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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Rare earth manganite perovskites

Maguire, Elaine T. January 1999 (has links)
The 'RMnO3': R = La, Nd, Pr, phases have been synthesised and characterised by a combination of electron probe microanalysis (EPMA), H2-reduction thermogravimetry (TG), x-ray (XRD) and neutron diffraction (ND). RMnO3 forms, at" 1400C, over the ranges: NdMn0, 95Oz to Nd0 88MnOz PrMn0.97O2 to Pr0 88MnOz LaMno 0.90Oz to La0.97MnOz Oxygen contents vary in air over the range 700 to 1400 C and can be varied further, either by high pressure Oz treatment or by reduction in H2. The structure of 'RMnO3' R = Nd, Pr is based on the GdFe03 structure with a Jahn-Teller distortion associated with the high proportion of Mn3+ ions present. The oxygen deficient 'LaMnOz' compositions also exhibit this structure consistent with earlier reports. By combining EPMA, TG, XRD and ND results various defect models describing the stoichiometry and structure of Mn-rich and R-rich, R = Nd, Pr compositions have been summarised. Both R = Nd and Pr systems exhibit very varied defect structures; depending on composition and heat treatment, vacancies can form on any one or any two of the three sublattices, R, Mn and O and the overall Mn oxidation state can include 2+, 3+ and 4+ contributions. For 'RMn03': R = La, Nd, Pr, data on their compositional ranges and defect crystal structures are presented in the form of novel phase diagram-defect structure maps from which the principal defect structure for a given stoichiometry can be easily obtained. The majority of the Pr-Sr-Mn-O pseudotemary phase diagram has been determined. EPMA was used to follow the progress of reaction and the conditions to achieve complete reaction established. Several solid solutions were evidenced, some previously unreported (3 - 6): 1) Pr1.xSrxMnO3 0[Special character omitted]x[Special character omitted]1.0 2) Pr1+xSr2.xMn2O7 0 [Special character omitted]x [Special character omitted] 0.4 3) SrxPr1-xO2 0[Special character omitted]x[Special character omitted]0.16 4) Sr1-xPrO3 0[Special character omitted]x[Special character omitted]0.15 5) Sr2.xMnxO4 6) Sr2.xPrxMnO4 The perovskite-like Pr1-xSrxMnO3 solid solution extends from PrMnOz to SrMnOz. The unit cell symmetry changes from orthorhombic to rhombohedral to tetragonal to cubic and finally to hexagonal as the Sr content increases. The limits of the Ruddlesden Popper (RP) n=2 Pr1+xSr2_xMn2O7 solid solution were determined: 0 [Special character omitted] x [Special character omitted] 0.4 and a tetragonal unit cell observed consistent with the literature. Synthesis of the RP compositions by solid state methods requires long heating times (up to 36 days) to produce homogeneous samples; qualitative EPMA of younger samples indicated an inhomogeneous distribution of Pr and Sr. Contrary to EPMA results, XRD of younger samples indicated that complete reaction had occurred and single phase compositions produced. It is suggested that the SrxPr1-.xO2 solid solution extends over the range 0 [Special character omitted] x [Special character omitted] 0.16 where similarly to the polymorphism of praseodymium oxides, compositions 0.03 [Special character omitted] x [Special character omitted]0.16 exhibit the cubic fluorite-type structure of Pr6O11 and x [Special character omitted] 0.03 is a mixture of cubic SrxPr1-xO2 and hexagonal SrxPr2.xO3. Perovskite-like SrPrO3 exhibits variable cation ratios; the Pr-rich boundary is Sr0.85PrOz. The lower Sr-rich boundary is yet to be identified. Similarly to 'RMnO3': R = La, Nd, Pr, the oxygen content of 'SrPrOz' is expected to vary. Therefore, various possible defect structures describing vacancies on the three sublattices, Sr, Pr and O could exist and charge compensation would be an interesting example of ionic and electronic mechanisms where Pr adopts the +4 and +3 oxidation states. Four layer hexagonal SrMnO3 exhibits variable Sr:Mn ratios but the solid solution limits are not yet known. The unreported Sr2-xPrxMnO4 solid solution has been observed but the solid solution limits are not yet known. The K2NiF4-type structure of Sr2Mn04 is retained at x = 0.75 and is expected to contain Mn3+ as Mn4+ is reduced to compensate Sr24 substitution by Pr3+.
2

