of Gd-modified BiFeO3
Introduction
Any multiferroic material shows at least two ferroic ordering, which can be a combination of ferroelectric, anti-ferromagnetic or ferroelastic order in the same phase, and a coupling between them over a certain range of temperature [1]. Such materials could electrically be polarized by applying an external magnetic field. Alternately, an external electric field could induce magnetization in the materials [2]. Therefore, multiferroic materials can provide an opportunity for potential applications in magnetic as well as electric field. Therefore, one can classify devices (employing such materials) into three categories: (a) devices that employ the ferroelectric or the magnetic properties separately, (b) devices that employ the ferroelectric and magnetic properties simultaneously but without magnetoelectric (ME) interaction, and finally, (c) devices whose action is based on ME effects [1], [2], [3], [4]. Realizing the importance of the materials several attempts have been made to discover the new multiferroic materials. Some known multiferroic have low transition temperature and small order parameter(s) with lots of leakage current. Recent discovery of multiferroic properties in BiFeO3 in a wide temperature region (above room temperature) has given hope to use the materials for multifunctional applications above room temperature. Bismuth iron oxide (BiFeO3) with rhombohedral (i.e., distorted perovskite of (ABO3) type [1] structure) has multiferroic properties in a wide range of temperature, and is one of the most promising single-phase multiferroic materials [2], [3] for devices. In last decade lots of research works on BiFeO3 (BFO) based multiferroics were carried out by making a suitable substitution at the Bi- and/or Fe-site of BFO. BFO shows simultaneous ferroelectricity (with high Curie temperature (Tc∼1103 K)), and ferromagnetism (with high Neel temperature (TN∼630 K)) in distorted perovskite or rhombahedral phase [5]. Unfortunately, the material possesses weak ferroelectric (polarization) and ferromagnetic order parameters (magnetization). In order to (i) enhance order parameters of the above phenomenon and (ii) reduce inherent problems (leakage current, structural distortion etc.) several attempts have been made by fabricating rare-earth modified single-phase BFO in different forms (ceramics, thin films single crystals, composites etc). It is found that a small amount of lanthanides, substituted at the Bi-site, has not changed the basic crystal structure of BFO [6], [7]. Room temperature X-ray structural study of BFO shows that it has rhombohedral (distorted perovskite) structure. The high tangent loss of BFO [8] is generally originated from the high conductivity causing higher leakage current. This major problem of the material creates difficulties in getting a high ferroelectric and ferromagnetic polarizations, and magnetoelectric (ME) coupling coefficient which are very much required for multifunctional applications. It has been observed that substitution of some rare-earth ions at the Bi+3site in BFO [6], [7] has improved the multiferrocity in the materials. It has also been observed that magnetic properties of BFO can also be changed by substituting some other rare earth ions such as Sm, Dy [9], [10] at the Bi-site of BFO. However, some works [11], [12] have already been done on Gd-modified BFO, but not much improvement in the above properties has been observed. Further, no detailed studies on dielectric and J–E characteristics of BFO were available on Gd-modified BFO. Therefore, in the present work some attempts have been made to enhance multiferroic ordering, conductivity, magnetization and coupling coefficient of Gd-modified BFO (BGFO).
Section snippets
Experimental procedures
The polycrystalline samples of Bi1−xGdxFeO3 (BGFO); (x=0.0, 0.05, 0.15, 0.25) were prepared using high-purity (>99.9%) ingredients: Bi2O3, Fe2O3 and Gd2O3 (M/s Loba Chemie Co., India) as raw materials. These oxides were first stoichiometrically weighed and mixed thoroughly using agate mortar and pestle in dry and wet (methanol) media for about 2 h. The homogeneous mixtures of the compounds were calcined at optimized temperatures ranging from 780 to 840 °C for 4 h using alumina crucibles. The
Results and discussion
Fig. 1 presents the XRD patterns of the Bi1−xGdxFeO3 (BGFO) (x=0, 0.05, 0.15, 0.25) ceramics sintered at different temperature from 820–870 °C for 4 h. A single-phase rhombohedral structure is found in all the BGFO samples. All the XRD peaks were indexed using a standard computer software program ‘PowdMult’, [13] and the lattice parameters of BGFO were determined as a function of Gd content. From the best agreement between observed (obs) and calculated (cal) inter-planner distance d (Σ(dobs−dcal
Conclusions
The Gd-modified BFO (BGFO) samples were prepared by a high-temperature solid-state reaction technique. Preliminary XRD structural analysis suggests that BGFO has the rhombohedral structure. Among the studied samples the lowest value of tan δ (3.97) and highest value of εr (3915) are observed for the BGFO samples with 15 mol% Gd doping. The saturated P–E hysteresis loops truly represent the ferroelectric nature of the studied samples and the lowering of coercive field. An increase in the remnant
Acknowledgment
The authors thank Professor B.K. Mahapatro and Dr. Dillip Mishra of IMMT, Bhubaneswar for their kind help in some experimental work.
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