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

CONTENTES

ABSTRACT: 1

CHAPTER I: Introduction and motivation 3

1 1 Preface 3

1 2 Brief historical review 3

1 3 Electronic structure for parent and Doped compounds 5

1 3 1 Parent compound (ABO3) 6

1 3 2 Doped manganites 7

1 3 2 1Valence distribution 8

1 4 Magnetoresistance (MR) 10

1 4 1 Colossal Magnetoresistance (CMR) 13

1 5 Interactions in manganites 14

1 5 1 Double Exchange (DE) 14

1 5 2 Superexchange 18

1 5 3 Lattice Polaron 20

1 6 Structural Distortions 21

1 6 1 Tolerance Factor 21

1 6 2 Jahn-Teller (JT) Distortion 22

1 7 Applications 25

1 8 Aim of this work 28

CHAPTER II: Previous work 29

2 1 Introduction 29

2 2 Crystal structure 30

2 2 1 Undoped (parent) compound LaMnO3 30

2 2 2 Doped compounds (La1-xAxMnO3) where A is divalent cation 32

2 2 3 Spin Valve Structure (Manganites / Insulator) 34

2 3 Magnetic and transport properties 37

2 3 1 Undoped Compound LaMnO3 (parent) 37

2 3 2 Doped compounds (La1-xAxMnO3) where A is divalent cation 39

2 3 3 Spin Valve (Manganites / Insulator) 40

2 4 Thermoelectric Power (TEP) 42

CHAPTER III: Theoretical approach 44

3 1 Preface 44

3 1 1 Crystal structure 44

3 1 2 Electronic configuration 46

3 1 3 Jahn-Teller effect 47

3 2 Exchange interactions in magnetism 50

3 2 1 Direct Exchange 50

3 2 2 Indirect Exchange: Superexchange 51

3 2 3 Double Exchange Model 52

3 3 Spin valve structure 55

3 4 Transport properties 56

3 4 1 Electrical Resistivity 56

3 4 1 1 Electrical resistivity in metal (Houg, 1972) 57

3 4 1 2 Insulators/semiconductors 57

3 4 1 3 Band insulators/semiconductors 58

3 4 1 4 Polarons 59

3 4 1 5 Diffusive Conductivity 61

3 4 1 6 Variable range Hopping 61

3 4 2 Phase transitions 63

3 4 3 Magneto-resistance Effect (Jain &Bery, (1972c)) 64

3 4 4 Thermoelectric power 65

3 4 4 1 Sources of thermal emf 66

3 4 4 1 1 Volumetric component of thermal emf 66

3 4 4 1 2 The junction component of thermal emf 66

3 4 4 1 3 Phonon drags of electrons 67

3 4 4 2 Thermoelectric power of metal 67

3.4.4.3 Thermoelectric power of degenerate semiconductors 67

3.5 Fundamentals of Magnetism 68

3.5.1 Magnetic properties 71

3.5.1.1 Curie-Weiss Law 73

3.5.1.2 Zero Field Cooling Magnetization 74

CHAPTER IV: Experimental techniques 76

4.1 Introduction 76

4.2 Synthesis 76

4.2.1 Measurement of thickness 77

4.3 Crystal structure 78

4.3.1 X-ray Diffraction examination (XRD) 79

4.3.2 Rietveld analysis 79

4.4 Surface morphology and elemental composition 79

4.4.1Scanning Electron Microscope (SEM) investigation 79

4.4.2Energy-dispersive X-ray spectroscopy (EDX) 79

4.5 Electrical resistivity measurements 80

4.6 Thermoelectric power (TEP) 83

4.7 Magnetization 87

CHAPTER V: Results and discussion 89

5.1 Introduction 89

5.2 Effect of composition 90

5.2.1 XRD characterization analysis and Crystal structure 90

5.2.1.1 The average crystallite size 92

5.2.1.2 Rietveld analysis 92

5.2.2 Surface morphology characterization 96

5.2.3 Magnetic Studies 100

5.2.4 Electrical Resistivity of (LBMO)1-x/(NiO)x composites in zero

magnetic field 103

5.2.5 Effect of applied magnetic field on the D.C electrical resistivity 106

5.2.6 Magnetoresistance 108

5.2.7 Conduction mechanisms 110

5.2.7.1 Ferromagnetic metallic region (T< Tms) 110

5.2.7.2 Paramagnetic semiconducting region 112

5.2.7.2.1 Variable range hopping model 112

5.2.7.2.2 Small Polaron hopping 115

5.2.8 Thermoelectric power 119

5.2.8.1 General 119

5.2.8.2 Effect of Composition 119

5.2.8.3 Thermoelectric Power at T< Ts 121

5.2.8.4 Thermoelectric power at T>Ts 123