Plonųjų manganitų sluoksnių sąveikos su mikrobange spinduliuote tyrimas / Interactions of thin manganites films with microwawe radiation

Kavaliauskas, Žydrūnas 16 June 2006 (has links)
In this work the experimental resuls of the investigation of microwave response of textured LCMO films are presented. It is shown that microwave pulse induced dc resistance response is an agreement with the temperature derivative of the film resistance.
3

Étude magnéto-optique des composés multiferroïques: DyMnO[indice inférieur 3] et TbMn[indice inférieur 2]O[indice inférieur 5]

Mansouri Sabeur January 2015 (has links)
Les matériaux multiferroïques sont des composés qui présentent la remarquable propriété de magnétoélectricité. Leur polarisation électrique est induite, renforcée ou orientée par le contrôle magnétique de leurs ordres de spins. Les matériaux multiferroïques qui font l’objet d’étude dans cette thèse sont les composés DyMnO[indice inférieur 3] hexagonal et TbMn[indice inférieur 2]O[indice inférieur 5] orthorhombique qui représentent chacun une famille de composés multiferroïques. Dans ce travail, nous présentons une approche magnéto-spectroscopique dans l’étude de ces matériaux multiferroïques. Dans cette approche, nous avons combiné plusieurs mesures spectroscopiques (Raman, infrarouge et champ cristallin) avec des mesures magnétiques afin de sonder leurs propriétés magnétoélectriques. La question centrale dans cette thèse est d’apprendre quels sont les paramètres et les mécanismes microscopiques qui contrôlent l’effet magnétoélectrique dans DyMnO[indice inférieur 3]-hex et TbMn[indice inférieur 2]O[indice inférieur 5]-ortho. À partir de la dépendance en champ magnétique de l’énergie de certains phonons infrarouges du DyMnO[indice inférieur 3], nous avons estimé la variation de sa polarisation électrique en fonction du champ magnétique appliqué. La polarisation électrique estimée se renforce en fonction du champ magnétique et reproduit un comportement similaire à la variation en champ magnétique de la polarisation électrique de HoMnO[indice inférieur 3]-hex. Le renforcement de la polarisation électrique est expliqué par une modulation magnétique des effets de transfert de charges entre les terres rares et les oxygènes apicaux. La dépendance en température des phonons Raman et infrarouge du TbMn[indice inférieur 2]O[indice inférieur 5] montre que ce matériau présente un fort couplage spin-réseau. L’étude de la dépendance en champ magnétique de ses signatures Raman et infrarouge a permis de distinguer des effets de distorsions structurales purement magnétiques qui contrôlent son effet magnétoélectrique. En particulier, nous avons montré que les phonons qui impliquent la liaison Mn[indice supérieur 3+] -O et ceux qui embarquent les ions Tb sont sensibles à la présence d’un champ magnétique. La dépendance en champ magnétique des fréquences de phonons Mn-O montre un saut énergétique entre 0 et 3 Tesla qui correspond exactement au saut de la polarisation électrique du TbMn[indice inférieur 2]O[indice inférieur 5]. Nous avons expliqué l’effet magnétoélectrique du TbMn[indice inférieur 2]O[indice inférieur 5] en termes d’un effet magnétostrictif dû au couplage d’échange symétrique Mn-Mn. Le rôle du magnétisme du Tb dans le renversement de la polarisation électrique du TbMn[indice inférieur 2]O[indice inférieur 5] se concrétise par le biais d’une interaction Tb-Mn qui modifie l’état magnétique du Mn[indice supérieur 3+][indice inférieur flèche vers le bas] - Mn[indice supérieur 4+][indice inférieur flèche vers le haut] - Mn[indice supérieur 3+][indice inférieur flèche vers le haut].
4

