Ferroelectric and ferromagnetic properties of Gd-modified BiFeO₃

Highlights

• The value of tan δ decreases whereas ε_r increases on increasing Gd concentration in BiFeO₃.

• A significant effect of Gd-substitution on coercive field and remnant polarization was observed.

• The magnetic moment, magneto-electric voltage and coupling coefficient of BiFeO₃ are enhanced on Gd-substitution in it.

Abstract

The polycrystalline samples of gadolinium (Gd)-modified bismuth iron oxide, Bi_1−xGd_xFeO₃ (BGFO) with x=0, 0.05, 0.15, 0.25 were prepared by a standard high-temperature solid-state reaction technique. A preliminary X-ray structural analysis was carried out to examine the structural deformation and stability of Gd-modified BiFeO₃. Room temperature surface morphology and textures of the samples were recorded by a scanning electron microscope, which reveal the uniform distribution of the plate-and rod-shaped grains in the samples. The grain size decreases with increase of Gd content in BGFO. Studies of dielectric and electric properties in a wide range of frequency (1 kHz–1 MHz) and temperature (30–400 °C) using complex impedance spectroscopic method have provided many new results. With higher Gd-concentration, dielectric constant and tangent loss of BGFO increases and decreases respectively at room temperature. Generally, the spontaneous and remnant polarizations are found to be enhanced on Gd substitutions in BFO. Studies of temperature dependence of electrical conductivity exhibit that all the samples follow Arrhenius-type of electrical conductivities behavior. The frequency dependence of ac conductivity suggests that the samples obey Jonscher's universal power law. The magneto-electric coupling coefficient of BFO is enhanced on increasing Gd content in BGFO.

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 (T_c∼1103 K)), and ferromagnetism (with high Neel temperature (T_N∼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).