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INVESTIGATIONS OF FERRITES PREPARED BY THE CITRATE PRECURSOR METHOD

by

AMARENDRA K. SINGH Department of Physics

Submitted

iii fulfilment of the requirements for the degree of DOCTOR OF PHILOSOPHY

to the

INDIAN INSTITUTE OF TECHNOLOGY, DELHI

DECEMBER, 2002

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CERTIFICATE

This is to certify that the thesis entitled 'INVESTIGATIONS OF FERRITES PREPARED BY THE CITRATE PRECURSOR METHOD' being submitted by Mr. Amarendra K. Singh to the Department of Physics, Indian Institute of Technology, Delhi for the award of the Doctor of Philosophy is a record of bonalide research carried out by him. lie has worked under our guidance and supervision, and has fulfilled the requirements for the submission of this thesis, which to Our knowledge has reached the requisite standard.

The results contained herein have not been submitted in part or full to any other university or institution for award of any degree or diploma.

(Prof. T.C. (;od) Department of Physics I1T, Maui. Khas

NCW Delhi 110016 INDIA

(Prof. R.G. Mcndiratta) Department of Physics

Ill, Maui. Khas New Delhi 110016 INDIA

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ACKNOWLEDGEMENTS

1 take great pleasure in expressing my gratitude to my research supervisors Prof. R. G.

Mendiratta and Prof. T. C. Goel for their immense interest, invaluable guidance and constant encouragement throughout the tenure of this work. I am indebted to them for their objective criticism, enthusiasm and dedication without which it would have been

impossible for this work to reach its fruition.

I am sincerely thankful to Dr. Anjali Varma for her constructive suggestions and helpful advice during the research period.

I am highly grateful to Prof. Vikram Kumar, Director SSPL, Delhi for providing some experimental facilities and Dr. Chandra Prakash and Dr. O.P. Thakur for helpful discussions. I acknowledge the University Grant Commission and I.I.T., Delhi for providing financial assistance during the work. I extend my sincere thanks to Dr A. K.

Tripathi and Dr. H. D. Sharma for giving their time and help at every stage of this work.

I am thankful to Dr. Pankaj Srivastava of Physics Department I. 1. T Delhi for the moral support I received from him. Thanks are also due to Dr. Seema Sharma, Sonalee, Pawan,

Ravindra and Radheshyam of my lab for their cooperation in some part of the present work. I MU also thankful to all of my friends and colleagues specially Rakesh, Navin, Manoj, Somnath, Puspendra, Dipanjan, Bala, Aloke, Srabjeet, Sandeep, Rajiv, Abhishek and Ved Varun for helping me in different way. I am personally grateful to my family

members for their constant inspiration and moral support.

(Amarendra K. Singh)

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ABSTRACT

In recent times, processing of ferrites by non-conventional solution techniques has gained importance with a view to obtaining high quality and high performance materials for various applications. Superiority of citrate precursor method over the conventional ceramic method for the preparation of ferritcs has been established. Mn-Zn ferrites are known to posses high initial permeability but low resistivity (-100 ohm-cm ). On the other hand, Ni-Zn ferrites posses higher resistivity (-10 ohm-cm) but relatively much lower initial permeability. For high frequency magnetic applications, ferrites with high saturation magnetization, permeability as well as high resistivity are needed. A these two ferrites is envisaged to meet these requirements. Although Ni-Zn and Mn-Zn ferrites have been investigated extensively, literature on the mixture of these two ferrites is scarce. In the present work following series of samples are prepared and investigated : (a) Mn,Ni0.5_,Zn0.5Fe204 with x 0.05, 0.1, 0.2, 0.3, 0.4.

(b) Mn,Nio4.,Zno j,Fe20,, with x 0, 0.1, 0.2, 0.3, 0.4.

(c) MnNi0.6-xZn04Fe204 with x — 0.1, 0.2, 0.3, 0.4, 0.5.

Further, to investigate the effect of substitution of Ni by Zn and Zn by Mn, Mno,Zn,Niog_,Fc20.4 with x — 0.2, 0.3 and Ni() 310,4n,Zno,7_,Fe204 with x 0.4, 0.5 have been also prepared and investigated. Citrate precursor method has been used for synthesizing ferrites. A comprehensive study of micro-structural (XRD, SEM), electrical (de resistivity, dielectric constant and dielectric loss), and magnetic (initial permeability

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and corresponding loss, saturation magnetization, Curie temperature, hysteresis parameters) properties are made.

