No description available.

In this work, the effect of reversible elastic lattice strain on the electronic properties of a) (Pr1−y Lay )0.7 Ca0.3 MnO3 (PLCMO) thin films and b) the interface layer of La0.7 Sr0.3 MnO3 (LSMO) with SrTiO3 (STO) has been determined using piezoelectric substrates. Lattice strain is known to effectively alter the electronic structure of compounds from the manganite family, since it shifts the balance of competing electronic interactions by changing bond angles and bond lengths.The PLCMO films have been repared by pulsed laser deposition (PLD) from a La0.7 Ca0.3 MnO3 (LCMO) and a Pr0.7 Ca0.3 MnO3 (PCMO) target. The metal- insulator phase boundary has been established to lie around y = 0.6. In films with y = 0.6, the piezoelectric release of tensile strain in the film plane induces a drastic reduction of the resistance, or a “colossal” elastoresistance. Resistive gauge factors as high as Γ = 1000 have been found. Consistent with the transport results, the release of tensile strain leads to an increase in both the Curie temperature and the magnetisation. The coexistence of the ferromagnetic metallic (FMM) and charge ordered insulating (COI) phases in PLCMO has been found to be strongly affected by the reversible substrate strain. Both the magnetisation and the resistance data in controlled strain states demonstrate a strong suppression of the ferromagnetic double exchange interaction by tensile strain. [La0.7 Sr0.3 MnO/SrTiO3 ] superlattices have been deposited on STO and piezoelectric PMN-PT (001) (PbMg1/3 Nb2/3 O3 )0.72 (PbTiO3 )0.28 substrates by PLD. X-ray reflectivity (XRR) measurements show clear Kiessig fringes as well as the larger interference maxima caused by the superlattice, giving qualitative proof of a well defined superlattice structure with sharp interfaces on both substrates. With decreasing LSMO layer thickness d, the samples show a sharp decrease of the Curie temperature, accompanied by a decrease of the saturation magnetisation and an increase of the coercive field around d = 5 nm. Reversible strain measurements on thicker superlattices (d = 16.7 nm) reveal a behaviour of the magnetisation similar to that of single thick films of LSMO. When d is decreased, the strain induced relative change in magnetisation ∆M/M0 shows a behaviour comparable to PLCMO thin films. This has been attributed to the increased volume fraction of the LSMO inter- face layer with STO, which displays a reduced magnetic order and phase-separated tendencies. From the absolute change in magnetisation per interface, the thickness of the so-called magnetically “dead” layer of the LSMO has been estimated to lie between 1.3 nm and 1.7 nm in the superlattices grown on PMN-PT.

Des films minces de pérovskites NdNiO3 et SmxCa1-xMnO3 ont été synthétisés par co-pulvérisation magnétron et leurs propriétés thermochromes ont été étudiées dans le domaine infrarouge. Dans un premier temps, l'étude du nickelate NdNiO3 a montré la possibilité de synthétiser la phase pérovskite en utilisant un recuit de cristallisation sous air à basse température (550°C). Nous avons mis en évidence une cristallisation compétitive entre la phase orthorhombique thermochrome et une phase rhomboédrique non thermochrome qui est largement influencée par les paramètres d'élaboration : pression de dépôt, épaisseur des couches, température de recuit... L'obtention de ces deux phases a permis d'étudier plus précisément les mécanismes d'oxydation et de réduction intervenant lors du dépôt et lors du traitement thermique respectivement. Parallèlement à cette étude, des couches minces de SmxCa1-xMnO3, ont été également synthétisées par co-pulvérisation. Après une optimisation des conditions expérimentales permettant d'obtenir des films stoechiométriques et des analyses structurales (MEB, DRX), les comportements électriques et optiques des films ont été suivis en fonction de la température (mesure 4 pointes et IRTF). Le composé Sm0,5Ca0,5MnO3 présente une température de transition proche de la température ambiante ainsi qu'un contraste optique dans le très proche et le moyen infrarouge. Les mesures de la transmittance optique dans le domaine visible-proche infrarouge ont permis d'évaluer la dépendance du gap optique avec la température. A 20°C, celui-ci est proche de 0,7 eV. Le manganite SmxCa1-xMnO3 est donc un bon candidat pour la régulation thermique et la furtivité infrarouge / NdNiO3 and SmxCa1-xMnO3 perovskites thin films were synthesized by magnetron co-sputtering process and the thermochromic properties have been studied in the infrared range. At first, the study of the NdNiO3 nickelate has established the timeliness to synthesise the perovskite phase using a subsequent annealing crystallisation in air at moderate temperature (550°C). We have