Phase coexistence in manganites

Chapman, James Christopher January 2005 (has links)
The doped perovskite manganite La1-xCaxMnO3 (0<x<1) has been extensively studied due to the interactions between the electronic, magnetic and crystal lattices, and the wide range of phases that can coexist. The region of greatest interest in the bulk material is around x~0.5, where there is often mesoscopic phase coexistence between a ferromagnetic metal (FM) and an antiferromagnetic insulator (AF). The first part of the dissertation describes a systematic study on a series of La1-xCaxMnO3 films deposited onto SrTiO3 (001) by pulsed laser deposition with compositions in the range 0.40<x<0.45. From electrical transport and magnetisation measurements, the limit of metallic behaviour was found to be x=0.41 whereas ferromagnetism was seen up to x=0.45. Although the transition temperatures of 150-200 K were comparable with the bulk material, the saturation moment at 20 K was about 40% of the fully spin-aligned value, which suggests the possibility of a phase separated mixture of FM and AF regions. The deviation from the bulk behaviour is thought to be due to substrate-induced strain altering the crystal symmetry and making the cubic ferromagnetic state less favourable. In the remainder of this work, the nature of phase separation in 60 nm La0.59Ca0.41MnO3 and La0.60Ca0.40MnO3 films is investigated. The effect of an external magnetic field is studied. A high-field magnetoresistance (Δρ/ρB=0) of 41% in fields of 400 mT was observed for a La0.60Ca0.40MnO3 film, which, while not as large as the values previously reported in the literature, is still significant. The magnetic history of the films was found to be very significant, with the zero-field resistivity depending on the highest field applied. The isothermal time dependence of the resistivity was found to be exponential, with a time constant in the range 100-1000 s. Possible mechanisms for the MR effect and the dependence on magnetic history are discussed.
5

A Spatially Resolved Spectroscopic Investigation Into The Ferromagnetic Metallic State In Hole Doped Manganites

Mitra, Joy 07 1900 (has links) (PDF)
No description available.
6

Experimental And Theoretical Studies Of Strongly Correlated Multiferroic Oxides

Ghosh, Anirban 03 1900 (has links) (PDF)
This thesis presents the synthesis and investigations of physical and chemical properties of multiferroic materials experimentally as well as theoretically. Multiferroics are materials in which at least two of the three ferroic orders, ferroelectricity, ferromagnetism and ferroelasticity occur in the same phase. Multiferroics, have the potential to be used as a four state as well as cross switchable memory devices. The thesis is organized into seven Chapters. Chapter 1 gives a brief overview of the different facets of multiferroics, explaining the origin of Multiferroicity and magnetoelectric coupling, their possible technological applications and the challenges involved. Chapter 2-4 concerns the experimental aspects and chapter 5-7 concerns the theoretical aspects. Chapter 2 deals with experimental investigations on nanoscale charge-ordered rare earth manganites. It shows with decreasing particle size the ferromagnetic interaction increases and the charge-ordering vanishes down to the lowest sizes. Chapter 3 describes magneto-dielectric, magnetic and ferroelectric properties of hexagonal LuMnO3. It also describes the Raman spectroscopy of this compound through the magnetic and ferroelectric transition temperatures. Chapter 4 deals with the anisotropic multiferroic properties in single crystals of hexagonal ErMnO3. In chapter 5 a brief introduction of density functional theory (DFT) is given. Chapter 6 deals with the magneto-structural changes, spin-phonon couplings and crystal field splittings for the different magnetic orderings LuMnO3. Chapter 7 elucidates the role of Lu d0-ness for the ferroelectricity observed of this compound.
7

Magnetization, Magnetotransport And Electron Magnetic Resonance Studies Of Certain Doped Rare Earth Manganites

