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The study of novel properties of materials in nanometer length scales has been an extensive area of research in the recent past. The field of nanosciece and nanotechnology deals with such studies and has gained tremendous importance because of the potential applications of these nanosystems in devices. Many of the bulk properties tend to change as a function of size, be it particle size in case of nanoparticles, or thickness in case of very thin films. Not only is it important to study these changes from the point of view of applications, but also the interesting physics behind such changes prompts further research and exploration in this area. In this thesis we try to see how changes in the length scales affect the properties of nanoparticles and how change in thickness affects the properties of thin films, along with making an effort towards measurements of conductivity in the nanoscale using the technique of electron magnetic resonance (EMR) signal shape analysis. Electron magnetic resonance is a general term used to combine both electron paramagnetic resonance (EPR) and ferromagnetic resonance (FMR). This thesis deals with mainly two kinds of systems viz., nanoparticles of doped rare earth manganites and thin films of the conducting polymer, vapor phase polymerized polyethylendioxythiophene (VPP PEDOT). The general formula for doped manganites is A1-xBxMnO3 where A is a rare earth trivalent cation like La3+, Pr3+, Nd3+..., and B is an alkaline earth divalent cation like Sr2+, Ca2+, Ba2+... These together with Mn and O form the distorted perovskite structure to which manganites belong. The phase diagram of doped manganites involves many interesting phases like ferromagnetic metallic, antiferromagnetic insulating and charge ordered insulating phases. The magnetic properties of the manganites are governed by exchange interactions between the Mn ion spins. These interactions are relatively large between two Mn spins separated by an oxygen atom and are controlled by the overlap between the Mn d-orbitals and the O p-orbitals. The changing Mn-O-Mn bond lengths and bond angles as a function of the radius of the A and B cations [1, 2], and the different magnetic interactions among the Mn3+ and Mn4+ ions together are responsible for the different phases that we see in manganites as a function of temperature and magnetic field. Manganites have potential applications in the field of spintronics because of their colossal magnetoresistance (CMR) [3] and half-metallic [4] properties. Studies on nanoscale manganites have shown that as size reduces, their electrical and magnetic properties change significantly[5]. By changing the morphology and grain size, the properties of CMR manganites can be tuned [6-9]. Phase separation seems to disappear in nanoparticles compared to bulk [10]. In the charge ordered manganites, size reduction is known to bring about suppression of charge order [11], emergence of ferromagnetism [12, 13] and even metallicity in some nanostructures [12]. The conducting polymer under study viz., VPP PEDOT is in a semiconducting phase at room temperature and becomes more insulating as temperature reduces. It is a technologically important polymer which has cathodically coloring property, can be used as a highly conducting electrode in organic solar cells and organic LEDs [14-16]. In the following we give a summary of the results reported in the thesis chapter by chapter. Chapter 1: This chapter of the thesis consists of an introduction to the physics of manganites and the technique of EMR. This includes a detailed account of previous EMR studies done on manganites, in particular nano manganites. There is a section about different line shapes observed in EMR of manganites, their origin and how to fit them to an appropriate lineshape function [17]. There is an introduction to the transport properties of conducting polymers, including how magnetic fields can affect the transport and the mechanism behind variable range hopping transport which is the dominant kind of transport in such polymeric systems. There is also a description of the different experimental methods and instruments used to study the systems in the thesis and their working principles. They are: EPR spectrometer, SQUID magnetometer, Janis cryostat with superconducting magnet, atomic force microscope (AFM) and transmission electron microscope (TEM). Chapter 2: This chapter deals with the method of contactless conductivity of nanoparticles using EMR lineshape analysis. It is difficult to measure the conductivity of individual nanoparticles by putting contacts. Other methods tend to include the contribution of grain boundaries which mask the grain characteristics [5]. We have introduced a new contactless method to measure the conductivity of nanoparticles in a contactless manner [18]. Metallic nanoparticles in which the skin depth is less than the size of the particles, exhibit an asymmetric EMR signal called the Dysonian [19]. Dysonian lineshape is an asymmetric lineshape with the so-called A/B ratio >1, where, A is the amplitude of the low field half of the derivative and B is the amplitude of the high field half. In a ferromagnetic conducting sample, the lineshape has contributions from the Dysonian part and also a part which arises due to magnetocrystalline anisotropy [20]. We have developed a method of deconvoluting the signals from conducting nanoparticles to take out the Dysonian part from them and measure the A/B ratio as a function of temperature. The A/B ratio thus determined can then be used to find out the ratio of the sample size to the skin depth using the work by Kodera [21]. The skin depth can be used to determine the conductivity by using the relationship  = (1/)1/2, where,  is the measuring frequency,  is the conductivity and  is the permeability. This technique has been used to determine the conductivity as a function of temperature (from 60 K to 300 K) of La0.67Sr0.33MnO3 (LSMO) nanoparticles of average size 17 nm. The method has been cross-checked by measuring the conductivity of bulk LSMO particles at 300 K by EMR lineshape analysis method and by standard four-probe method, which give conductivity values close to each other within experimental error. Chapter 3: In this chapter, we report a novel phenomenon of disappearance of electron-hole asymmetry in nanoparticles of charge ordered Pr1-xCaxMnO3 (PCMO). In bulk PCMO there is asymmetry in electric and magnetic properties seen on either side of x = 0.5. In the samples with x = 0.36 (hole doped: called PCMH) and x = 0.64 (electron doped: called PCME), the bulk sample has opposite g-shifts as observed in EPR signals [22]. PCME sample shows g-value less than and PCMH sample shows g-value greater than the free electron g-value at room temperature. This is explained using the opposite sign of the spin-orbit coupling constant for the