The x-ray diffraction patterns of the samples calcined at 500°C show all the expected intense lines of the spinel structure. No additional lines other than spinel structure are seen. An increase in lattice parameter is observed when Ni is substituted by Mn (keeping Zn fixed) and Zn (keeping Mn fixed), and Zn by Mn (keeping Ni fixed). The results of theoretical and experimental densities have been also discussed. The surface morphology of the fractured surfaces of samples are studied. The effect of sintering temperature has been also investigated. It has been observed that grain size increases with increase in sintering temperature.

DC resistivity of all compositions has been investigated. It has been observed that resistivity decreases with increase in Mn concentration (keeping Zn). The observed temperature variation curves of some of the samples show two linear regions. Activation energy corresponding to both regions has been calculated. Possible conduction mechanisms contributing to this process have been discussed. Variation of resistivity with sintering temperature establishes a correlation between microstructure and conductivity.

It is observed that resistivity increases with increase in zinc concentration up to x = 0.5 and then decreases. An initial increase followed by the subsequent decrease is observed with increase in Mn contents (for a fixed Ni contents). The observed behaviour is explained in terms of hopping and site preference of ions in the lattice.

ii

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Dielectric properties of Mn-Ni-Zn ferrites have been investigated as a function of frequency, temperature, composition and sintering temperature. Increase in dielectric constant is observed with increase in Mn concentration (keeping Zn fixed) except for x = 0.3 of the series Mn,Nio,5_,Zno.5Fe204. Dispersion in dielectric constant with frequency in the range 100 liz to 1 MHz is observed. Resonance peaks were observed in tank vs. frequency curves for some of the samples. Shift in resonance frequency towards higher frequency is observed with increase in temperature. Peak height also increases with increase in temperature. Possible mechanisms contributing to these processes have been discussed. From the temperature variation of dielectric relaxation, activation energies for various samples have been calculated and compared with those obtained from d.c. resistivity measurements. The main contribution to dielectric relaxation intensity is observed to be due to space charge polarization.

Mn-Ni-Zn ferrites with different compositions have been synthesized by the citrate precursor method and investigated for their magnetic properties. The initial relative permeability is first observed to increase with increase in Mn concentration (keeping Zn fixed) up to certain concentration and then a decrease is observed. An initial increase followed by a subsequent decrease of saturation magnetization with increase in Mn concentration is observed. Curie temperature is observed to decrease continuously with increase in manganese contents. Frequency variation of complex initial permeability indicates that resonance peak due to domain wall oscillations is at a frequency of above 13 MHz. Saturation magnetization is observed to increase by about 22% when the sintering temperature is increased from 1200°C to 1400°C. Initial Permeability and B-H

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loops are observed to increase and constrict respectively with increase in sintering temperature. Initial permeability is observed to increase with increase in zinc concentration (keeping Mn fixed). Saturation magnetization increases with increase in zinc up to x — 0.4 and then starts decreasing. Canting effect is observed for higher zinc concentrations. Initial increase followed by a decrease is observed in saturation magnetization with increase in Mn concentration (keeping Ni fixed).

The work is reported in seven chapters. The first chapter describes a short review of the historical developments of ferrites and structure of ferrites. The chapter also establishes the motivation and aim of the present work.

Chapter II contains the relevant theoretical background and formulation necessary for the study of the micro-structural, electrical and magnetic properties of these ferrites.

Chapter III deals with the methodology of sample preparation and characterization techniques used for studying the micro-structural, electrical and magnetic properties.

Chapter IV discusses the experimental values and the theoretically calculated values of the densities obtained from the x- ray diffraction studies. It also presents the surface morphology obtained from scanning electron microscopy.

Chapter V describes the results and discussion of the electrical properties. Variations of de resistivity with temperature, composition and sintering temperature are discussed. The results of variations of dielectric constant and loss as a function of frequency, temperature, composition and sintering temperature are reported.

Chapter VI discusses the magnetic properties. Initial permeability and corresponding loss studies as a function of composition, frequency and sintering temperature are reported.