Sharma, Ajay 03 1900 (has links)
Study of rare-earth manganites has been a very active research area in the last few years in condensed matter physics. This is due to the interesting phenomena such as (1) colossal magneto resistance (2) charge, orbital and spin ordering and (3) phase separation exhibited by these materials as a function of doping, pressure and temperature [1-3]. There is a lot of experimental data available in literature on different doped manganites, but no satisfactory and complete theoretical understanding is available yet. Though different theoretical models proposed are able to explain certain individual physical properties, a unified theory is missing which can comprehensively explain the full phase diagram. The study of such complex systems requires a probe that is sensitive to various interactions observed in manganites such as spin-spin interactions, spin-lattice interactions, spin-orbit interactions, crystal field interactions and the magnetic environment of the spins. Electron paramagnetic resonance (EPR) being sensitive to these interactions is an ideal probe for investigating these strongly correlated systems. A number of EPR studies have been reported in the paramagnetic phase of manganites, throwing light on the complex spin dynamics present in the manganites [4-10]. There are a few reports in the ferromagnetic state of manganites [11-12]. In recent years, a few studies reporting the observation of phase separation using EPR have also been published [13-15]. Charge ordering phase is the other interesting phase, which is not understood from EPR point of view [16-19]. Recently there are a few reports on suppression of CO phase by reducing the particle size from micro to nano range [20-22]. In this thesis we present the results of Electron Magnetic Resonance (EMR) (EPR in the paramagnetic phase and FMR: ferromagnetic resonance in the ferromagnetic phase) studies supported by magnetization and magneto-transport studies of the following : (1) various magnetic phases in the two electron doped manganite Ca1-xCexMnO3 (CCMO) (2) Charge ordered phase vs. ferromagnetic metallic phase as a function of Cr and Ni doping at the Mn site of Nd0.5Ca0.5MnO3 (NCMO) and comparison between the effect of the two dopants, and (3) a study of nano-sized particles (with different particle size) of Cr doped NCMO. Chapter 1 of the thesis consists of a brief introduction to the general features of manganites describing various phenomena and the interactions underlying them. Further we have written a detailed overview of EPR studies in manganites describing the current level of understanding in the area. In this chapter we have also described the experimental methodology and the analysis procedure adopted in this work. Chapter 2 reports the magnetization, transport and electron paramagnetic resonance studies (EPR) on two electron-doped manganites Ca1-xCexMnO3 (0.075 ≤ x ≤ 0.20). The various compositions of CCMO were prepared by solid-state synthesis and characterized by different techniques like XRD, SEM, EDX, and ICPAES. Our magnetization and transport results are consitent with the earlier reports [23-25]. For compositions x ≥ 0.13, all the EPR parameters viz. intensity, linewidth and the resonance field show signatures of a CO phase and at low temperature coexistence of two magnetic phases. x = 0.1 composition shows the most interesting results. Though the EPR intensity and resonance field indicate the presence of a CO phase, the EPR linewidth shows behaviour of a spin-disordered phase which we attribute to a possible spin-liquid phase [26]. The linewidth for x = 0.11 composition shows a combination of a CO and a spin-disorderd phase. For low composition x = 0.075, we observe a weak ferromagnetic phase and later on at low temperatures an antiferromagnetic phase. We do not observe the CO phase for this composition. In chapter 3, we present the magnetization, magnetotransport and EMR studies on Cr doped NCMO (0.0 ≤ x ≤ 0.10) [27]. The samples were prepared by solid-state synthesis and characterized by various techniques like XRD, SEM, EDX, and ICPAES. The magnetization studies show that the Cr doping induces ferromagnetic phase at low temperatures. With the increase of Cr doping the magnetization increases at the expense of the CO phase and for higher doping CO phase disappears completely. The Cr doping induces insulator-metal transition and with increase of Cr doping the metallic phase increases. The doped samples show high CMR, almost 100%, near the TC. The EMR studies in the paramagnetic phase indicate a CO phase for low Cr doping and the presence of short-range dynamical CO-OO correlations for higher Cr doping, which were not observed in magnetization studies. We observe two EPR signals at low temperatures for the Cr doped samples. For 3% doping, the two signals appear well above TC whereas for higher doping (5%, 10%) the two signals were observed in the FM phase. We rule out the possibility of the two-signal behaviour arising from the coexistence of two magnetic phases. For higher doping, the presence of two signals in FM phase can be attributed to magnetic anisotropy. With increase of Cr doping, magnetic anisotropy decreases which is also supported by reduction of magnetic anisotropy in magnetization measurements. But it cannot explain the observation of two signals above TC in the 3% doped sample. In chapter 4, we present the magnetization, magnetotransport and EMR studies on Ni doped NCMO (0.0 ≤ x ≤ 0.10). The samples were prepared by solid-state synthesis and characterized by various techniques like XRD, SEM, EDX, and ICPAES. The magnetization studies show that the Ni doping induces ferromagnetic phase at low temperatures. With the increase of Ni doping, though the CO phase is suppressed, the FMM phase also weakens which is different from the behaviour observed in Cr doped NCMO. The Ni doping induces insulator-metal transition and with increase of Ni doping, the metallic phase weakens. The magnetic anisotropy increases with increase of Ni doping as obtained from magnetization measurements and the EMR data also corroborates the same fact. The EMR studies in the paramagnetic phase indicate a CO phase for low Ni doping and the presence of short-range dynamical CO-OO correlations for higher Ni doping, which were not observed in magnetization studies. We observe two signals in the FM phase, which again can be attributed to the magnetic anisotropy. In chapter 5, we present EMR studies on nano-particles of Cr doped NCMO for x = 0.03. We have prepared nano-particles of three different sizes by the sol-get route. The samples were characterized by various techniques like XRD, SEM, EDX, and ICPAES. The particle sizes are 50, 100, 200 nm. We also compare the results of nano samples with the bulk samples. The ac susceptibility measurements show that the FM phase increases with the reduction of particle size. The EMR measurements show that the magnetic anisotropy decreases with decrease of particle size. The EMR linewidth in the paramagnetic phase increases with the decrease of particle size. The EMR intensity also increases with the reduction of particle size consitent with the magnetization results. The EMR results show that the reduction of particle size is one more way of inducing FM phase more effectively. Also the CO phase gets suppressed with the reduction of particle size. The two-signal feature is observed for all the particles. For nano-sized particles, the two signals appear in FM phase whereas in bulk sample they appeared well above TC. For 50 nm sized particles, the two signals appear well below 40 K. Thus we conclude that with decrease of particle size, the magnetic anisotropy decreases. The thesis concludes with a brief writeup summarizing the results and indicating possible future directions of research in the area.
8

