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.

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

1.3.2.1Valence 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 < T < )

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

5.3.7.1Thermoelectric behavior at low temperature (T

5.3.7.2Thermoelectric behavior at high temperature (T>Ts)

5.3.8 Power Factor

Summery and conclusions

1. Samples Preparation

2. Structural analysis

3. Magnetic studies

4. Electrical properties

5. Magnetoresistance

6. Thermoelectric power (TEP)

Research Objectives and Topics

This thesis aims to synthesize and investigate the structural, electrical, and magnetic properties of (La0.7Ba0.3MnO3)1-x/(NiO)x composites for spin-valve applications. The research specifically focuses on the impact of NiO doping and various annealing temperatures on the performance of these materials to enhance magnetoresistance (MR) and thermoelectric efficiency.

• Synthesis of polycrystalline manganite-based composites using solid-state reaction methods.
• Structural characterization using X-ray diffraction (XRD), Rietveld refinement, and Scanning Electron Microscopy (SEM).
• Analysis of magnetic phase transitions and the impact of the insulating NiO phase on ferromagnetic order.
• Investigation of conduction mechanisms through electrical resistivity measurements and theoretical modeling (SPH and VRH models).
• Study of thermoelectric properties and the role of phonon and magnon drag effects.

Excerpt from the Book

1.4 Magnetoresistance (MR)

Magnetoresistance is a property of some magnetic materials which has an important role in the rapid development of new technologies. For instance, magnetic sensors are based on this property. Magnetoresistance MR is defined as the change in the electrical resistance produced by the application of an external magnetic field. It is usually given as a percentage in the next form:

MR (%) = Δρ/ρ0 x 100% = (ρH - ρ0)/ρ0 x 100% (1.2)

where ρ0 is the resistivity in the absence of magnetic field and ρH is the resistivity under the applied magnetic field (H) and Δρ is the difference between them. There can be many different physical effects causing magnetoresistance; some of the most common ones are shown in table (1-1). In the mid 19thcentury it was pointed out that the electric resistance in magnetic materials depends on the orientation of an applied magnetic field relative to the orientation of the crystal itself. This phenomenon (Thomson, 1857) was given the name anisotropic magnetoresistance. On the other hand, the ordinary magnetoresistance, which is related to the Hall Effect, originates from the impact of the Lorentz-force on moving charge carriers. In absolute numbers, the magnitudes of the anisotropic and the ordinary magnetoresistances are moderate and typically not more than a few percent (see table. (1-1)). In the end of the 1980’s it was discovered that multi-layers of magnetic and nonmagnetic-metallic materials could show a magnetoresistance of much higher-magnitude that previously observed. The prefix giant was then used to describe the magnetoresistance as "Giant magnetoresistance" (GMR) (Baibich et al., 1988). As we mentioned before GMR was discovered by the French scientist Albert Fert and the German scientist Peter Grünberg at the same time in 1988 and they had Noble prize for their efforts (2007).

Summary of Chapters

CHAPTER I: Introduction and motivation: This chapter introduces the fundamentals of mixed-valence manganites, their general formula, and the significance of the double exchange mechanism and colossal magnetoresistance in modern technology.

CHAPTER II: Previous work: This chapter provides a literature review of experimental findings regarding crystal structure, magnetic properties, and transport behavior in various doped and undoped manganite systems.

CHAPTER III: Theoretical approach: This chapter outlines the physical models and mathematical frameworks used to analyze crystal structure, electronic configuration, transport mechanisms, and magnetism in these oxide systems.

CHAPTER IV: Experimental techniques: This chapter details the sample preparation process, measurement setups for resistivity, thermoelectric power, and magnetization, as well as the analytical techniques employed for data evaluation.

CHAPTER V: Results and discussion: This chapter presents the comprehensive experimental data on crystallographic, magnetic, and transport properties of the synthesized composites, along with the impact of annealing and theoretical fitting results.

Keywords

(LBMO)1-x/(NiO)x, Composites Structure, Magnetic properties, Electronic transport, Spin-valve, Thermopower, Manganites, Magnetoresistance, Rietveld analysis, Small polaron hopping, Variable range hopping, Perovskite.

Frequently Asked Questions

What is the core focus of this research?

The research explores the structural, electrical, and magnetic properties of (La0.7Ba0.3MnO3)1-x/(NiO)x composites to improve their viability for spin-valve applications.

What are the primary thematic fields covered in this study?

The study covers condensed matter physics, specifically focused on perovskite oxides, magnetotransport, structural characterization, and thermoelectric behavior of composite systems.

What is the central aim or research question of this thesis?

The main objective is to synthesize oxide manganite spin-valve structures for high-sensitivity magnetic read heads and sensors, investigating how NiO doping and annealing influence performance metrics like magnetoresistance.

Which scientific methodology is utilized in this study?

The work employs solid-state reaction methods for synthesis, X-ray diffraction (XRD) and Rietveld refinement for structural analysis, and physical measurements for magnetization and resistivity, complemented by empirical fitting models for conduction mechanisms.

What topics are addressed in the main body of the work?

The main body covers historical developments of manganites, theoretical foundations of transport (small polaron hopping, variable range hopping, double exchange), experimental setup procedures, and a detailed discussion of result data regarding composition effects and annealing impacts.

Which keywords best characterize this work?

Keywords include (LBMO)1-x/(NiO)x, Composites Structure, Magnetic properties, Electronic transport, Spin-valve, Thermopower, Manganites, Magnetoresistance, Rietveld analysis, Small polaron hopping, Variable range hopping, and Perovskite.

How does the introduction of NiO affect the parent LBMO compound?

The NiO phase acts as an insulator, and its incorporation into the LBMO matrix generally increases the zero-field resistivity, influences the metal-semiconductor transition temperature, and can open new parallel conductive channels at higher doping levels.

What role does the annealing process play according to the experimental results?

Annealing influences grain size, connectivity, and defect concentrations, which in turn leads to a resistivity reduction in pure LBMO and specific modifications to the magnetoresistance and thermoelectric responses of the composites.

• 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.