two different kinds of charge carriers. But when the size of PCMH and PCME is reduced to nanoscale (average size ~ 20 nm), the g-shift was seen on the same side i.e., positive and almost equal g-shift in both cases. This points towards a disappearance of electron-hole asymmetry at nanoscale. This positive g-shift is analyzed in the two cases in the light of disappearance of charge ordering and emergence of ferromagnetism in these systems, since emergence of ferromagnetic hysteresis is noticed at low temperatures in both nano PCMH and nano PCME. In nano PCMH, charge ordering completely disappears and in nano PCME it weakens. Exchange bias is seen in both the systems, suggestive of core-shell structure [23] in the nanoparticles. Other competing factors include spin-other orbit interactions and size reduction induced metallicity [12] which can average out the anisotropies in the system, causing the asymmetry to disappear. Chapter 4: This chapter deals with thickness induced change in transport mechanism in VPP PEDOT thin films. Two samples were studied with average thickness of 120 nm (VP-1) and 150 nm (VP-2). The average room temperature conductivity of VP-1 was found to be 126 Scm-1 and VP-2 was 424 Scm-1. The transport mechanism in VP-1 is seen to be 2-dimensional variable range hopping (VRH) [24]. However, as the thickness increases by 30 nm, the transport mechanism in VP-2 is found to be 3-dimensional VRH. The low temperature magnetotransport is analyzed in the two systems and it shows that there is wavefunction shrinkage in both the systems at 1.3 K [24]. The DC transport results are cross checked with AC transport data at 5 different temperatures in the frequency range of 40 Hz to 110 MHz. The data can be analyzed by using the extended pair approximation model [25]. The AC transport shows the presence of a critical frequency 0 which marks the transition from the frequency independent to a frequency dependent region. The value of 0 decreasing with decreasing temperature suggests that the system is becoming more insulating and it supports the DC transport model of VRH. The morphological studies were done using AFM which revealed higher grain size for VP-2, confirming the direct correlation of the average grain size with the conductivity of the sample. Chapter 5: summarizes the main conclusions of the thesis, also pointing out some future directions for research in the field.

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Les travaux présentés dans ce manuscrit consistent à élaborer par la méthode céramique des oxydes ferromagnétiques de type pérovskite et à étudier leurs propriétés physiques (structurales, magnétiques, magnétocaloriques..). Nous avons commencé ce travail par la synthèse de séries de manganites à base de praséodyme ( Pr0.6-xEuxSr0.4MnO3 et Pr0.6-xErxSr0.4MnO3 ) et de lanthane ( La0.6Sr0.2Ba0.2-x□xMnO3 et (La0.6Sr0.2Ba0.2MnO3)1-x /(Co2O3)x ) en utilisant la méthode solide-solide à haute température. Les échantillons élaborés ont été caractérisés par diffractométrie de poudre RX. Les diffractogrammes obtenues ont été affinés par la méthode Rietveld en utilisant le logiciel Fullprof. L'affinement structural a montré que les manganites synthétisés se présentent sous forme de phases pures avec des raies fines et intenses sans phases parasites et cristallisent dans des structures déformées.Des mesures magnétiques (M(T) et M(H)) ont permis d'obtenir des informations sur le comportement magnétique à basse température, les transitions magnétiques et l'évaluation de l'aimantation à saturation. L'aimantation en fonction de la température montre que ces manganites présentent des transitions ferromagnétiques - paramagnétiques et que leurs températures de Curie diminuent sous l'effet de la substitution dans le cas des composés à base du praséodyme et sous l'effet de l'introduction des lacunes dans le système basé sur le lanthane. Les isothermes M (H) confirment le comportement ferromagnétique à basses températures des échantillons étudiés. A partir de ces mesures et en utilisant les relations de Maxwell, on a déterminé les variations d'entropie magnétique ∆Sm et on a évalué l'effet magnétocalorique présent dans ces matériaux. Via la connexion entre la chaleur spécifique et l'aimantation, on a déterminé la variation de la chaleur spécifique ∆Cp dans tous ces échantillons en exploitant les résultats ∆Sm. Nos résultats confirment que les grandeurs caractéristiques de l'effet magnétocalorique sont très sensibles au champ magnétique appliqué, d'où l'étude de leur dépendance en champ magnétique présente un très grand intérêt. Cette dépendance en champ magnétique de la variation d'entropie magnétique peut être exprimée selon une loi de puissance de type ∆Sm ~ a (µ0H)n où n est appelé exposant local. Cette étude permet donc d'une part, d'identifier les matériaux qui se comportent de façon similaire et les voies d'amélioration de ces propriétés et elle constitue, d'autre part, un outil intéressant permettant d'extrapoler ces propriétés dans des conditions non accessibles au laboratoire. / The studies presented in this manuscript deal with the synthesis and characterization of ferromagnetic perovskite oxides. Four material systems have been described in this work ( Pr0.6-xEuxSr0.4MnO3 et Pr0.6-xErxSr0.4MnO3 ) et lanthanum ( La0.6Sr0.2Ba0.2-x□xMnO3 et (La0.6Sr0.2Ba0.2MnO3)1-x /(Co2O3)x ) . Our samples have been synthesized using the solid-state reaction method at high temperatures. Rietveld refinement of the X-ray diffraction patterns using Fullprof program shows that all our samples are single phase and crystallize in the distorted structures. Magnetic measurements show that all our samples exhibit a paramagnetic–ferromagnetic transition with decreasing temperature. The Curie Temperature TC shifts to lower values with increasing substitution in the Pr0.6-x(Eu or Er)xMnO3 system and under the effect of barium deficiency in the La0.6Sr0.2Ba0.2-x□xMnO3 system. From the magnetization isotherms at different temperatures, magnetic entropy change ∆Sm and relative cooling power RCP have been evaluated. By means of the connection between the specific heat and the magnetization was determined the variation of the specific heat ΔCp in these samples using the results ΔSm. Our results confirm that the characteristic values of the magnetocaloric effect are very sensitive to the applied magnetic field, where the study of their dependence on magnetic field has a very great interest. For fixed temperatures, the magnetic field dependence of magnetic entropy change ∆Sm is accounted for by the n exponent, which may be derived by a numerical fitting to the formula ∆Sm ~ a (µ0H)n where a is a constant. This study allows one hand, identify materials that behave similarly and ways to improve these properties and it is, on the other hand, a useful tool to extrapolate these properties under conditions not accessible in the laboratory.