demonstrated a competitive crystallisation between the thermochromic orthorhombic phase and a non-thermochromic rhombohedral phase largely influenced by experimental parameters: deposition pressure, layer thickness, annealing temperature... The presence of these two phases allowed to clarify more precisely the oxidation mechanisms involved during the deposition process and the reduction mechanisms occurring during the heat treatment. At the same time, the SmxCa1-xMnO3 system was synthesised also as thin films. The first step includes developing the experimental conditions to obtain stoichiometric films using two separate targets. After structural analyses (SEM, XRD), the electrical and optical behaviours of films were analysed using the four probe configuration and the FTIR spectroscopy versus temperature respectively. As result, the metal-insulator transition of the Sm0,5Ca0,5MnO3 compound is close to the room temperature and the film shows an interesting optical contrast in the near and mid-infrared ranges. The optical transmittance measurements in the visible-near infrared range were used to evaluate the optical gap dependence with temperature. At room temperature, the optical gap is close to 0.7 eV. Finally, SmxCa1-xMnO3 manganite is a good candidate for thermal regulation and infrared furtivity

Perovskite rare-earth manganites of the form R1-xAxMnO3 (R – rare earth ion or Bi, A – Ca,Sr) have drawn an overwhelming research interest during the last few years owing to their extraordinary physical properties. Some of the interesting phenomena exhibited by the manganites are (a) colossal magneotresistance (CMR) (b) charge, orbital and spin ordering and (c) phase separation at nano and micron scale. The manganites are strongly correlated systems in which the charge, spin and orbital degrees of freedom are coupled. 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 on single crystal, bulk polycrystalline and thin film forms of the samples [6-9]. 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 [10-14]. The physical properties of antiferromagnetic insulating charge ordered manganites have been well investigated by using numerous experimental techniques on bulk solids. It is known that the charge ordered (CO) phase is ‘melted’ resulting in a ferromagnetic, metallic phase on application of high magnetic fields, electric fields, impurity ion doping, high energetic ion irradiation and by pressure [15-17]. However, no attempts have been made on the fabrication and the physical property investigations on nanoscale charge ordered manganites. Hence, we have undertaken to study the properties of charge ordered manganites prepared at nanoscale using various experimental probes. In this thesis we present the results on magnetization, magnetotransport and Electron Magnetic Resonance (EMR) (electron paramagnetic resonance in the paramagnetic phase and ferromagnetic resonance in the ferromagnetic phase) studies of the following nanoscale compounds and compare the properties with those of their bulk counterparts; (a) highly robust antiferromagnetic insulating CE –type charge ordered manganite Pr0.5Ca0.5MnO3 (PCMO) (b) highly robust antiferromagnetic insulating CE- type charge ordered manganite Nd0.5Ca0.5MnO3 (NCMO) (c) moderately robust A-type charge ordered manganite Pr0.5Sr0.5MnO3 (PSMO) (d) highly robust insulating anti-ferromagnetic charge ordered manganites Bi0.5Ca0.5MnO3 (BCMO) and Bi0.5Sr0.5MnO3 (BSMO) and (e) a CMR manganite Pr0.7Pb0.3MnO3 (PPMO). Chapter 1 of the thesis contains a brief introduction to the general features of manganites describing various interesting phenomena and the interactions underlying them. Further, we have written a detailed review on the properties of nanometric CMR manganites of various sizes and shapes. In this chapter, we have also described the experimental methodology and the analysis procedure adopted in this work Chapter 2 reports the fabrication of nanowires and nanoparticles of Pr0.5Ca0.5MnO3 (PCMO) and the results obtained from magnetization, magnetotransport and electron magnetic resonance measurements performed on nanoscale PCMO along with their comparison with those of the bulk sample. Here, the nanowires of PCMO were prepared by hydrothermal method and the nanoparticles of mean sizes 10, 20 and 40 nm were prepared by polymer assisted sol-gel method. Solid state reaction method was used to prepare the micron sized PCMO bulk material. Different techniques like XRD, TEM, EDAX and ICPAES have been used to characterize the samples. The novel result of the present investigation is the weakening of charge order and switch over from the anti-ferromagnetic phase to ferromagnetic phase in PCMO nanowires [18]. In addition, the charge order is seem to have completely suppressed in 10 nm PCMO