Spatially Resolved Studies Of Electronic Phase Separation And Microstructure Effects In Hole Dopped Manganites

Kar, Sohini 03 1900 (has links)
The main focus of this thesis is in understanding the role of phase separation and microstructure in determining the physical properties of manganites. We also aim to be able to tune certain material properties using appropriate control mechanisms. For this, an understanding of the local electronic properties of manganites is essential. We thus set out to study the local electronic states in manganites using a highly sensitive probe: the scanning tunneling microscope (STM). The chapter 1 of the thesis gives an introduction to manganites, and of how manganites are susceptible to various perturbations due to closely lying ground states and an intricate interplay of their charge, spin and lattice degrees of freedom. Chapter 2 of this thesis gives a detailed account of various experimental methods used in the current investigation. In particular, we describe the design and fabrication of a variable temperature ultra-high vacuum scanning tunneling microscope (UHV-STM) which was used to carry out spatially resolved measurements on various manganite systems. This chapter also describes sample fabrication techniques by which strain and microstructure of thin films can be controlled. Other characterization techiniques, such as tranport and magnetotransport measurements, are also described in detail. Chapter 3 presents our investigation of the role of microstructure and phase separation on the DOS and local electronic properties of manganite thin films. We describe various spatially resolved STM/STS measurements carried out on La0.67Sr0.33MnO3 and La0.67Ca0.33MnO3 films having different micrsotructure and varying degrees of phase separation. We also present a theoretical model used in interpreting STS data to account for finite temperature effects and explain the existing data in this context. We use this model to gain insight into the behaviour of the DOS at EF near the MIT where thermal smearing can often give rise to misleading inferences. Chapter 4 presents our investigation on the density of states in a typical charge ordered manganite system, Pr1-xCaxMnO3. We describe STS measurements carried out on this system to study the occurrence and evolution of the charge ordering (CO) gap as a function if temperature as well as tunneling current. We report the observation of destabilization of the CO gap using tunnel current injection by an STM tip. Chapter 5 presents our investigation into the controlled and localized “nanoscale” phase separation in Pr1-xCaxMnO3 (PCMO) using an STM tip. The investigations were carried out on PCMO single crystal and PCMO epitaxial films. Our results raise the possibility of nano-fabrication of metallic nanoislands in a CO matrix using an STM tip. We demonstrate some examples of this and also raise the relevance of intrinsic phase separation in this context. We show that the “melting” of CO using tunnel current injection by an STM tip is analogous to the magnetic field-induced melting of CO on a microscopic scale. Chapter 6 summarizes the important results of this thesis work and suggests the scope for future experiments.
9

Phase Transitions And Magnetic Order In Multiferroic And Ferromagnetic Rare Earth Manganites