5.3 Effect of annealing treatment on the composites 126

5.3.1 Preface 126

5.3.2 Structural analysis 126

5.3.2.1 XRD characterization analysis and Crystal structure 126

5.3.2.2 The surface morphology study 130

5.3.3 Magnetization 133

5.3.4 D.C electrical resistivity 135

5.3.5 Magnetoresistance 139

5.3.6 Conduction mechanisms above and below Tms 141

5.3.6.1 Conduction mechanisms below Tms 141

5.3.6.2 Conduction mechanisms above Tms 143

5.3.7 Effect of annealing temperature on thermoelectric power 153

5.3.7.1Thermoelectric behavior at low temperature (T<Ts) 156

5.3.7.2Thermoelectric behavior at high temperature (T>Ts) 156

5.3.8 Power Factor 161

Summery and conclusions 163

1. Samples Preparation 163

2. Structural analysis 163

3. Magnetic studies 164

4. Electrical properties 165

5. Magnetoresistance 166

6. Thermoelectric power (TEP) 167

References 170

Abstract

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 2 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.

1. Introduction and Motivation

1.1 Preface

Manganites has a general formula Rel-xDxMnO3, where RE is a rare earth cation (RE = La, Pr, Nd, Sm, etc.) and D is a divalent alkaline-earth cation (D = Ca, Sr, Ba, etc.). The manganese ions play an important role in manganites due to the two oxidation states Mn3+ and Mn4+ that known as mixed valence depending on the substitution parameter x. One of the main features of these materials is the close relationship between magnetism and electrical transport properties. Furthermore, the discovery of huge negative magnetoresistance (MR) near the Curie temperature, Tc. This phenomenon called “colossal” magnetoresistance (CMR) which is expected to have application in advanced technology. The mixed valence manganites have been used in different applications such as; a magnetic storage media, magnetic sensors, and electric field effect devices,……etc. The great attention increased due to the potential application in spintronic (Fiebig M. et al., (2002) and Hill N. A. and Rabe K. M., (1999)) and in Ferro electromagnet materials (Sharan A. et al., (2004)).

1.2 Brief historical review

In 1950 the manganites were first described by Jonker and Van santen. They prepared a polycrystalline samples of (La,Ca)MnO3, and (La,Sr)MnO3 manganites noticed a ferromagnetism and a significant increase in conductivity near Curie temperature with a variation in lattice parameters as a function of hole doping. One year later, Zener (1951) explained this unusual correlation between magnetism and transport properties as shown in Fig. (1-1) by introducing a novel concept, so-called “double exchange” mechanism (DE). Zener’s pioneering work was followed by more detailed theoretical studies by (Anderson and Hasegawa (1955)) and de Gennes (1960). In 1954 Volger observed a noticeable decrease in the resistivity of a sintered ceramic La0.8Sr0.2MnO3 when placed in a magnetic field. After those original experiments Wollan and Koehler (1955) characterized and drawn the first magnetic structure of La1-xCaxMnO3 using extensive neutron diffraction study. Interest in the manganites and their properties came about after the discovery of the colossal magnetoresistance effect (CMR) by Jin et al. (1994) in strained annealed La0.67Ca0.33MnO3 thin films. He observed a thousand- fold change in resistivity and observed magnetoresistance (MR) values larger than those giant magnetoresistance (GMR) obtained by Helmholt et al., (1993) in La2/3Ba1/3MnO3 and called it colossal magnetoresistance (CMR). Since the discovery of manganites, in 1950, there has been thousands papers published on the subject. The field is broad and remains very active.