Studies on perovskite rare earth manganites of general formula R1-xAxMnO3 (where R is a trivalent rare earth ion such as La3+, Pr3+ etc. and A is a divalent alkaline earth ion such as Ca2+, Sr2+, Ba2+, have been a very active research area in the last few years in condensed matter physics. Manganites have a distorted perovskite crystal structure with R and A ions situated at the cube corners, oxygen ions at the edge centers of the cube and Mn ions at the centres of the oxygen octahedra. In these manganites the Mn ions are found to be in mixed valence state i.e., in Mn3+ and Mn4+ states. In the octahedral crystal field of oxygen ions the single ion energy levels are split into t2g and eg levels. Mn3+ being a Jahn-Teller ion, the eg level is further split due to the Jahn-Teller effect. A strong Hund’s coupling between the spins in the t2g and eg levels renders the Mn3+ ions to be in the high spin state. The interplay of competing super exchange between Mn ions which determines the antiferromagnetism, orbital ordering and insulating behavior and double exchange between Mn ions which leads to ferromagnetism and metallicity gives rise to very complex phase diagrams of manganites as a function of composition, temperature and magnetic field. The strength of these interactions is determined by various factors such as the A-site cation radius and the Jahn-Teller distortion due to the presence of Mn3+ ions. The strongly coupled charge, spin, lattice and orbital degrees of freedom in manganites gives rise to complex phenomena such as colossal magnetoresistance (CMR), charge order (CO) and orbital order (OO) and phase separation (PS) etc. The properties of these materials are sensitive functions of external stimuli such as the doping, temperature and pressure [1-5] and have been extensively studied both experimentally and theoretically in single crystal, bulk polycrystalline and thin film forms of the samples [6-9]. Charge ordering is one of the fascinating properties exhibited by manganites. Charge ordering has historically been viewed as a precursor to the complex ordering of the Mn 3d orbitals, which in turn determine the magnetic interactions and these magnetic interactions are the driving force for charge localization and orbital order. This ordering of Mn3+ / Mn4+ charges can be destabilized by many methods. An external magnetic field can destabilize the charge ordered phase and drive the phase transition to the ferromagnetic metallic state [10-11]. Other than magnetic field, charge ordering can also be ‘melted’ by a variety of perturbations like electric field [12, 13], hydrostatic and chemical pressure [14-16], irradiation by X-rays [17], substitution at the Mn -site [18 -21] and A-site [22]. Of these, A-site substitution with bigger cations like barium is particularly of great interest since it does not interrupt the conduction path in the “MnO3” frame work Recently attention has been drawn towards the properties of nanoscale manganites. The nanoscale materials are expected to behave quite differently from extended solids due to quantum confinement effects and high surface/volume ratio. Nanoscale CMR manganites have been fabricated using diverse methods in the form of particles, wires, tubes and various other forms by different groups. It has been shown that the properties of CMR manganites can be tuned by reducing the particle size down to nanometer range and by changing the morphology [23-27]. As mentioned above, charge order is an interesting phase of manganites and these CO mangnites in the form of nanowires and nanoparticles show drastic changes in their properties compared to bulk. In contrast to the studies on the CMR compounds, there are very few reports on charge ordering nano manganites except on nanowires of Pr0.5Ca0..5MnO3 [28] and nanoparticles of Nd0.5Ca0.5MnO3 [29] and Pr0.5Sr0..5MnO3 [30]. This thesis is an effort in understanding certain aspects of charge order destabilization by two different methods, namely, doping bigger size cation (barium) in A-site (external perturbation) and by reducing the particle size to nano scale ( intrinsic). For this purpose we have selected the charge ordering system Pr1-xCaxMnO3 (PCMO) with composition x = 0.43. The reason behind choosing this composition is the observation [31] that CO is particularly weak for this value of x. We have prepared bulk, nanoparticles and nanowires of Pr0.57Ca0.41Ba0.02MnO3 manganite and have carried out microstructure, magnetic, magneto transport and EMR measurements to understand the nature of CO destabilization and also to understand other aspects such as magneto transport and magnetic anisotropy . Apart from destabilization of the charge order in PCMO we have also studied the bismuth based manganite Bi0.5Ca0.5MnO3. The reason behind choosing this system is the robust charge order of Bi0.5Ca0.5MnO3 compared to rare earth based manganites. So far no attempt has been made in comparing the electron paramagnetic resonance properties of bismuth based manganites with those of the rare earth based manganites. We have studied the magnetic, transport and electron paramagnetic resonance properties of Bi0.5Ca0.5MnO3 prepared by solid state reaction method and compared the results with those of Pr0.5Ca0.5MnO3 . In the following we present a chapter wise summary of the thesis. Chapter 1 of the thesis contains a brief introduction to the general features of manganites describing various interesting phenomena exhibited by them and the underlying interactions . Chapter 2 contains a detailed review of EPR studies on manganites describing the current level of understanding in the area. In this chapter we have also described the different experimental methodology adopted in this thesis. Chapter 3 reports the effect of a small amount (2%) of barium doped in the charge ordered antiferromagnetic insulating manganite Pr0.57Ca0.43MnO3. The samples were prepared by solid state synthesis and charecterized by various techniques like XRD, EDXA. The results of magnetization, magnetotransport and EPR/EMR experiments on both Pr0.57Ca0.43MnO3 and Pr0.57Ca0.41Ba0.02MnO3 are compared. The magnetization studies show that barium doping induces ferromagnetic phase in place of the CO-antiferromagnetic phase of