Magnetoresistance enhancement of (LBMO)1-x/(NiO)x composites based on Spin Valves

Master's Thesis , 2016 , 196 Pages

Physics - Nuclear Physics

The objective of this thesis is to study the structural, electrical and magnetic properties of (La0.7Ba0.3MnO3)1-x/(NiO)x composites, where 0 ≤ x ≤ 0.20 step 0.05 wt.%, that show promising properties for spin- valve applications.

The polycrystalline composites were prepared using the standard solid state reaction method. Besides to the as- prepared condition, the annealing process was taken into account as a variation in annealing temperature to see its influence on their properties of this system.

An analysis of the structural properties was carried out by means of X-ray diffraction and the Rietveld technique. The doping process wasn't change the structural properties. The composites undergo ferromagnetic (FM) to paramagnetic (PM) transition at TC.

Magnetization decreases with doping at x=0.05, 0.15 and 0.20 wt. %. M(T) relation in low temperature (x>0) show an obvious knee, which due to the presence of NiO.

The temperature dependence of resistivity showed a metallic behavior below the transition temperature Tms and above this temperature, the behavior become semiconducting. There were various mechanisms that governed the conduction above and below Tms. The semiconducting region was characterized by two main conduction mechanisms, the small polaron hopping (SPH) and the variable range hopping (VRH). In the metallic region at T < Tms, there were different contribution of mechanisms that govern this region as grain boundaries, domains and temperature independent process. In addition to some interactions as electron-electron interaction, electron-phonon interaction and spin wave scattering process which is a temperature range, composition and annealing temperature dependent.

The temperature dependence of resistivity was measured under the effect of 0.6T magnetic field for the as prepared and the annealed composites. The magnetoresistance was calculated and found to be affected by NiO content and annealing temperature. Thermoelectric power data (TEP) explains hole and electron contribution in conduction, and this also was found to be Ni amount and annealing temperature dependent. The TEP data analysis below temperature peak (Ts) confirmed the presence of some conduction mechanisms in electrical measurements as phonon drag, while the activation energy was determined from the region above Ts.

Inhaltsverzeichnis (Table of Contents)

• CHAPTER I: Introduction and motivation
• 1.1 Preface
• 1.2 Brief historical review
• 1.3 Electronic structure for parent and Doped compounds
• 1.3.1 Parent compound (ABO3)
• 1.3.2 Doped manganites
• 1.3.2.1 Valence distribution
• 1.4 Magnetoresistance (MR)
• 1.4.1 Colossal Magnetoresistance (CMR)
• 1.5 Interactions in manganites
• 1.5.1 Double Exchange (DE)
• 1.5.2 Superexchange
• 1.5.3 Lattice Polaron
• 1.6 Structural Distortions
• 1.6.1 Tolerance Factor
• 1.6.2 Jahn-Teller (JT) Distortion
• 1.7 Applications
• 1.8 Aim of this work
• CHAPTER II: Previous work
• 2.1 Introduction
• 2.2 Crystal structure
• 2.2.1 Undoped (parent) compound LaMnO3
• 2.2.2 Doped compounds (La1-xAxMnO3) where A is divalent cation
• 2.2.3 Spin Valve Structure (Manganites / Insulator)
• 2.3 Magnetic and transport properties
• 2.3.1 Undoped Compound LaMnO3 (parent)
• 2.3.2 Doped compounds (La1-xAxMnO3) where A is divalent cation
• 2.3.3 Spin Valve (Manganites / Insulator)
• 2.4 Thermoelectric Power (TEP)
• CHAPTER III: Theoretical approach
• 3.1 Preface
• 3.1.1 Crystal structure
• 3.1.2 Electronic configuration
• 3.1.3 Jahn-Teller effect
• 3.2 Exchange interactions in magnetism
• 3.2.1 Direct Exchange
• 3.2.2 Indirect Exchange: Superexchange
• 3.2.3 Double Exchange Model
• 3.3 Spin valve structure
• 3.4 Transport properties
• 3.4.1 Electrical Resistivity
• 3.4.1.1 Electrical resistivity in metal (Houg, 1972)
• 3.4.1.2 Insulators/semiconductors
• 3.4.1.3 Band insulators/semiconductors
• 3.4.1.4 Polarons
• 3.4.1.5 Diffusive Conductivity
• 3.4.1.6 Variable range Hopping
• 3.4.2 Phase transitions
• 3.4.3 Magneto-resistance Effect (Jain &Bery, (1972c))
• 3.4.4 Thermoelectric power
• 3.4.4.1 Sources of thermal emf
• 3.4.4.1.1 Volumetric component of thermal emf
• 3.4.4.1.2 The junction component of thermal emf
• 3.4.4.1.3 Phonon drags of electrons
• 3.4.4.2 Thermoelectric power of metal
• 3.4.4.3 Thermoelectric power of degenerate semiconductors
• 3.5 Fundamentals of Magnetism
• 3.5.1 Magnetic properties
• 3.5.1.1 Curie-Weiss Law
• 3.5.1.2 Zero Field Cooling Magnetization
• CHAPTER IV: Experimental techniques
• 4.1 Introduction
• 4.2 Synthesis
• 4.2.1 Measurement of thickness
• 4.3 Crystal structure
• 4.3.1 X-ray Diffraction examination (XRD)
• 4.3.2 Rietveld analysis
• 4.4 Surface morphology and elemental composition
• 4.4.1Scanning Electron Microscope (SEM) investigation
• 4.4.2Energy-dispersive X-ray spectroscopy (EDX)
• 4.5 Electrical resistivity measurements
• 4.6 Thermoelectric power (TEP)
• 4.7 Magnetization
• CHAPTER V: Results and discussion
• 5.1 Introduction
• 5.2 Effect of composition
• 5.2.1 XRD characterization analysis and Crystal structure
• 5.2.1.1 The average crystallite size
• 5.2.1.2 Rietveld analysis
• 5.2.2 Surface morphology characterization
• 5.2.3 Magnetic Studies
• 5.2.4 Electrical Resistivity of (LBMO)1-x/(NiO)x composites in zero magnetic field
• 5.2.5 Effect of applied magnetic field on the D.C electrical resistivity
• 5.2.6 Magnetoresistance
• 5.2.7 Conduction mechanisms
• 5.2.7.1 Ferromagnetic metallic region (T< Tms)
• 5.2.7.2 Paramagnetic semiconducting region
• 5.2.7.2.1 Variable range hopping model (Tms
• 5.2.7.2.2 Small Polaron hopping
• 5.2.8 Thermoelectric power
• 5.2.8.1 General
• 5.2.8.2 Effect of Composition
• 5.2.8.3 Thermoelectric Power at T< Ts
• 5.2.8.4 Thermoelectric power at T>Ts
• 5.3 Effect of annealing treatment on the composites
• 5.3.1 Preface
• 5.3.2 Structural analysis
• 5.3.2.1 XRD characterization analysis and Crystal structure
• 5.3.2.2 The surface morphology study
• 5.3.3 Magnetization
• 5.3.4 D.C electrical resistivity
• 5.3.5 Magnetoresistance
• 5.3.6 Conduction mechanisms above and below Tms
• 5.3.6.1 Conduction mechanisms below Tms
• 5.3.6.2 Conduction mechanisms above Tms
• 5.3.7 Effect of annealing temperature on thermoelectric power
• 5.3.7.1 Thermoelectric behavior at low temperature (T
• 5.3.7.2Thermoelectric behavior at high temperature (T>Ts)
• 5.3.8 Power Factor

