Nd and Sc co-doped BiFeO₃ nanopowders displaying enhanced ferromagnetism at room temperature

Highlights

• Will study the structure of materials obtained.

• A well crystallized BiFeO₃ doped material was obtained without further annealing.

• The obtained nanoparticles have sizes less than 60 nm.

• Enhanced ferromagnetic materials was obtained.

Abstract

We have developed a novel synthetic route for the preparation of single phase Nd_xBi_1 − xFe_0.95Sc_0.05O₃ (NBFSO) nanopowder materials by a surfactant-assisted combustion-derived method. Rietveld fitting of the Powder X-ray diffraction data showed the nanopowder structure evolves from a distorted rhombohedral BiFeO₃ crystalline structure (R3c, x = 0) to a orthorhombic structure (Pbnm, x = 0.10). Differential thermal analysis and thermogravimetric analysis (DTA/TGA) showed a crystallization temperature of 200 °C. Transmission electron microscopy (TEM) images revealed the presence of clusters formed by fine nanoparticles less than 60 nm in diameter. From Raman spectroscopy, the change from rhombohedral structure to cubic structure was observed by a drastic intensity reduction of the A₁⁻² and A₁⁻³ Raman modes, with the A₁⁻¹ and A₁⁻² modes gradually merging together, indicating the merge of the orthorhombic phase. Despite the antiferromagnetic nature of bulk BiFeO₃, the NBFSO nanopowders obtained displayed a ferromagnetic hysteresis loop, with coercivities of 0.08 T and remanent magnetizations of 0.65–4.05 Am²/kg when measured at room temperature. The increasing and uncompensated spins at the surface of nanoparticles and the canted internal spin by the tilt of FeO₆ octahedral units and the structure transition appear to be the main reason for observed this ferromagnetic behavior.

Introduction

In the last years ABO₃ perovskite structured BiFeO₃ (BFO) compound and related materials have attracted much attention due to the fact that it is a well-known Pb-free and eco-friendly material that exhibits simultaneous electric and magnetic orders over a broad range above room temperature. This concurrence of properties happens because it has a Curie temperature >800 °C and a Neel temperature of about 370 °C [1], [2], [3], [4], [5], [6]. As a partially covalent oxide, BFO has a rhombohedrally distorted perovskite structure belonging to the space group R3c [7]. It exhibits antiferromagnetism at room temperature due to the local spin ordering of the Fe³⁺ which forms a spiral magnetic spin cycloid with a periodicity of ∼62 nm [6]. This modulation inhibits the observation of weak ferromagnetism and also the linear magnetoelectric effect [8]. The co-existence of Fe²⁺ and oxygen vacancies makes the BFO compound suffer from large leakage currents, which limits its applications, preventing at the same time the multifunctional applications of BFO as a dielectric [9]. So far, most studies on BFO have been performed on two-dimensional epitaxial thin films grown on various substrates [10], [11], [12], [13], where the epitaxial strain manifests as to alter some important properties including crystal lattice of the structure, polarization, and magnetization. However, more recent approaches have been focused on polycrystals as well as substrate-free nanostructures such as low-dimensional nanostructures, especially zero-dimensional materials like nanoparticles (NPs) [14], [15], [16], [17], [18], [19]. At the same time, studies on the finite size effect of BFO have been carried out by different authors and some interesting properties (e.g., shift of Neel temperature, gas sensing properties, etc.) have been reported [20], [21], [22], [23], [24]. In order to improve magnetic and electrical properties, partial substitution of Bi³⁺-ions for rare-earths at the A-site, transition metal ions like Co²⁺ and Sc³⁺ at B-site or co-substitution with Pr/Mn, Pr/Sc, Nd/Sc ions at A/B sites respectively has been carried out [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]. As a first consequence of these ions substitutions, either the magnetic or the ferroelectric behavior has been improved.

Concerning the fabrication method, Selbach et al. synthesized BFO NPs through a modified Pechini method using nitrates as metal precursors [35]. Although pure phase BFO was obtained in this study, a non-desired extra phase produced by the decomposition of the precursor was also present. Ghosh et al. synthesized nanosized bismuth ferrite using a soft chemical route with tartaric acid as a template material and nitric acid as an oxidizing agent [36], [37]. However, the crystallinity of the resulting BFO NPs was unsatisfactory and the existence of an impurity Bi₂O₃ amorphous phase in the host was evident in the low temperature product of 400 °C. In contrast with the issues mentioned above, our research work shows that if an excess oxidant agent is added to the solution, crystallinity of BFO NPs can be improved. Herein, we reported this new combustion derived route for synthesizing nanoparticles at low temperatures, about 200 °C. By using this fabrication route, we have synthesized Nd and Sc co-doped BiFeO3 (x = 0, 0.05, 0.10) compounds. Structural and magnetic properties of the obtained NṔs have been investigated and discussed.