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This chapter also discusses the saturation magnetization, Curie temperature and hysteresis parameters.

Chapter VII presents the summary of the work done and recommendations for further work in this field.

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CONTENTS

Page No.

ACKNOWLEDGEMENTS

CHAPTER I. INTRODUCTION

1.1 Introduction

1.2 Motivation and Aim of the Present Work 5

1.3 Structure of Ferrites 8

CHAPPTER IL INVESTIGATED PARAMETERS

I

Introduction 1 I

2.1 Calcination 11

2.2 Sintering 12

2.3 Resistivity 13

2.4 Dielectric constant 15

2.5 Dielectric loss factor 17

2.6 Saturation Magnetization 18

2.7 Curie Temperature 20

2.8 Initial Permeability 21

Frequency dependence of Initial Permeability 22 Sintering temperature effect on Initial Permeability 23 Compositional dependence of Initial Permeability 24 2.9 Magnetocrystalline Anisotropy 24

2.10 Hysteresis Parameters 25

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2.11 Magnetic Losses 26

Eddy Current Losses 26

Hysteresis Losses 27

Residual Losses 28

CHAPTER III. EXPERIMENTAL DETAILS

29

Introduction 29

3.1 Sample Preparation 29

3.2 Structural Characterization 30

X-ray Diffraction 30

Density 31

Scanning Electron Microscopy 32

3.3 Electrical Properties 32

DC Resistivity 33

Dielectric Measurements 34

3.4 Magnetic Properties 35

Saturation Magnetization 35

Curie Temperature 36

Initial Permeability 36

Hysteresis Loop 36

CHAPTER IV. STRUCTURAL PROPERTIES

39

Introduction 39

4.1 X-ray Diffraction 39

4.2 Lattice Parameter 39

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4.3 Density 46 4.4 Scanning Electron Micrographs 46

CHAPTER V. ELECTRICAL PROPERTIES

54

5.1 Effect of Mn-substitution (keeping Zn fixed)

Introduction 54

5.1.1 DC Resistivity

Effect of Composition 55

Effect of Sintering Temperature 55

Effect of Temperature 58

5.1.2 Dielectric Constant 63

Temperature and Frequency Variation of Dielectric Constant 63

Compositional Effect 70

Effect of Sintering Temperature 72

5.1.3 Dielectric Loss 73

5.2 Effect of Ni-substitution by Zn (keeping Mn fixed)

82

and of Zn-substitution by Mn (keeping Ni fixed)

Introduction 82

5.2.1 Effect of Ni-substitution by Zn (keeping Mn fixed)

DC Resistivity 82

Dielectric Relaxation Intensity 85 5.2.2 Effect of Zn-substitution by Mn (keeping Ni fixed) 87

DC Resistivity 87

Dielectric Properties 88

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C PTER VT. MAGNETIC PROPERTIES

91

6.1 Effect of Mn-substitution (keeping Zn fixed)

91

Introduction 91

6.1.1 Saturation Magnetization 92

Effect of Composition 92

Effect of Sintering Temperature 95

6.1.2 Initial Permeability 96

Compositional Dependence 96

Frequency Dependence 99

Sintering Temperature Effect 99 6.1.3 Initial Permeability Loss 108

6.1.4 Curie Temperature 108

6.1.5 Hysteresis Parametrs 109

6.2. Effect of Ni-substitution by Zn (keeping Mn fixed) and of Zn-substitution by Mn (keeping Ni fixed)

Introduction 120

6.2.1 Effect of Ni-substitution by Zn (keeping Mn fixed) 120 6.2.1.1 Saturation Magnetization 120

6.2.1.2 Curie Temperature 121

6.2.1.3 Initial Permeability 121

6.2.2 Effect of Zn-substitution by Mn (keeping Ni fixed) 123 6.2.2.1 Saturation Magnetization 123

6.2.2.2 Initial Permeability 124

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CHAPTER VII. CONCLUSIONS AND RECOMMENDATIONS

125

7.1 Conclusions 125

7.2 Recommendations of Future Work 129

REFERENCES 131

LIST OF PUBLICATIONS 141

BIODATA OF THE AUTHOR 143

References

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