Structural and magnetic properties of Bi and Fe sites co-substituted bismuth ferrite

Abstract

Abstract

Magnetoelectrics, due to their coupled magnetic and electric order parameters, have potential applications in the areas of data storage, sensors and actuators, etc. The known magnetoelectrics show very low magnetoelectric coupling or show the coupling only at very low temperatures. This makes them impossible to use for the practical applications. There are very few room temperature magnetoelectrics and they have very low magnetoelectric coupling constant. Perovskite oxides (ABO3) are the much investigated systems because of their various interesting properties such as ferromagnetism, ferroelectricity, ferroelasticity etc. Even though ferromagnetism and ferroelectricity are mutually exclusive, in perovskite oxides they can coexist, if magnetism can originate from one metal cation and ferroelectricity from the other. BiFeO3 (BFO) belongs to this category, where ferroelectricity is from Bi and weak ferromagnetism from Fe, and is one the few single-phase room temperature multiferroics, which received tremendous attention. Though it can show high ferroelectric properties, it is antiferromagnetic, and due to the poor magnetic parameters, the magnetoelectric coupling is very low. To improve the magnetoelectric properties, magnetic properties of BiFeO3 need to be improved. Many studies have been reported on attempts to enhance the multiferroic characteristics of BFO by different methods like reducing the size to nano dimensions below the spin periodicity, substitution at the Bi and Fe sites which destroys the spin cycloid structure, etc. Substitution by suitable metal ions of comparable size is a very effective method to alter the properties of BiFeO3. Fe-site substitution by transition metal ions is widely studied to enhance the magnetic properties by the destruction of the spin cycloid structure due to the structural distortions as well as the magnetic contribution from the substituents. On the other hand, Bi- site substitution by rare earth ions and divalent ions can enhance the properties due to the structural distortions. Furthermore, in the case of divalent ion co-substituted compounds, in addition to the structural distortions, there will be charge compensation by the formation of Fe4+ and/or oxygen vacancies which may also improve the magnetic properties.

Co-substitution, Bi1-xAxFe1-yMyO3 (x = y or x ≠ y), which is the simultaneous substitution at the Bi-site and the Fe-site in BFO, is attracting much attention, with possibilities of simultaneous tuning of the magnetic and electrical properties. The literature on the detailed magnetic properties of co-substituted BiFeO3 is rare and therefore, the aim of this work is to study the detailed structural and magnetic properties of the divalent ion and Mn co-substituted BiFeO3. In this work, BiFeO3 is co-substituted at the Bi and Fe sites with divalent metal ions (Ca, Sr and Ba) and Mn. Mn4+ formed for the charge compensation is expected to lead to ferromagnetic double exchange interaction, and thus enhancing the magnetic properties. The structural, magnetic and dielectric properties are studied using powder XRD, Raman spectroscopy, SEM, XPS, SQUID VSM, etc. Chapter 1 deals with the general introduction to magnetic, ferroelectric and multiferroic properties of materials. Importance of BiFeO3, being one of the few room temperature multiferroics, have been discussed. Literature review on the structural, magnetic and ferroelectric properties of BiFeO3 is included. Various approaches to enhance the multiferroic properties of BiFeO3 emphasizing on the substitution effects in BiFeO3 are discussed in detail.

Chapter 2 describes the synthesis method and the experimental techniques used. Solid state synthesis method was used to prepare different materials. The details of various characterization methods like X-ray diffraction, Raman spectroscopy, SEM, XPS, magnetic and dielectric measurements are included. A brief discussion on the Rietveld refinement of the XRD patterns using the GSAS-EXPGUI program is also discussed.

Chapter 3 describes the structural, magnetic and dielectric properties of Bi1- xAxFe1-xMnxO3 (A= Ca, Sr and Ba). Effect of ionic radii of the substituent ‘A’ on the structural, magnetic and dielectric properties of Bi1- xAxFe1-xMnxO3 has been investigated. Ca2+ has almost similar ionic radii to Bi3+, whereas Sr2+ and Ba2+ are larger ions. For Ca-Mn co-substitution, XRD studies shows rhombohedral structure (R3c) for x ≤ 0.1 and orthorhombic structure (Pbnm) for x ≥ 0.2 and for Sr-Mn co-substitution R3c to R3 ̅c structural change is observed around x = 0.1, whereas for Ba-Mn co-substitution, R3c to tetragonal (P4mm) structural change is observed around x = 0.25. Raman studies also supported this structural transition. Decrease in the unit cell parameters and the unit cell volume indicated the presence of smaller Mn4+ in all the compositions. Room temperature magnetization of the samples increased with increasing the degree of co-substitution, showing a maximum remanence and coercivity at x = 0.15 for all the three co-substituted systems. Low temperature magnetization data implied a spin-glass-like nature of the samples. Among Ca/Sr/Ba-Mn co-substituted samples, Ca-Mn co-substituted samples showed better magnetic and dielectric properties around the morphotropic phase boundary (MPB) region, x = 0.15. The anomalous properties around x = 0.15 can be due to the MPB region around this composition or due to various exchange interactions among Fe3+, Mn3+, Mn4+ etc.

Chapter 4 discusses the detailed structural, magnetic, dielectric, and magnetodielectric properties of Bi1- xCaxFe1-xMnxO3. Large number compositions were synthesised near the MPB region. Rietveld refinement of the XRD patterns revealed rhombohedral structure (R3c) for x ≤ 0.11 and orthorhombic structure (Pbnm) for x ≥ 0.2. Compositions with 0.12 ≤ x ≤ 0.175 showed R3c+Pbnm mixed phase. The magnetization of the samples increased with increasing co-substitution, showing a maximum remanence and coercivity at x = 0.175. Dielectric properties showed a maximum at x = 0.15 and magnetodielectric data showed a maximum at x = 0.1. Changes in the various structural parameters like Fe-O-Fe bond angle, rhombohedral angle, tilt angle, etc, along with the higher Mn3+ content around the MPB region could be leading to the enhanced properties in this region. The higher magnetic, dielectric and magnetodielectric parameters around the MPB region suggested possible magnetoelectric coupling.

Chapter 5 discusses the structural, magnetic and dielectric properties of Bi1-xCaxFe1-yMnyO3 (x ≠ y). Five different series of compositions were prepared with the general formula, Bi1-xCaxFe1-yMnyO3 (0.1≤ y≤ 0.5). The Mn content was fixed and the Ca content was varied to study the effect of Ca on the properties of the co-substituted samples. The objective was to identify whether other combinations of Ca and Mn can show better properties than that obtained for Bi0.85Ca0.15Fe0.85Mn0.15O3. Rhombohedral (R3c) to orthorhombic (Pbnm) structural change was observed around x = 0.15 irrespective of the Mn content. Room temperature magnetization of the samples increased with increasing the calcium content with a maximum coercivity and remanence observed in the range x = 0.15 to 0.25 for different series. Magnetization of the samples increased with Mn content whereas the coercivity and remanence decreased. Dielectric constant showed a maximum value in the range of x = 0.1 to 0.25 for different series. Among the various compositions studied in the series Bi1-xCaxFe1-yMnyO3, Bi1-xCaxFe0.9Mn0.1O3 showed better magnetic properties around x = 0.15.

Chapter 6 is a summary of the work discussed on different co-substituted compositions in the previous chapters. The possible reasons for the weak ferromagnetism shown by the co-substituted BiFeO3 are discussed. Scope for future research based on the present results is also discussed.