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Title of Thesis

Affia Aslam
Institute/University/Department Details
Department of Physics/ Quaid-i-Azam University Islambad
Number of Pages
Keywords (Extracted from title, table of contents and abstract of thesis)
colossal magnetoresistive perovskites, dopants, perovskite manganites, cmr, ferromagnetic parent system, thermoelectric power

The thermoelectric, magnetic and transport measurements have been made on bulk perovskite manganites popularly known as CMR materials with a view to understanding the phase changes in these materials with different types of structural and compositional changes. The materials studied include; La1-xCaxMnO3+δ (0.15‰ x‰ 0.50) La1-xCaxMnO1-y FeyO3, La0.49X0.01Ca0.50MnO3+δ and Sm0.50Sr0.50MnO3 {undoped and (Sm,Nd)0.05Sr0.05MnO3 and Sm0.05 (Sr,Ca) 0.05MnO3}.

The thermopower for Lal-xCaxMnO3 (0.15‰ x‰ 0.50) has shown a strong temperature dependence and is found to be positive or negative depending on the doping level and temperature. In high temperature paramagnetic region TEP data for all the compositions indicate that transport mechanism is dominated by hopping of small polarons while in low temperature ferromagnetic regime the main contribution in thermopower is through the magnon drag. Further our results show that electronic changes accompanying the transformations due to thermal cycling in some metastable composition (La0.50Ca0.50MnO3+δ) are extended well into the region above the charge ordering temperature. This suggests that the micro-structural changes accompanying the thermal cycling leave their imprint in the paramagnetic insulating state as well.

The effect of a magnetic dopant such as Fe that tends to create short-range clusters and break the ferromagnetic ordering of the host system has been investigated in detail. For purely ferromagnetic parent system (La0.65Ca0.35MnO3), the ferromagnetic and metallic transition temperatures are lowered and the thermoelectric power (TEP) shows an increasingly positive trend with the addition of Fe. TEP data indicates the decrease in the density of active holes, i.e. holes that can participate in the hopping process, with increasing Fe content and suggests the role of magnetic scattering due to the clusters formed by the antiferromagnetically coupled Fe. For the parent composition lying just at AFM/FM boundary in the x-T phase diagram (i.e La0.85Ca0.15MnO3), the replacement of Mn ions with Fe results in a spin glass phase at lower temperature that tends to increase the magnetic scattering contribution to the TEP strongly.

The effect of disorder induced by smaller cations doping has been investigated In a system close to charge ordered insulator-ferromagnetic boundary (La0.50Ca0.50MnO3+δ). It is observed that close to the charge ordered insulator-ferromagnetic phase boundary, the effect of disorder is to weaken the charge ordering that is more sensitive to disorder, whereas it leaves the more robust Double Exchange unaffected. Thereby extending the region in phase space over which the ferromagnetic phase is stable.

Further effect of change of one electron bandwidth has been studied in systems such as (Sm0.50Sr0.50MnO3) that lie close to the ferromagnetic-antiferromagnetic phase boundary by doping with larger and smaller size cations. The ground state for Sm0.50Sr0.50MnO3 composition is a mixture of ferromagnetic and A-type antiferromagnetic phases. Substitution of larger size cation (Nd) in place of Sm raises the ferromagnetic transition temperature while at the same time stabilizing the AFM phase. This latter behaviour is explained in terms of decrease of size mismatch factor between cations. However substitution of smaller size cation (Ca) in place of Sr almost eliminates the ferromagnetic phase leaving the system in a charge ordered state, which is typically accompanied by the CE-type of orbital ordering exhibiting very large values of resistivity.

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4353.24 KB
S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents
340.95 KB
2 1 Introduction 1
583.25 KB
  1.1 Important Features Of Manganites 1
  1.2 Basics Of Thermoelectric Power 15
3 2 Experimental Setup 24
475.48 KB
  2.1 Thermoelectric Power Measurements 24
  2.2 AC Susceptibility 31
  2.3 Resistively Measurements 36
4 3 Preparation And Some Basic Characterization Of Samples 42
565.13 KB
  3.1 Introduction 42
  3.2 Samples Preparation 42
  3.3 Crystal Structural Analysis By X-Ray Diffraction 44
  3.4 Determination Of Oxygen Content By Titration Analysis 46
  3.5 Thermoelectric Power Measurement Of La 1-X Ca x Mno 3+ ” ( 0.15‰X‰0.45) 51
5 4 Effect Of A Magnetic Impurity (Fe) On Tep Of La 1-X Ca x Mno 3 65
414.48 KB
  4.1 Introduction 65
  4.2 Tep Behavior Of La 0.65 Ca 0.35 Mn 1-X Fe x o 3 ( 0‰X‰0.07) 67
  4.3 Tep Behavior Of La 0.85 Ca 0.15 Mn 1-X Fe x o 3 (X=0 ,0.05 ) 76
  4.4 Summary And Conclusion 80
6 5 Disorder Effect Induced By Doping Of Trivalent Cations In La 0.5 Ca 0.5 Mno 3+” 82
430.74 KB
  5.1 Introduction 82
  5.2 Resistivity Measurement 85
  5.3 Magentization Measurements 89
  5.4 Thermoelectric Power Measurements 93
  5.5 Conclusions 96
7 6 Phase Separation In Sm 0.05 Sr 0.05 Mno 3 : Effects Of Cation Dopants 99
1743.68 KB
  6.1 Introduction 99
  6.2 Magnetic Properties 101
  6.3 Transport Properties 111
  6.4 Discussion And Conclusions 119
  6.5 Final Conclusion 122
  6.5 References 123