Harikrishnan, S 04 1900 (has links)
Recent findings of multiferroicity and magnetoelectric effects in rare earth manganites have fuelled research in this class of materials. These multiferroics can be structurally divided into two classes – orthorhombic and hexagonal. Especially attractive are TbMnO3, HoMnO3 and DyMnO3. Since the ionic radius of Dy is at the boundary that separates the orthorhombic and hexagonal RMnO3, DyMnO3 can be synthesized in both the structures using different synthesis conditions. In this thesis, DyMnO3 single crystals (both hexagonal and orthorhombic) prepared using optical floating zone furnace are studied through structural, magnetic and thermal properties. The influence of rare earth ion on the magnetic phase transitions is revealed in magnetisation, ac susceptibility and specific heat studies. Moreover, doping RMnO3 (small R) with alkaline earth ions creates an arena to test the interesting physics of spin-glass-like phenomena in manganites that arises due to quenched disorder. In this regard, 50% strontium diluted DyMnO3 could be an ideal system to study the effects of quenched disorder and structural/magnetic inhomogeneities that govern the magnetic phases in manganites. Structural phase-coexistence and ensuing anomalous magnetism in Pr–based manganite Pr0.6Sr0.4MnO3 are also presented in this thesis. Details of how the thesis is organized into eight chapters and a brief summary of each chapter follows: Chapter 1 is an introduction to the physics of manganites which progresses into multiferroics and eventually discusses the spin-glass-like effects arising due to size mismatch. A discussion on the phase-coexistence and its effect on physical properties are also presented. Eventually, the scope of the thesis is outlined in the last section. Chapter 2 outlines the basic experimental methods employed in this thesis work. Chapter 3 describes the details of crystal growth by optical floating zone method. DyMnO3 crystals in both hexagonal and orthorhombic structures are grown by employing the ambience of argon and air respectively. The crystals in the two crystallographic variants are characterized by X ray diffraction, Energy dispersive X ray analysis and Inductively coupled plasma atomic emission spectroscopy. The crystal structures are refined using Rietveld method with FULLPROF code and found to be P63cm for hexagonal and Pnma for orthorhombic DyMnO3. Details of crystal growth of Dy1−xSrxMnO3 are also presented. The change in ambience has no effect in the crystal structure of this doped manganite. A comparison of the growth of undoped and doped systems is given. In a later section, the crystal growth and structure refinement of Pr0.6Sr0.4MnO3 are discussed and the optimized growth parameters are tabulated for various manganite systems grown in the present work. Chapter 4 deals with the magnetic and thermal characterization of hexagonal and orthorhombic DyMnO3 single crystals. Magnetic measurements reveal the importance of rare earth magnetism in these compounds. The antiferromagnetic transition to a stacked triangular antiferromagnet is discernible from the specific heat studies of hexagonal DyMnO3, which is masked in the bulk magnetisation measurements. Various magnetic transitions pertaining to the antiferromagnetic sinusoidal – spiral – incommensurate magnet, are evident in the magnetisation and specific heat of orthorhombic DyMnO3 which belongs to the class of non-collinear magnets. Chapter 5 deals with basic investigations on the spin-glass-like state in Dy0.5Sr0.5MnO3. Preliminary dc magnetisation shows indication of spin-glass state as a split in field-cooled and zero-field-cooled magnetisation cycles. Further, the failure of scaling of M(T) with H/T indicates the absence of superparamagnetism in Dy0.5Sr0.5MnO3. The dynamic susceptibility and its