nanoparticles as observed from the magnetization measurements. These results are particularly very significant as one needs magnetic fields of ~ 27 T to melt the charge ordered phase in PCMO. Size induced insulator-metal transition TM-I is observed in nanoscale PCMO at low temperatures accompanied by ferromagnetism. CMR of 99.7% is obtained at TM-I and at a field of 11 T. EMR studies have confirmed the presence of ferromagnetic phase at low temperatures. Temperature dependent EMR line width and intensity have shown the presence of CO phase in PCMO10 though static magnetization measurements have shown the absence of CO phase. It is found that the EMR linewidth increases with the decrease of particle size. Chapter 3 reports the fabrication of nanoparticles of Nd0.5Ca0.5MnO3 (NCMO) and the results obtained from magnetization, magnetotransport and electron magnetic resonance measurements performed on nanoscale NCMO along with their comparison with those of bulk NCMO. The nanoparticles of NCMO of mean sizes 5, 20 and 40 nm were prepared by polymer assisted sol-gel method. Solid state reaction method was used to prepare the micron sized NCMO bulk material. Different techniques like XRD, TEM, EDAX and ICPAES have been used to characterize the samples. A striking result of this particular investigation is the complete suppression of charge ordered phase in 5 and 20 nm NCMO nanoparticles as observed from the magnetization measurements [19]. Size induced insulator-metal transition TM-I is observed in nanoscale NCMO at low temperatures accompanied by ferromagnetism in accordance with Zener double exchange meachanism. CMR of 99.7% is obtained at TM-I and at a field of 11 T. EMR studies have confirmed the presence of ferromagnetic phase at low temperatures. Temperature dependent EMR line width and intensity have shown the presence of residual CO fluctuations in NCMO5 though the static magnetization measurements have shown the absence of CO phase. It is found that the EMR linewidth increases with the decrease of particle size. Low temperature X-ray diffraction measurements on NCMO20 indicate the absence of CO phase. But the preliminary results obtained from the optical spectroscopy measurements indicate the evidence for the presence of CO phase. In Chapter 4, we report the investigations on the nanoscale PSMO. PSMO nanoparticles of sizes 20, 40 and 60 nm are prepared by polymer precursor sol-gel method. PSMO nanowires of diameter 50 nm and lengths of a few microns have been prepared by hydrothermal method. The bulk polycrystalline PSMO is obtained by crushing the single crystal of the same prepared by float zone method. Various techniques like XRD, TEM, VSM, transport measurements and EMR spectroscopy have been employed to characterize and to study the size dependent magnetic, transport and electron magnetic resonance properties and to compare them with those of the bulk. Our results show that there is a disappearance of anti-ferromagnetic charge ordering phase and the appearance of a ferromagnetic phase at low temperatures in all PSMO nanoparticles and nanowires. Metal like behaviour is observed in the size induced ferromagnetic phase in nanoparticles. The EMR linewidth increases with the decrease of particle size. A comparison with the properties of the bulk material shows that the ferromagnetic transition at 265 K remains unaffected but the anti-ferromagnetic transition at TN = 150 K disappears in the nanoparticles. Further, the temperature dependence of magnetic anisotropy shows a complex behaviour, being higher in the nanoparticles at high temperatures, lower at lower temperatures in comparison with the bulk [20]. In Chapter 5, we present the fabrication, characterization and the results obtained from the magnetization and EMR measurements carried out on BCMO and BSMO nanoparticles and compare the results with those of the bulk. X-ray diffraction gives evidence for single phasic nature of the materials as well as their structures. Mono-dispersed to a large extent, isolated nanoparticles are seen in the transmission electron micrographs. High resolution electron microscopy shows the crystalline nature of the nanoparticles. Superconducting quantum interferometer based magnetic measurements from 10 K to 300 K show that these nanomanganites retain the charge ordering nature unlike the Pr and Nd based nanomanganites. The CO in Bi based manganites is thus found to be very robust consistent with the observation that magnetic fields of the order of 130 T are necessary to melt the CO in these compounds. These results are supported by electron magnetic resonance measurements [21]. In Chapter 6, we present our results on the effect of particle size on the magnetic properties of Pr0.7Pb0.3MnO3 (PPMO). PPMO nanoparticles of two different sizes (~5 nm and 30 nm) were prepared by the polymeric precursor sol-gel method. The samples are characterized by different techniques like XRD, TEM, SQUID magnetometry, EMR and optical spectroscopic measurements. It is found that the nanoparticles crystallize in the cubic perovskite structure. TEM measurements show that the 5 nm particles are uniform in size. They are also crystalline as seen by HREM and XRD measurements. SQUID magnetometry measurements have shown that the Curie temperature increases (from 220 K to 235 K) with the increase of particle size. Saturation magnetization is higher for the smaller particles studied. We have observed only one EMR signal down to 4 K in both the nanoparticles (5 and 30 nm) in contrast to the two EMR signal behaviour observed in bulk PPMO [22]. It is found that the EMR linewidth increases with the decrease of particle size in the paramagnetic phase. Temperature dependent optical spectroscopy measurements performed on 5 nm PPMO nanoparticles indicate that the insulator-metal transition temperature TM-I = 230 K, is not very different from TM-I = 235 K of the bulk sample [23] The thesis concludes with a brief writeup summarizing the results and pointing out possible future directions of research in the area.

After the prediction of magnetoelectric effect in Cr2O3, in early 1960's, D. Asrov became the first to experimentally verify this phenomenon. After the pioneering work on magnetoelectric materials in 1960's and 1970's, the discovery of large magnetoelectric effect in orthorhombic rare-earth manganite TbMnO3 has revived great interest in magnetoelectric materials, especially during the last decade. Magnetoelectric multiferroics have great potential in applications such as novel memory storage devices and sensors. As a result of extensive theoretical and experimental investigations conducted on rare-earth magnetoelectric manganites, TbMnO3 has become a prototype magnetoelectric multiferroic material. Orthorhombic rare-earth manganites RMnO3 (R = Gd, Tb and Dy) exhibit improper ferroelectricity where the origin of ferroelectricity is purely magnetic in nature. RMnO3 exhibit diverse and complex magnetic interactions and phases. Doped manganites of the type R1-xAxMnO3 (A = Ca, Sr and Ba) present a rich magnetic and electronic phase diagram. The doping concentration, average ion-size and size mismatch (i.e. disor-der) at A-site, all contribute to determine the ground state. A variety of magnetic phases, competing with each other, are responsible for many functional properties like magnetoelectric effect, colossal magnetoresistance (CMR), magnetostriction and magnetocaloric effect (MCE). In this context, studies of magnetoelectric materials are of great relevance from technical as well as fundamental aspects. Notably, complexity of electronic (and magnetic) phases and experimental difficulties in acquiring reliable measurement-data easily are the most concerning issues in establishing a clear understanding of magnetoelectric materials. In the magnetic phase diagram of RMnO3, GdMnO3 lies on the border between A-type antiferromagnetic and cycloidal antiferromagnetic ground states. Cycloidal spin arrangement is responsible for the induction of ferroelectricity in these materials. There are disparate opinions about the ground state of GdMnO3 (whether the ground state is ferroelectric or not). Understanding of the influence of rare-earth magnetic sublattice on magnetism in GdMnO3 (at low temperature) lacks clarity till date. Neutron scattering studies on GdMnO3 due to high absorption cross-section of Gd ion, yield little success in determining the nature of complex magnetic phases in this material. Interestingly, an earlier report on strontium-substituted gadolinium manganite Gd0.5Sr0.5MnO3 demonstrated the spontaneous electric polarization and related magnetoelectric effect. It was hypothesized that the observed ferroelectricity could be improper and electronic in nature. Strontium doping facilitates quenched disorder that leads to interesting magnetic phases and phase transitions. In order to understand the physical properties of gadolinium manganites and to unravel the relationship between them, it is essential to investigate high quality single crystals of these materials. This thesis deals with growth and investigation of several important physical phenomena of gadolinium manganites such as magnetic, electric, magnetoelectric and magnetocaloric properties. The thesis is organized in seven chapters. A brief summary of each chapter follows: Chapter:1 This chapter provides general introduction to magnetoelectric effect and multiferroicity. The term multiferroicity refers to simultaneous existence of magnetic and electric ordering in a single phase material. Magnetoelectric multiferroics have