Zielsetzung und Themenschwerpunkte (Objectives and Key Themes)

This dissertation aims to investigate the structural, magnetic, electrical, and thermoelectric properties of La0.7Ba0.3MnO3 (LBMO) and its composites with NiO. The study focuses on understanding the impact of composition and annealing treatment on the physical properties of these materials.

• Influence of composition on the structural, magnetic, electrical, and thermoelectric properties of LBMO/NiO composites.
• Investigation of the conduction mechanisms in LBMO/NiO composites.
• Impact of annealing treatment on the structural, magnetic, electrical, and thermoelectric properties of LBMO/NiO composites.
• Analysis of the thermoelectric power and power factor of LBMO/NiO composites.
• Exploration of the potential applications of LBMO/NiO composites in thermoelectric devices.

Zusammenfassung der Kapitel (Chapter Summaries)

Chapter I provides an introduction to the research topic, outlining the historical background, electronic structure, magnetoresistance phenomena, and relevant interactions in manganites. It also discusses structural distortions, applications, and the specific aims of this work.

Chapter II reviews previous work on the crystal structure, magnetic and transport properties of LaMnO3 and its doped compounds, including the spin valve structure. This chapter provides a foundation for understanding the current research.

Chapter III presents the theoretical framework used in the study, covering topics such as crystal structure, electronic configuration, Jahn-Teller effect, exchange interactions in magnetism, spin valve structure, transport properties, phase transitions, magneto-resistance effect, and thermoelectric power.

Chapter IV describes the experimental techniques used in the study, including synthesis, characterization methods such as X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Energy-dispersive X-ray spectroscopy (EDX), electrical resistivity measurements, thermoelectric power (TEP), and magnetization measurements.

Chapter V presents the results and discussion of the study, focusing on the effect of composition and annealing treatment on the structural, magnetic, electrical, and thermoelectric properties of LBMO/NiO composites. It explores the conduction mechanisms in these materials and analyzes the thermoelectric power and power factor.

Schlüsselwörter (Keywords)

The main keywords of this dissertation include: La0.7Ba0.3MnO3 (LBMO), NiO composites, structural properties, magnetic properties, electrical resistivity, thermoelectric power, conduction mechanisms, annealing treatment, power factor, colossal magnetoresistance (CMR), double exchange (DE), Jahn-Teller distortion, and spin valve structure.

• Author: Aml Mahmoud (Author)
• Publication Year: 2016
• Pages: 196
• Catalog Number: V357254
• ISBN (eBook): 9783668487659
• ISBN (Book): 9783668487666
• Language: English
• Tags: (LBMO)1-x/(NiO)x, Composites Structure, Magnetic properties, Electronic transport, Spin-valve, Thermopower.