the pristine sample at low temperatures as reported earlier by Zhu et al.,[31]. The transport studies show insulator to metal transition. The EPR parameters viz line width, intensity and ‘g’ value of Pr0.57Ca0.43MnO3 and Pr0.57Ca0.41Ba0.02MnO3 are compared. The magnetization and EPR studies reveal that the CO transition temperature TCO has shifted to a slightly lower value accompanied by a small decrease in the strength of the charge order. Thus a small amount of barium affects the CO phase of Pr0.57Ca0.43MnO3 and it also induces a ferromagnetic metallic phase at low temperature. Another most important and unexpected result of EMR experiment is the observation of high field signals, i.e. two EMR signals are observed at low temperatures in the ferromagnetic phase of Pr0.57Ca0.41Ba0.02MnO3. The appearance of the high field signals are understood in terms of the effects of magneto crystalline anisotropy. Chapter 4, reports the microstructure, magnetization and EMR studies of Pr0.57Ca0.41Ba0.02MnO3 nanoparticles prepared by sol-gel method. We have mainly focused on the effect of size on the charge ordered phase. The samples were characterized by different techniques like XRD, EDXA and TEM. The obtained particle size of the samples are 30, 60 and 100 nm respectively. We have compared the magnetic, magneto transport and EMR results of these nano samples with the bulk properties. The 30 nm particles do not show the CO phase whereas the 60 and 100 nm particles show CO signatures in DC- magnetization measurements. The EPR intensity also shows a similar trend. These results confirm that charge ordering can also be destabilized by reducing the particle size to nano scale. But the EPR linewidth which reflects the spin dynamics shows a change in the slope near the CO temperature and there by indicates the presence of premonitory charge ordering fluctuations in smaller particles. We also observed that the EMR linewidth increases with the decrease of particle size. Another striking result is the disappearance of high field signals in all the nanosamples. This is understood in terms of a decrease in the magnetic anisotropy in nanoparticles. Part of the result of this chapter is published [32]. Chapter 5, reports the morphological, magnetic and electron paramagnetic resonance studies of Pr0.57Ca0.41Ba0.02MnO3 nanowires. Recently our group has studied the nanowires of Pr0.5Ca0..5MnO3 [28]. In the nanowire sample of Pr0.5Ca0..5MnO3 only a partial suppression of CO is observed. This raises the question about the incomplete suppression of the CO in the nanowires: is this a consequence of the material being microscopic in one dimension and is it necessary to have a 3-dimensional nano material to have full suppression of the charge order ? In the present work we attempt to provide an answer to this question. PCBM nanowires of diameter 80-90 nm and length of ∼ 3.5 μm were synthesized by a low reaction temperature hydrothermal method. We have confirmed the single phase nature of the sample by XRD experiments. Scanning electron microscopy (SEM) and trasmission electron microscopy (TEM) were used to characterize the morphology and microstructures of the nanowires. The surface of nanowires was composed of particles of different grain size and interestingly some particles were hexagonal in shape. The bulk PCBM manganite exhibits charge order at 230 K along with a ferromagnetic transition at 110 K. However, SQUID measurements on PCBM nano-wires show a complete melting of the charge ordering and a ferromagnetic transition at 115 K. The magnetization observed in the nanowires was less compared to that in the bulk. EPR intensity measurements also support this result. Characteristic differences were observed in linewidth and ‘g’ factor behaviors of nanowires when compared with those of the bulk. EPR linewidth which reflects the spin dynamics shows a slope change near the CO temperature (like in nanoparticles) possibly due to charge order fluctuations in nanowires. The high field signals were absent in nanowires as well. Part of the result of this chapter is published [33]. Chapter 6 deals with the magnetic and electron paramagnetic resonance studies on Pr0.5Ca0.5MnO3 and Bi0.5Ca0.5MnO3. These manganites are prepared by solid state reaction method and characterized by different techniques like XRD and EDXA. Further, we have compared the results of magnetization and electron paramagnetic resonance properties of Pr0.5Ca0.5MnO3 with those of Bi0.5Ca0.5MnO3 manganite in the temperature range of 10- 300 K. The two charge ordered manganites show significant differences in their behavior. The temperature dependence of the EPR parameters i.e. line width, central field and intensity of Bi0.5Ca0.5MnO3 are quite different from the rare earth based manganite i.e. Pr0.5Ca0.5MnO3. Linewidth of BCMO is large compared to PCMO manganite and interestingly the temperature dependence of the central fields (CF) of PCMO and BCMO show opposite behavior. The CF of PCMO decreases with decrease in temperature as found in a large number of other CO systems, whereas CF of BCMO increases with decrease in temperature. This unusual behavior of resonance field is attributed to the different magnetic structure of BCMO system at low temperatures. Chapter 7 sums up the results reported in the thesis. The insight gained from the present work in understanding the destabilization of charge order by chemical doping and size reduction is discussed as well as the differences in the properties of bismuth and rare earth manganites. Further, we have indicated possible future directions of research in this area.

Electrical conductance in the ferromagnetic insulating (FMI) phase of manganites has been experimentally investigated. The investigations were performed on single crystals of compositions La0.82Ca0.18MnO3 and Nd0.7Pb0.3MnO3. The nature of electrical conductance is determined to be Shklovskii–Efros variable range hopping (SE–VRH). Further, at high bias levels, non–linear conductance (NLC) is observed. A “hot electron” model, that quantitatively explains the bias and temperature dependence of the NLC, consistent with the SE–VRH nature of electrical conductance, is presented. The limits of validity of the model are discussed.

