I= EFFECTS OF COMPETING INTERACTIONS AND PHASE SEPARATION IN DOPED RARE-EARTH MANGANITES
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Title of Thesis
EFFECTS OF COMPETING INTERACTIONS AND PHASE SEPARATION IN DOPED RARE-EARTH MANGANITES

Author(s)
Wiqar Hussain Shah
Institute/University/Department Details
Department of Physics/ Quaid-i-Azam University Islamabad
Session
2004
Subject
Physics
Number of Pages
157
Keywords (Extracted from title, table of contents and abstract of thesis)
doped rare-earth manganites, colossal magnetoresistive, ferromagnetism, antiferromagnetism, perovskites, polaronic system, fe doping

Abstract
The magnetic and transport properties of the doped manganites colossal magnetoresistive (CMR) compounds with the perovskite structure are mainly characterized by a competition between ferromagnetism and antiferromagnetism, and between a metallic and insulator behavior. The effects of the magnetic and transport measurements of the CMR compound have been investigated by different DC and AC techniques.

The magnetic and transport behavior of La1-xCaxMnO3+ δ (x=0.48, 0.50, 0.52 and 0.55 and δ =0.015) compositions close to charge ordering, was studied through resistivity DC magnetization and AC susceptibility measurements. With time and thermal cycling (T<300 K) there is an irreversible transformation of the low-temperature phase from a partially ferromagnetic and metallic to one that is less ferromagnetic and highly resistive. For instance, an increase of resistivity can be observed by thermal cycling, where no effect is obtained for lower Ca concentration. The time changes in the magnetization are logarithmic in general and activation energies are consistent with those expected for electron transfer between Mn ions. The data suggest that oxygen non-stoichiometry results in mechanical strains in this two-phase system, leading to the development of irreversible metastable states, which relax towards the more stable charge-ordered and antiferromagnetic micro domains. This behavior is interpreted in terms of strains induced charge localization at the interface between FM/AFM domains in the antiferromagnetic matrix. Charge, orbital ordering and phase separation play a prominent role in the appearance of such properties, since they can be modified in a spectacular manner by external factor, making the different physical properties metastable. Here we describe two factors that deeply modify those properties, viz. the doping concentration and the thermal cycling. The metastable state is recovered by the high temperature annealing. We also measure the magnetic relaxation in the metastable state and also the revival of the metastable state (in a relaxed sample) due to high temperature (800 Cº) thermal treatment.

The effect of Fe doping in La0.65Ca0.35Mn1-xFexO3 (where 0 ‰x ‰ 0.10) compound on the Mn site in the ferromagnetic phase has been studied in detail. The XRD results showed that all compounds crystallized in a tetragonal phase and no appreciable change is observed in the lattice parameters with increasing Fe concentration. Resistivity measurements of the compound from ambient temperature down to 77 K exhibit a peak at temperature Tp, which decreases with increasing Fe content. Substantial rise in resistivity corresponding to the Tp and increase spin disorder are also observed with increasing doping. Two models, variable range hopping (VRH), and polaronic have been used to explain the DC transport mechanism in the insulating region above Tp. The VRH model shows better fit to the resistivity data as compared to the polaronic model. The localization length is found to decrease by increasing Fe concentration. The activation barrier, W, has been calculated and found to increase with the increase of Fe content. In the metallic region (T

The effects of partially replacing Mn with Fe ions in the canted AFM La0.85Ca0.15MnO3 system are also studied. On replacing 5% of the Mn with Fe ions the system exhibits a transition to a re-entrant spin glass like phase with a ferromagnetic transition at 170 K and spin freezing transition at T -100 K. The system remains insulating throughout both the ferromagnetic and glassy phases with however a sharp resistivity increases at low temperatures (T -150 K). The ZFC and FC magnetization deviate from each other below Tc and closely logarithmic relaxation appears at our experimental time scales (3600 s). The ZFC magnetization has a maximum at 160 K, whereas the FC magnetization continues to increase, although less sharply, also below this temperature. The spin freezing temperature shifts upwards with increasing frequency and magnetic relaxation effects are observed. However conventional spin glass features such as a memory effect are not observed and the spin freezing temperature is relatively insensitive to applied magnetic fields.