analysis using the theory of critical slowing down yield exponents pertaining to the spin-glasses. However, a four-order magnitude change is observed in the characteristic spin-flip time. This leads to the assumption that in Dy0.5Sr0.5MnO3 the spin entities are not atomic spins as in canonical spin-glasses but clusters of spins. The specific heat is analysed for signatures of spin-glass state and is found that a linear term in temperature is essential in fitting the observed data. The crystalline electric fields of Dy ion is also analysed attempting multiple Schottky-levels instead of two. Chapter 6 concerns with the aging experiments performed in the spin-glass-like state in Dy0.5Sr0.5MnO3. Striking aging and chaos effects are observed through these measurements. However, owing to the clusters of spins present, deviations from the typical time-dependent behavior seen in canonical spin-glass materials are anticipated in Dy0.5Sr0.5MnO3. In fact, the relaxation measurements indicate that the glassy magnetic properties are due to a cooperative and frustrated dynamics in a heterogeneous or clustered magnetic state. In particular, the microscopic spin flip time obtained from dynamical scaling near the spin-glass transition temperature is four orders of magnitude larger than microscopic times found in atomic spin-glasses. Magnetic viscosity, deduced from the waiting time dependence of the zero field cooled magnetisation, exhibits a peak at a temperature T<Tsg. Waiting time experiments prove that the dynamics is collective and that the observed memory effects are not due to superparamagnetism of separate magnetic entities. Chapter 7 discusses the Electron paramagnetic resonance (EPR) studies on single crystals of DyMnO3 in hexagonal as well as orthorhombic structures. The interesting effect of strontium dilution on the frustrated antiferromagnetism of DyMnO3 is also probed using EPR. The lineshapes are fitted to broad Lorentzian in the case of pure DyMnO3 and to modified Dysonian in the case of Dy0.5Sr0.5MnO3. The linewidth, integrated intensity and geff derived from the signals are analysed as a function of temperature. The EPR results corroborate well with the magnetisation measurements. The study clearly reveals the signature of frustrated magnetism in pure DyMnO3 systems. It is found that antiferromagnetic correlations in these systems persist even above the transition. Moreover, a spinglass-like behaviour in Dy0.5Sr0.5MnO3 is indicated by a step-like feature in the EPR signals at low fields. Chapter 8 deals with the magnetic and electrical properties of Pr0.6Sr0.4MnO3 single crystals. This crystal undergoes two prominent phase transitions – a paramagnetic to ferromagnetic at Tc~300 K and a structural transition at Tstr ~ 64 K. These phase transitions are evident in the static magnetisation as well as in frequency-dependent susceptibility. In these measurements, the structural transition is associated with a sizeable hysteresis typical of a first-order transition. The M–H curves below Tc show clear indication of anomalous magnetism at low temperatures: the virgin curve lies outside the subsequent magnetisation loops. These observations are explained by assuming structural coexistence of a high–temperature orthorhombic and a low–temperature monoclinic ferromagnetic phases. The nature of static magnetisation data is analysed in the critical region. Modified Arrott’s plots yielded perfect straight lines with the isotherm at ~ 300 K passing through the origin. The exponent values thus should be very close to those expected for the universality class of Heisenberg ferromagnets. The temperature dependence of resistivity also shows critical nature with an exponent belonging to the Heisenberg class. The thesis concludes with a chapter on General conclusions and future scope on these systems.
10