This thesis deals with the investigations on the nonlinear electrical and manganetotransport properties of polycrystalline multi-phase ceramic composites of Zno/pervoskite manganite. Multifunctional properties are studied such as the enhanced low-field magnetoresistance(LFMR). magnetically tuneable low-voltage nonlinear current-voltage (I-V) characteristics with larger nonlinearity coefficients suitable for semiconducting and magnetoelectric devices. A brief introduction on the structure-property correlations, electronic and magnetic structures, nonlinear electrical conduction, phase separation, grain size and grain boundary effects on transport properties of manganites are presented. The nonlinear current-voltage characteristics of ZnO based varistors are also summarized. The thesis describes the synthesis of the ceramics and the methodology of different techniques utilized in characterizing the samples. The phase conversions in calcium manganite with changing Ca/Mn ratios as well as the oxygen non-stoichiometry and their influence on electrical transport properties were studied. The realization of low-voltage varistors prepared from ZnO+ CaMnO3 ceramic composites was described. An energy band model consisting of n-p-n heterojunctions of n-ZnO1-γ:Mn/p-CMZO/n-ZnO1 γ:Mn has been proposed in order to explain the large nonlinearity coefficients obtained at low field-strengths of 1.8 to 12 V/mm. The detailed investigationos on the structural identification and physico-chemical analyses of Ca4Mn7Zn3O21-δ(CMZO) phases having the beta-alumina or magnetoplumbite-type structures were carried out. The thesis also embodies the magnetically tuneable nonlinear I-V characteristics and the magnetotransport properties of ZnO/La(Sr)MnO3 and ZnO/La(Ca,Sr)MnO3 ceramic composites. The present investigations demonstrate that the ferromagnetic insulating (FMI) La06 Sr04Mn1-yZnyO3(y = 3 to 8 at.%) when present as minor phase in ZnO1- γ:Mn ceramics enables in attaining magnetically tunealbe nonlinear I-V characteristics. Wherein, the dominant ZnO1- γ:Mn phase remains paramagnetic. The results also indicate that the prevalence of ferromagnetism in ZnO1-γ:Mn is not significant for realizing magnetically tuneable I-V curves. The controversial results related to the existence of ferromagnetism in ZnO(doped)leading to diluted magnetic semiconductors(DMS) have been investigated. Another novel aspect of the present work is the low-field magnetoresistive(LFMR) property of ZnO/La(Sr)MnO3 and ZnO/La(Ca.Sr)MnO3 ceramic composites which been explained on the basis of spin-polarised tunneling across the intergrain regions. The influence of Zn2+ as a diamagnetic substitutent in modifying the crystallographic phase content, electrical transport and magnetic properties of Lao6Sro4MnO3 were studied in detail. The results point towards the fact the large decrease of Tc and Ms at lower Zn contents(≤ 8 at.%)is due to the dominant role played by the excess oxygen vacancy (Vo) as an electron donor in p-type Lao6Sro4Mn1-yZnyO3-δ rather than the charge compensatively predictable values. The modifications of electronic and magnetotransport properties were carried out on Lao6Sro4MnO3 substituted with diamagnetic ions such as Mg2+ - Al3+ - Ti4+ - Nb5+ - Mo6+ or W6+ at Mn-sublattice. The TEM studies including HREM results point to the fact the large ΔT(= Tc-TM-1)is accountable in terms of charge conduction within the electronically heterogeneous phase mixtures of charge ordered insulating (CO1) bi-stripes prevailing within the charge disordered FMI phases.

In this thesis I present studies of some transport and spectroscopic properties for two di erent materials exhibiting large magnetoresistance. Both of these materials are oxides of transition metals, showing exotic magnetic and transport properties. Despite these similarities, they are very different in many other aspects. One of them is an oxide of Manganese, along with a rare-earth metal, and exhibits large magnetoresistance under certain conditions, when doped by an alkaline earth metal. They are known as doped rare-earth manganites. The other material, Sr2FeMoO6, exhibits large magnetoresistance in the parent compound, without any doping, but only in the polycrystalline state. The manganites, on the other hand, show magnetoresistance under appropriate conditions in both single crystal and in polycrystalline state. Moreover, manganites exhibit several Metal-Insulator Transitions (MIT) as a function of doping, temperature and magnetic field. Sr2FeMoO6, on the other hand, is usually always metallic. In the first chapter, a brief introduction is provided regarding different types of magnetoresistance (MR) phenomena observed in different materials, namely Anisotropic MR (AMR), Giant MR (GMR), Collosal MR (CMR), Tunneling MR (TMR), Powder MR (PMR) etc. Out of these, CMR and PMR are found in doped manganites, while Sr2FeMoO6 exhibits PMR only. Next, a brief overview of the structure, properties and theories for both of these materials is provided. For the case of doped manganites, a short introduction is given for a novel two-fluid hamiltonian (called l - b model) which was proposed recently by Ramakrishnan et. al.. This model reproduces several exotic transport and magnetic properties of manganites which were inexplicible by earlier theories. The model was solved within the Dynamical Mean Field Theory (DMFT) framework by Hassan et. al.. A brief description of this DMFT solution is given. Many of the DMFT results for this model have been used in the subsequent chapters. In the second chapter, the hysteresis behaviour of the magnetoresistance and the magnetization (M ) of powdered Sr2FeMoO6 is considered in detail. In a recent experi- ment by Sarma et. al., it was found that this material, when powdered exhibits an exotic variety of PMR. In ordinary PMR, the hysteresis behaviour of the MR is supposed to follow that of M, in the sense that the coercive fields should be identical in both cases. Also, the MR is supposed to be roughly proportional to the square of the magnetization. However, in the experiments by Sarma et. al. on cold-pressed Sr2FeMoO6 powder, it was observed that the M R did not appear to be determined purely by the magnetization. Rather, the coercive fields for the hysteresis of the MR was almost 6 times that of M . Moreover, the quantity M R/M2, instead of remaining constant with changing magnetic field, itself