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S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents
502.8 KB
2 1 Introduction 1
815.59 KB
  1.1 Rare-Earth Manganite Perovskites 1
  1.2 Exchange Interactions In Manganites 2
  1.3 The Jahn Teller Effect 9
  1.4 Conduction Process In Polaronic System 9
  1.5 Charge, Orbital, And Magnetic Ordering In Manganites 12
  1.6 Lattice Effect In Rare-Earth Manganites 15
  1.7 Effects Of Fe Doping On The Magnetic And Transport Properties Of Manganites 17
  1.8 Survey Of Experimental Results For Rare-Earth Manganites 18
  1.9 Why Manganites Are Interesting 21
  1.10 Motivation Of This Research Work 22
3 2 Design And Construction Of Experimental Set-Up 24
515.44 KB
  2.1 Designing And Construction Of DC Solenoid Magnet 24
  2.2 Design And Construction Of DC Resistivity Measuring System 26
  2.3 M( T) Probe And Measuring System 28
  2.4 Ace Susceptometer And Techniques For AC Susceptibility Measurements 28
  2.5 Computer Interfacing (Lab View ) 33
4 3 Fabrication And Characterization Of The Samples 37
350.17 KB
  3.1 Preparation Of The Samples 37
  3.2 Titration Analysis For Determining Oxygen Stochiometry 40
  3.3 Basic Magnetic And Transport Characterization Of L 1-X ca x mno 3+” And La 0.65 ca 0.35 m 1-X fe x o 3 Compounds 43
  3.4 Dc Resistivity Measurements, Field Effects 44
5 4 Irreversible Metastable Behavior In Doped Rare-Earth Manganites Close To Charge Ordering 49
1344.64 KB
  4.1 Introduction 49
  4.2 Mangetization Of L 1-X ca x mno 3+” (X=0.48 ,0.50,0.52,0.55 ) Compounds 50
  4.3 Iodometric Analysis L 1-X ca x mno 3 +” 51
  4.4 Transport Properties Of L 1-X ca x mno 3+” 52
  4.5 Effects Of Thermal Cycling On Resistivity Of La 0.50 ca 0.50 Mno 3+” 53
  4.6 Temperature Dependence Of Magnetization For L 1-X ca x mno 3+ ” : Effects Of Thermal Cycling 59
  4.7 Suppression Of Metastable Behavior By Large Magnetic Fields 62
  4.8 Dc Magnetic Relaxation For L 0.50 ca 0.05 mno 3+” 64
  4.9 Dynamic (Ac) Response Of The L 1-X ca x mno 3+” Manganites 69
  4.10 Revival Of Metastable Behavior 78
  4.11 Effect Of Oxygen Annealing On Ac Susceptibility Measurements 79
  4.12 Magnetic Relaxation Via Ac Susceptibility For L 0.05 ca 0.05 mno 3+” 81
  4.13 Summary And Conclusions 86
6 5 Effects Of Fe Doping On The Transport And Magnetic Behavior In La 0.65 ca 0.35 m 1-X fe x o 3 Compounds 89
1179.39 KB
  5.1 Introduction 89
  5.2 Transport Properties Of La 0.65 ca 0.35 m 1-X fe x o 3 Compounds 90
  5.3 Magnetoresistance Measurements For La 0.65 ca 0.35 m 1-X fe x o 3 102
  5.4 Effects Of Fe Doping On Magnetorsistance 106
  5.5 Dc Magnetization Measurements Of Fe Doped Manganites 108
  5.6 Dynamic Response Of La 0.65 ca 0.35 m 1-X fe x o 3 Manganites 113
  5.7 Summary And Conclusions 121
7 6 Re-Entrant Spin Freezing Behavior In Fe Doped La 0.85 ca 0.15 mn 0.95 fe 0.05 o 3 Compound 124
2356.32 KB
  6.1 Introduction 124
  6.2 Transport Properties Of La 0.85 ca 0.15 mn 0.95 fe 0.05 o 3 126
  6.3 Arrot Plots From Magnetization Isotherms 128
  6.4 Dc Magnetization In Fc & Zfc Mode For La 0.85 ca 0.15 mn 0.95 fe 0.05 o 3 131
  6.5 Ac Susceptibility Studies Of La 0.85 ca 0.15 mnfe 0.05 o 3 & La 0.85 ca 0.15 mn 0.95 fe 0.05 o 3 133
  6.6 Ac Magnetic Relaxation In Thermo Remnant Moment ( Trm ) State 141
  6.7 Dc( Isothermal Remanent ) Moment Relaxation 142
  6.8 Memory Effects In Spinglass Region 144
  6.9 Non-Linear Harmonics Susceptibility 145
  6.10 Summary And Conclusions 150
  6.11 References 152