Understanding the structure-magnetic properties correlation in transition metal oxides and rare-earth-based double perovskites

Abstract

From precious stones to fundamental scientific interests, oxides have been a major part of research in the field of chemistry, physics, material science, electronics, electrical engineering, etc. Transition metal oxides show a wide range of exotic properties which includes high-TC superconductivity, ferromagnetism, gas sensing, magnetoresistance, magnetocapacitance. Perovskite structures are a stable structure and hence can accommodate different transition metal ions, unraveling interesting physics and numerous functionalities and applications in the field of magnetism, ferroelectricity and electrical transport. Perovskites find many important technological applications. Keeping exotic features of simple and complex oxides in mind, we have focused on the study of transition metal and multifunctional rare-earth-based double perovskite oxides.Among all transition metal oxides, CuO nanostructures have received huge scientific interest due to their extremely important unique properties. This is the building block of high-TC superconductors and giant magnetoresistance compounds. CuO is only binary oxide type-II multiferroic. CuO nanoparticles are widely used in optoelectronics, photodetectors, gas/bio-sensors, field emissions, supercapacitors, photocatalysis, magnetic storage media, and Li-ion batteries. In comparison with other metal oxides such as TiO2, ZnO, SnO2, CuO has an interesting magnetic response. The magnetic properties of CuO can be manipulated by various dopants such as Fe, Mn, or Fe/Li co-substitution to achieve desired functionality and applications. However, wide controversies exist in explaining the origin of room temperature ferromagnetism in this system. Magnetic properties depend on the synthesis methods, annealing, and processing conditions. Therefore, variation in synthesis routes may be responsible for differing reports on the magnetic nature of CuO. Hence, to understand the origin of ferromagnetism, further studies on electronic and magnetic nature of CuO are explored in this thesis.Rare-earth-based double perovskites, A2BBꞌO6 have drawn huge scientific attention in recent years, mainly due to the flexibility of various elemental combinations at A and B/Bꞌ sites and the consequent tailoring of their functionalities. This has stimulated the study of the complex and potentially richer double perovskites. The wide varieties of double perovskites were formed with many suitable elements at B, B′ cations. However, there are chances that B and B′ cations interchange with each other leading to antisite disorder, which strongly influences the magnetic and magneto-transport properties and may restrict the promise of rich functionality in double perovskites. So, the role of antisite disorder needs to understand to explore its functionalities. Out of various combinations, R2NiMnO6 compounds offer attractive research possibilities in multiferroic and multifunctional materials field. Such materials have not been studied in details and lack of functional characterizations. The materials require further exploration in examining their electronic and magnetic ground states.Among multifunctional R2NiMnO6 double perovskite oxides, La2NiMnO6 has recently drawn significant attention due to its importance both in terms of understanding of fundamental physics and potential for device applications. Near room-temperature ferromagnetism, cation ordering, spin−phonon coupling, magnetoelectricity, and magnetoresistance: these are the properties which make La2NiMnO6 multifunctional and an interesting candidate for fundamental study. It has been studied widely for its magnetic and dielectric behaviors. However, there are discrepancies regarding the origin of these properties in the material. The explanations found in the literature regarding the origin of the magnetic ground state, irreversibility in the zero-field-cooled (ZFC) and field cooled (FC) magnetization data, differ significantly. Discrepancies also exist regarding the valence states of Ni and Mn ions in the compound. It appears from the literature that the sample synthesis conditions may be significantly responsible for the reported discrepancies in the magnetic properties of La2NiMnO6. It is expected that the variation of Ni/Mn ratio in La2NiMnO6 may influence that cation ordering resulting in modification of its magnetic ground states. With an increase in Mn content in the samples, at least a fraction of Mn4+ should convert to Mn3+.Similarly, at least a fraction of Ni2+ should convert to Ni3+ for Ni excess samples. The influence of such ionic fraction on the magnetic interactions in perovskites is rarely discussed in the literature. Hence, there is a large scope of research in La2Ni1-xMn1+xO6 where one may tune antisite disorder and magnetic properties of La2Ni1-xMn1+xO6 compounds by controlling x.A primary topic of this thesis is to study the effect of annealing temperature, antisite disorder and cation ratio on the magnetic properties and spin−phonon coupling of La2NiMnO6. These studies are crucial to understand the origin of the multifunctional coupled features of La2NiMnO6 double perovskites. Additionally, replacing Ni by Fe, this thesis also studies La2FeMnO6, which is known to be a FiM material with finite antisite defects. An ordered polycrystalline La2FeMnO6 is targeted in this work which is difficult to synthesize and produces a more ideal magnetic nature. Also, effects of R3+ on the electronic and magnetic properties of R2NiMnO6 need to be clarified. The detailed knowledge of the oxidation and spin states of Mn and Ni ions is critical to understanding the broad magnetic response of these systems. To examine the short-range order and the physics of the La2NiMnO6, La2FeMnO6, R2NiMnO6 series, techniques which directly probe local atomic structure, such as x-ray-absorption spectroscopy are necessary. Uniformly distributed, homogeneous, and single-phase materials are synthesized using sol−gel method to understand the magnetic mechanism involved in these samples.