has a hysteresis loop. Apart from establishing the exotic nature of the PMR, the experiment also tries to determine whether the MR originates from intra-grain or inter-grain tunneling. In the second chapter we present a simple toy model to reproduce the experimental results, and provide theoretical explanations. A combination of Monte Carlo and transfer matrix methods are used to simulate the hysteresis behaviour of the M R as well as of M . We show that the observed data can be understood if it is as- sumed firstly that the MR arises predominantly from inter-grain rather than intra-grain tunneling, and that the inter-grain boundaries are themselves magnetic with a coercive field higher than that of the grains. In order to motivate the use of Monte Carlo method for studying hysteresis, a brief survey of main results obtained for some simple models using this technique is also provided. In the third chapter, we study the doping and temperature dependence of core-level photoemission spectra in doped rare-earth manganites. In some recent experiments on Strontium doped (LSMO) and Barium doped (LBMO) samples, it has been observed that the M n2p3/2 core-level spectra shows an intriguing spectral weight transfer over a range of several eV , as a function of doping (x) and temperature (T ), in the ferromagnetic metallic phase. Specifically, there appears a shoulder adjacent to the main peak on the side of lower binding energy, which increases in weight and intensity as the doping increases or the temperature decreases. In LSMO samples, another shoulder was noticed on the higher binding energy side also. Moreover, in data obtained from LBMO samples, the spectra at different temperatures was subtracted from the spectra at/above Tc, and then this difference spectrum was integrated. The integrated weight, when normalized by the weight at the lowest temperature, appears to follow the square of the measured magnetization almost exactly. In order to understand the experimental data, we extended the aforementioned l - b model to include a core-level, and the attractive interaction due to a core-hole on the local valence levels. The impurity problem arising in DMFT, consisting of a single impurity site coupled to a bath, was tailored for the photoemission problem, by including this extra core-level at the impurity site. The hybridization parameters for the bath were determined self-consistently from the DMFT, and then the single particle spectral function for the core-hole was determined. This spectral function is proportional to the photo emission intensity. We found that our calculations reproduced the observed spectral weight transfer as a function of x and T both in trends and in magnitude. The integrated difference spectra weight was found to follow the square of the DMFT magnetization, just as in the experiment. Linear discretization of the conduction bath was used for all the above-mentioned cases. In one particular case, a logarithmic discretization was also undertaken for comparison, and also to obtain the exponents of the edge singularities in the theoretical spectra. In the fourth chapter, the possibility of Anderson Localization in manganites is in- vestigated, using the l - b model. According to this model, a large fraction of the valence electrons are polaronically self-trapped even in the ferromagnetic metallic phase. Due to strong on-site Coulomb interaction, these polarons provide a strongly scattering background, which can localize the mobile-electron band states close to the band edges. Since the fraction of valence electrons which are truly mobile is small, hence the Fermi energy lies close to the lower band edge. Hence, there is a possibility of an Anderson Insulator phase where all charge carriers are localized. To investigate this, we studied the behaviour of the mobility edges as a function of doping. DMFT alone does not include the physics of localization. Hence, in order to obtain the mobility edges, we combined the DMFT results with the Self-consistent Theory of Localization (STL), using a simplified prescription called Potential Well Analogy (PWA) due to Economou et. al.. We found that there is indeed an Anderson Insulator phase in a certain region of doping, which would otherwise have been supposed to be metallic based on purely DMFT results. Finally, we have compared this result, obtained using effective field theories, with an actual real space simulation of the l - b model at T=0. In this case, the mobility edge trajectories were obtained by studying the Inverse Participation Ratio (IPR), as a function of band energy and doping. In the concluding chapter, the principal results presented in this thesis are summa- rized. The limitations of the approach or approximations used are discussed, and future possibilities for overcoming these limitations outlined.

Le but de ce travail était de réaliser une étude systématique de la structure électronique et magnétique de la famille des manganites semi-dopés du Ca: Ln0.50$Ca0.50MnO3 (Ln = terre rare). Nous avons particulièrement focalisé notre attention sur l'ordre de charge et l'ordre orbital généralement présents à cette composition. Nous avons dérivé un modèle microscopique de structure nucléaire et magnétique à partir de techniques de diffraction neutronique sur les poudres et de calculs ab-initio afin de comparer les résultats expérimentaux et les modèles numériques et comprendre ainsi le rôle de l'inhomogénéité chimique et magnétique dans ces systèmes. La modification de l'état de spin électronique et du métal de transition par le dopage correspond à une modification structurale de la géométrie du polyèdre de coordination des atomes autour du métal de transition. Ceci induit des modifications structurales coopératives observables par diffraction de neutrons. En contraste avec l'ordre induit par le dopage chimique, un désordre chimique intrinsèque est associé à l'élément de dopage sur le site A du perovskite (formule générale ABO3) où sont placés les ions trivalents (RE3+) et bivalents (Ca2+). Ce désordre est dû à la différence de rayon ionique et d'affinité chimique entre ces ions. Afin d'étudier systématiquement l'effet de la substitution au niveau du site A et la relation entre les propriétés structurales (nucléaires et magnétiques) et les propriétés magnétiques macroscopiques, plusieurs échantillons (spécificiés auparavant) ont été synthétisés et caractérisés par