THERMOELECTRIC AND MAGNETIC PROPERTIES OF SOME COLOSSAL MAGNETORESISTIVE (CMR) PEROVSKITES: EFFECTS OF DIVALENT AND TRIVALENT CATION DOPANTS

Affia, Aslam (2006) THERMOELECTRIC AND MAGNETIC PROPERTIES OF SOME COLOSSAL MAGNETORESISTIVE (CMR) PEROVSKITES: EFFECTS OF DIVALENT AND TRIVALENT CATION DOPANTS. Doctoral thesis, Quaid-i-Azam University Islambad.

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Abstract

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.

Item Type: Thesis (Doctoral)
Uncontrolled Keywords: colossal magnetoresistive perovskites, dopants, perovskite manganites, cmr, ferromagnetic parent system, thermoelectric power
Subjects: Q Science > QC Physics
Depositing User: Muhammad Khan Khan
Date Deposited: 09 Sep 2016 03:57
Last Modified: 09 Sep 2016 03:57
URI: http://eprints.hec.gov.pk/id/eprint/1188

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