1.3.1 Parent compound (ABO3)

Our starting with an undoped (parent) compounds, (ABO3). It is in the ideal case, cubic perovskite structure Fig. (1-3). A- ion locates at the corners of the cube, O- ion in the center of the cube faces and the manganese ion (B-site) would be located in the environment of six oxygen ions, which form a regular octahedron MnO6 (Bebenin N. G., (2011)). Depending on the mismatch between the sizes of the A-site, Bsite ion and some external conditions such as pressure and temperature the structure can be deformed, so that the whole system deviates from the perovskite structure. The degree of deformation can be represented by the geometric tolerance factor t* will discuss in detail later.

The perovskite structure in real lanthanum manganites is always distorted, so that the symmetry of the lattice is lower than cubic. The distortions of the cubic structure occur for two reasons. First, oxygen octahedra are turned relative to each other, and second the octahedra themselves can undergo distortions as a result of the Jahn–Teller effect (will be discuss in detail later). A significant role is also played by the presence of vacancies, especially, in oxygen sites. As a result, the lattice can be orthorhombic, rhombohedral, or (in rare cases) monoclinic (Huang Q. et al., (1998)). Thus the structure plays a great role in determining electron transport and magnetic properties in these oxides.

1.3.2 Doped manganites

Experimentally "doping" is the process that leads to the semiconductor-metal transition for LaMnO3. When doped with Ca2+ or Sr2+ these compounds could be called "hole doped manganites" attributed to the hole that is created in the eg level. This partial doping with a divalent cation causes a structural transition from orthorhombic to 8 rhombohedral or cubic structure (Rezlescu E. et al., (2008)) due to the differences in the size of the various atoms and these substitutions could change conductivity and phase stability. Hole doped manganites have a fascinating property under the application of magnetic field. A huge change in resistivity under the applied magnetic field noticed by Helmolt R. V. et al. (1993) and this phenomenon called giant magnetoresistance (GMR). Similar situation also occur for doping with Barium, the Lanthanum manganite LaMnO3 suffers a chemical substitution of La ions for Ba ions. However, since lanthanum is a trivalent ion and barium is divalent (La3+, Ba2+), the substitution leads to the change of some of the manganese ion valence from Mn3+ to Mn4+ (Salamon Myron B., and Marcelo Jaime (2001)). The resulting crystal is mixed-valent La1-xBaxMnO3, where x is the percentage of doping. The Ba and La are randomly occupying the A sites of perovskite-type structure.

1.3.2.1 Valence distribution

We described above the basic structure of the parent compound LaMnO3. So far we have been focused on the structure of the unit cell. Experimentally LaMnO3 is an insulator and its transition to the conducting state is provided by doping. The mixed-valence oxides can be regarded as solid solutions between end members such as LaMnO3 and CaMnO3 with formal valence states (La3+Mn3+O32-) and (Ca2+Mn4+O32-), leading to mixed-valence compounds such as (La1- x3+Cax2+) (Mn1-x3+ Mnx4+) O32-. The nominal electronic configuration of Mn3+ and Mn4+ are 3d4 and 3d3 respectively. For instance, La1-xSrxMnO3 compound is realized through a chemical substitution, e.g. La3+ → Sr2+, that is, by placing a divalent ion into the local La3+ position. The substitution La3+ → Sr2+ leads to the change in manganese-ion valence:

Mn3+ → Mn4+. The four-valent Mn ion loses its e2g electron Fig. (1-4). The missing electron can be described as a creation of a hole. At doping the Sr2+ ion goes into the center of the cubic cell (cf. Fig. (1-3). As to the hole itself, it is spread over the unit cell, being shared by eight Mn ions.

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