Spectral And Transport Properties Of Falicov-Kimball Related Models And Their Application To Manganites

Pakhira, Nandan 04 1900 (has links)
From the time of the unexpected discovery of the insulating nature of NiO by Verwey half a century ago, Oxide materials have continued to occupy the centre stage of condensed matter physics. The recent discovery of high temperature superconductivity in doped cuprates has given a new impetus to the study of the strongly correlated electron systems. Besides, the occurrence of Colossal Magneto-Resistance (CMR) in doped rare earth manganite has also created renewed interest in these rather old systems. Understanding of the rich and complex phase diagram of these materials and their sensitivity to small perturbations e.g. external magnetic field of a few Tesla, temperature, change in isotope etc. are of great theoretical interest and also these materials have many potential technological applications. A common feature of all these oxide materials is that the transition metal ions have partially filled d-shells. Unlike s and p-electrons which gives rise to hybridized Bloch states, the d-electrons retain their atomic nature in a solid. This gives rise to strong Coulomb interaction among d-electrons which may be comparable or more than its kinetic energy. The strong correlation effects are evident from the experimental fact that the undoped parent compounds are insulators rather than metals as suggested by band theory, which favours a metallic state for systems with one electron per unit cell since this gives rise to partially filled bands (and hence a metallic state). These insulators termed Mott insulators, arise solely due to strong electron-electron correlations as compared to the band insulators which arise due to complete filling of one electron bands thereby giving rise to a gap (band gap)in the excitation spectra. The delicate competition between the kinetic energy and the Coulomb energy for d-electrons is broadly responsible for the wide variety of phenomena like Mott metal-insulator transition (MIT), magnetic transitions, charge ordering, orbital ordering, ferro/antiferroelectricity, and most interestingly the observation of high Tc superconductivity in doped cuprates. In this thesis we will restrict our interest to one such class of oxide materials, namely the doped rare earth manganites. In Chapter 1 we give a brief overview of the structure and basic interactions present in the doped manganites. Also, in the same Chapter we give a brief introduction to the phenomenology of manganites, particularly its phase diagram in the doping and temperature plane and various experimental features, e.g. the wide variety of phase transitions and phenomena particularly the observation of CMR, charge ordering and incipient meso-scale phase separations etc.. Then we briefly introduce a recently proposed microscopic model which is believed to be a minimal model which, for the first time, includes the three most important interactions present in the manganites namely the following -1)coupling of the orbitally degenerate eg electrons to local lattice distortions of Jahn-Teller type which gives rise to two species of electrons. The one denoted by by ℓ is associated with Jahn-Teller effects and hence is localized whereas the other denoted by b is an extended state and propagates through the lattice. 2) The strong Hund’s couplingof ℓ and b electrons to the t2g core spin and 3) the strong Coulomb correlation between the two species of electrons. Additionally, the model includes a new doping dependent ferromagnetic exchange between the t2g core spins which can arise from “virtual double exchange” mechanism which will be discussed in great detail in Chapter 1 . Finally, we give a brief account on Dynamical Mean Field Theory (DMFT) and Numerical Renormalization Group (NRG) as an impurity solver for the single impurity problem arising under single site DMFT approximation. In Chapter 2 we study the effect of inter-site ℓ - b hybridization on the ‘ℓ - b’ model. The single impurity problem arising under DMFT approximation has close connection with the Vigman-Finkelshtein (VF)model. Then we briefly introduce the VF model and bring out its close connection with the impurity problem. We consider both the particle-hole symmetric as well as the U → ∞ particle-hole asymmetric cases. We derive various spectral functions at T = 0K and discuss the nature of fixed points under various circumstances. We explicitly show that for the particle-hole symmetric case the Hamiltonian flows from X-ray edge singularity fixed point to Free Electron fixed point under Renormalization Group transformation. This is evident from the spectral properties of the model. We write down the effective Hamiltonian at the free electron fixed point. For the particle-hole asymmetric case the model flows from X-ray edge singularity fixed point to Free Electron/Strong Coupling fixed point with additional potential scattering terms. We write down the effective Hamiltonian at this fixed point and derive various leading order deviations. We found all of them to be irrelevant in nature also most interestingly the quasi-particles describing the under lying Fermi liquid state are found to be asymptotically non-interacting. We also calculate the Fermi liquid parameter, z, by analyzing the energy level structure of a non-interacting Hamiltonian with effective renormalized parameter. Also, we consider the case of ‘self consistent bath hybridization’ without ℓ - b hybridization for Bethe lattice with infinite coordination. Low energy qualitative features are found to be same but some of the high energy features get qualitatively modified. In Chapter 3 we discuss the transport properties of doped manganites in the insulating phases and also the Hall effect in the metallic phase. In the first part of this chapter we calculate the resistivity based on the ‘ℓ - b’model and try to fit it to the semiconducting form: ρ(T )= ρ0(T /T0)−nexp[Δ(T )/kBT ] and extract the “transport gap”, Δ(T ). This gap can be characterized in terms of the “spectral gap” which can be defined for the ℓ - b model. It is found that the transport gap in the paramagnetic phase can be characterized in terms of the near constant “spectral gap” in this phase whereas the same in the ferromagnetic phase can be characterized in terms of the zero temperature spectral gap. In the last part of this chapter we calculate the Hall resistivity (ρxy) of these materials in the metallic phase. Ρxy is found to be negative and linear in applied field -quite consistent with the experimental findings but this fails to explain the positive linear Hall resistivity at low temperatures and its crossover as a function of field and temperature. We then present a reasonable explanation for this discrepancy and support it by calculating the Hall density of states for a two band “toy model” involving inter species hybridization. In Chapter 4 we calculate the optical conductivity, σ(ω), in ℓ - b model. σ(ω) arises from two independent processes. One of the processes involves ‘b’ electrons only and termed as ‘b - b channel’ and this gives rise to a Drude peak in the low frequency region. another process termed as the ‘ℓ - b channel’ involves hopping of an ℓ-electron to a neighbouring empty site and transforms into a ‘b’like state. This process gives rise to a broad mid-infrared peak. The total conductivity is the sum of contributions from these two incoherent channels. Calculated σ(ω) for metallic systems shows lot of similarities with experimental observations particularly the temperature evolution of the mid-infrared peak and the spectral weight transfer between the two peaks. But for the insulating systems the calculated optical conductivity showed trends similar to more recent experimental observations on some insulating systems (x =0.125) but contradicts with earlier experimental observations on some other insulating system (x =0.1). Finally, in the concluding chapter, we summarize results from all the chapters and also sketch some possible future directions of investigations.

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