des mesures magnétiques macroscopiques. Les résultats de diffraction de neutrons sont une étape fondamentale vers la compréhension de la relation entre les propriétés structurales et macroscopiques et représentent de fait une source de motivation pour l'étude de la structure magnétique et des phénomènes de CO/OO par des simulations ab-initio. Dans la thèse, suite à la description du diagramme de phase des manganites les plus étudiés (La1-xCaxMnO3), leurs propriétés physiques, les concepts de base et les modèles les plus importants tels que l'effet Jahn-Teller, l'interaction de super-échange, le double-échange et le modèle de Zener seront introduits. Une brève introduction à toutes les techniques expérimentales utilisées dans cette étude sera présentée. Les propriétés magnétiques macroscopiques ont été mesurées en fonction de la température á l'aide d'un SQUID qui sera également décrit au cours dans la partie du techniche expérimentaux. La technique microscopique principale utilisée pour cette thèse a été la diffraction neutronique sur poudre. Celle-ci, ansi que les instruments utilisés à l'institut Laue Langevin à Grenoble, D20 et D1A, seront décrits ainsi que la méthode de Rietveld utilisée pour affiner les données expérimentales et en extraire les informations structurales sous-jacentes. Les résultats expérimentaux correspondant à l'étude systématique sur les échantillons de manganites de Ln0.50Ca0.50MnO3 (Ln = P.R., ND, TB, Dy, Ho, TM, Yb et Lu), seront présentés suite à une description de leur préparation par réaction à l'état solide. La susceptibilité magnétique mesurée jusqu'à 530 K présente un pic large à températures élevées correspondant à la températures d'ordre de charge (TCO). A partir de ces résultats, nous définissons la nature des corrélations magnétiques au-dessus et en-dessous de cette température dans le cadre du modèle des polarons de Zener comme proposé par Daoud-Aladine. Nous présenterons également dans ce chapitre les détails de la structure nucléaire et magnétique pour tous les échantillons en fonction de la température. Tous nos échantillons présentent une configuration magnétique de type pseudo-CE à la plus basse température correspondant à un état fondamental de type CE avec un effet de canting. Les déformations dues au dopage et à l'effet du rayon ionique seront également analysées. Nous décrirons les calculs ab-initio realisés à l'aide de la théorie de la DFT pour modéliser la série des Ln0.50Ca0.50MnO3. Les caractéristiques de ces matériaux sont obtenues à partir de la résolution de l'équation de Schrodinger pour les électrons du système. La théorie de la DFT sera présentée ainsi qu'une discussion sur l'interprétation de l'énergie d'échange-corrélation et des approximations nécessaires à son évaluation. A cet effet, le programme VASP, utilisé pour les calculs, sera présenté ainsi que ses différents fichiers d'entrée et de sortie. Les résultats des simulations seront décrits et comparés aux résultats expérimentaux. En effet, ces calculs ont été effectués pour confirmer les résultats expérimentaux mais également pour accéder à d'autres quantités significatives comme la densité d'états électroniques. Les simulations ont été effectuées avec la DFT spin-polarisée, la fonctionnelle d'échange-corrélation GGA-PBE, et, quand nécessaire, avec une correction d'Hubbard dans l'approche GGA+U, pour prendre en considération la corrélation électronique forte dans les manganites. Nous avons choisi comme système deux manganites purs: CaMnO3 et NdMnO3, ayant déjà fait l'objet d'études précédentes (Filippetti and Picozzi), afin de valider notre protocole de simulation. Deux systèmes semi-dopés -- Nd0.5Ca0.5MnO3 et Lu0.5Ca0.5MnO3 -- ont ensuite été considérés. Nous avons choisi ces systèmes pour deux raisons: (i) les composés semi-dopés contenant le La et le Pr ont déjà fait l'objet de travaux antérieurs (Picozzi, Anisimov}, et il nous a donc semblé naturel de poursuivre la série de lanthanide avec le système Nd-Ca; (ii) nous avons choisi les composés de LuCa parce que le Lu, à l'instar de La, est saturé au niveau de ses orbitales 4f et qu'il présente de surcroit le plus petit rayon ionique dans la série de lanthanides.

Los materiales objeto de estudio en esta Tesis son óxidos de Mn del tipo La2/3A1/3MnO3 (A=Ca, Sr), que presentan una transición ferro-paramagnética acompañada de una transición metal-aislante a temperaturas del orden de TC=300K. La aplicación de un campo magnético favorece la aparición del orden ferromagnético produciendo un desplazamiento de la transición hacia temperaturas más elevadas. Esto genera una fuerte disminución de la resistividad, dando lugar a un importante efecto magnetoresistivo (magnetoresistencia colosal). Dado el complicado entramado de los diferentes fenómenos físicos concurrentes en las manganitas, estos óxidos tienen un gran interés tanto desde el punto de vista básico como del desarrollo de nuevos dispositivos de electrónica de espín. Dentro de este marco, esta tesis está organizada en tres apartados : 1. Una gran parte del trabajo experimental ha consistido en el desarrollo de un sistema de crecimiento de láminas delgadas mediante pulverización catódica de radiofrecuencia. Los distintos elementos del sistema (calefactores, portasustratos, magnetrones, etc) se controlan por ordenador, lo cual permite crecer muestras de forma automática. 2. La segunda parte del trabajo consiste en controlar el crecimiento de láminas del compuesto La2/3Ca1/3MnO3 a nivel nanoscópico. La influencia de las condiciones de depósito sobre los distintos parámetros claves del material (parámetros de celda, temperaturas de transición, resistividad, rugosidad, etc) ha sido estudiada de exhaustivamente y se ha conseguido la fabricación de muestras de alta calidad de forma reproducible. 3. La parte más importante de la investigación se ha dedicado a determinar el papel de las intercaras sobre las propiedades de capas de manganita. Como es bien sabido, la interacción responsable del ferromagnetismo en este material es muy sensible a las distorsiones estructurales, lo cual, combinado con la alta polarización de los portadores de carga, lleva a una magnetoresistencia de bajo campo muy fuerte en cerámicas nanométricas. Esta alta polarización también da lugar a una fuerte respuesta a campo débil en uniones túnel. Nuestra contribución consiste en la fabricación de intercaras artificiales de varios tipos y en la correlación de la magnetoresistencia obtenida con el análisis estructural. Las propiedades magnetoelectrónicas de las intercaras entre una manganita y un aislante (SrTiO3, LaAlO3 o NdGaO3) también han sido estudiadas. En estas regiones la temperatura de Curie TC y la conductividad eléctrica son más bajas que en el bulk, lo cual se atribuye a la coexistencia de fases con distintas propiedades electrónicas. Por otra parte, fenómenos más fundamentales como la magnetoresistencia anisotrópica y el efecto Hall anómalo también han sido objeto de estudio y se han intentado correlacionar con el acoplamiento espín-órbita y el estado magnético cerca de TC. / This thesis reports on properties of Mn oxides of the type La2/3A1/3MnO3 (A=Ca, Sr) which exhibit a ferromagnetic-to-paramagnetic transition concomitant of a metallic-to-insulator transition around TC=300K. The application of an external magnetic field favours the ferromagnetic ordering of Mn moments which leads to a shift of TC to higher temperatures, which in turn produces a decrease of the resistivity. This gives rise to a strong magnetoresistive effect called colossal magnetoresistance. In these compounds, many interactions have similar energy scales and compete to determine the ground state, making manganites very interesting materials from the basic point of view as well as for the development of new devices for spin electronics. Within this frame, this thesis is focused on the three following points : 1. The biggest effort in the experimental work has been dedicated to the set-up of a radiofrequency sputtering system for the fabrication of thin films of oxides. The different elements of the system (substrate heaters and holders, magnetrons, etc) are computer-controlled, which allows the growth of films and heterostructures in a fully automated fashion. 2. The second part of the work consisted in controlling the growth of thin films of mixed valence manganites, mainly La2/3Ca1/3MnO3, at the nanoscopic scale. The influence of several growth parameters on the key characteristics of the material (cell parameters, transition temperatures, resisitivity, roughness, etc) has been extensively studied and a reproducible fabrication of high-quality films has been achieved. 3. The main part of the work is focused on the role of interfaces on the properties of manganite films. It is well know that the interaction responsible for ferromagnetism in these compounds is very sensitive to structural distortions which, combined with a high polarisation of the charge carriers, leads to a strong low-field magnetoresistance in nanometric ceramic powders. This high polarisation also gives rise to a large low-field response in tunnel junctions. To probe these effects, we have fabricated artificial interfaces of several types and correlated their magnetotransport response with data from structural analysis. We have also studied the magnetoelectronic properties of the interface between a manganite film and an insulator (SrTiO3, LaAlO3, NdGaO3). A marked decrease of TC and of the electrical conductivity compared to bulk material is observed. This correlates with the detection of multiphase separation between regions with different electronic properties at these interfaces. Besides, fundamental transport phenomena such as anisotropic magnetoresistance and anomalous Hall have been investigated. Their magnitude and temperature dependence have been analysed and a semi-quantitative explanation involving the spin-orbit interaction and the electronic texture close to TC has been proposed. / Cette Thèse a été consacrée à l'étude d'oxydes de Mn La2/3A1/3MnO3 (A=Ca, Sr) présentant une transition ferromagnétique-paramagnétique concomitante d'une transition métal-isolant aux environs de TC=300K. L'application d'un champ magnétique externe favorise un ordre ferromagnétique du moment des ions Mn, ce qui déplace TC vers les hautes températures et induit une forte diminution de la résistivité. Ce fort effet magnétorésistif est appelé magnétorésistance colossale. Dans ces composés, de nombreux interactions sont en compétition ce qui les rend très intéressants du point de vue fondamental comme pour le développement de nouveaux dispositifs en électronique de spin.Dans ce cadre, cette thèse est structurée autour des trois points suivants :1. Un grand effort expérimental a été fourni pour développer un bâti de pulvérisation cathodique radiofréquence destiné à l'élaboration de couches minces d'oxydes. Les différents éléments du système (chaufferettes, porte-substrats, magnétrons) sont pilotés par ordinateur ce qui permet de fabriquer des couches minces de façon totalement automatique.2. La seconde partie du travail a consisté à contrôler la croissance de couches minces de La2/3Ca1/3MnO3 à l'échelle nanoscopique. L'influence des paramètres de croissance sur certaines caractéristiques du matériau (paramètres de maille, températures de transition, résistivité, rugosité de surface, etc) a été étudiée de façon extensive de sorte que des couches de très bonne qualité ont pu être obtenues de façon reproductible.3. Le rôle des interfaces sur les propriétés physiques de couches de manganite a fait l'objet de la majeure partie de cette thèse. En effet, l'interaction responsable du ferromagnétisme de ces composés est très sensible aux distorsions structurales, ce qui, du fait de la forte polarisation de spin des porteurs, donne lieu à une forte magnétorésistance à faible champ dans des céramiques nanométriques. Cette forte polarisation conduit également à une forte réponse magnétorésistive dans des jonctions tunnel. Nous nous sommes donc intéressés aux propriétés d'interfaces et avons fabriqué des interfaces artificielles de plusieurs types, et relié leur propriétés de magnétotransport aux résultats des analyses structurales. Par ailleurs, nous avons étudié les interfaces entre une manganite et un isolant (SrTiO3, LaAlO3, NdGaO3). Celles-ci possèdent une Tc et une conductivité beaucoup plus faibles que le matériau massif, ce qui est lié à la coexistence de régions présentant des propriétés électroniques différentes (séparation de phase). Par ailleurs, nous nous sommes également intéressés à des phénomènes de transport intrinsèques comme la magnétorésistance anisotrope et l'effet Hall extraordinaire dont nous avons essayé de comprendre la dépendance en température en la reliant au couplage spin-orbite et à la texture électronique près de TC.