shown great potential for several applications. They exhibit cross coupling between the electronic and magnetic order parameters, hence basics of various magnetic interactions (and magnetism) are brie y discussed in the rst section of the chapter. It is followed by a brief discussion about the principle of magnetoelectric effect. Magnetoelctric coupling is broadly classified into two types namely, direct coupling and indirect coupling. In the former, the emphasis is given on linear magnetoelectric effect. The concept of multiferroicity is introduced in the next section followed by a brief overview and application potential of multiferroics. Further, classi cation scheme of multiferroic materials is discussed. The concept of improper ferroelectricity and description of subcategories namely, magnetic ferroelectric, geometric ferroelectric and electronic ferroelectric are documented. Magnetic ferroelectric category is considered the most relevant; featuring the type of ferroelectric material as GdMnO3 referred in this thesis. The microscopic theory for mechanism of ferroelectricity in spiral antiferromagnets is presented. While brie ng the thermodynamic background of the magnetocaloric effect, indirect estimation of two important characteristics namely, isothermal magnetic entropy change (∆SM ) and adiabatic change in temperature (∆Tad) under the application of magnetic field are dealt with. In the last part of the chapter, motivation and scope of the thesis is discussed. Chapter:2 This chapter outlines various experimental methodologies adopted in this work. It describes the basic principles of various experimental techniques and related experimental apparatuses used. The chapter starts with the synthesis tech-niques used in the preparation of different compounds studied. The principle of oat-zone method, employed for single-crystal growth, is described. Orientation of single crystals was determined using a home-built back- reflection Laue set up. The basics of Laue reflection and indexing procedure for recorded Laue photographs are described. Various physical properties (electric, magnetic, thermal, magnetoelectric and magnetocaloric properties) were studied using commercial as well as home-built experimental apparatuses. Design and working principle of all the experimental tools are outlined in this chapter. Fabrication details, interfacing of measurement instruments and calibration (standardization) of equipment used in this work are described in appropriate sections. Chapter:3 Chapter-3 describes the investigation of various physical properties of high quality single crystals of magnetoelectric multiferroics, GdMnO3. Synthesis of GdMnO3 is carried out using solid state synthesis route. Single phase nature of the material is confirmed by X-ray powder diffraction technique. Single crystals of GdMnO3 are grown in argon ambience using oat-zone method. As grown crystals are oriented with the help of back-reflection Laue method. GdMnO3 exhibits incommensurate collinear antiferromagnetic phase below 42 K and transforms to canted A-type antiferromagnetic phase below 23 K. Magnetic and specific heat studies have revealed very sharp features near the magnetic transitions which also confirm the high quality of the single crystal. dc magnetization studies illustrate the anisotropic behavior in canted A-type antiferromagnetic phase and clarifies the influence of rare-earth magnetic sub-lattice on overall magnetism (at low temperature). Application of magnetic field (above 10 kOe) along `b' axis helps formation of the cycloidal antiferromagnetic phase. Here, spontaneous electric polarization is induced along `a' axis. The temperature variation plot of dielectric constant, ϵa (under ap- plied magnetic field along `b' axis) shows sharp anomalies in the vicinity of magnetic ordering transitions suggesting magnetodielectric effects. Magnetic field tuning of electric polarization establish the magnetoelectric nature of GdMnO3. Magnetocaloric properties of single crystals of GdMnO3 are investigated using magnetic and magnetothermal measurements. The magnitude of the giant magnetocaloric effect observed is compared with that of other rare-earth manganite multiferroics. Magnetocaloric studies shed light on magnetic ordering of rare-earth ion Gd3+. The phenomenon of inverse magnetocaloric effect observed at low temperature and under low fields is possibly linked to the ordering of Gd3+ spins. Complex interactions between the 3d and 4f magnetic sublattices are believed to influence magnetocaloric properties. Chapter:4 The details of synthesis and single crystal growth of Gd0.5Sr0.5MnO3 using oat-zone method are presented in Chapter 4. Single phase nature of the material is veri ed by carrying out powder x-ray diffraction analysis and confirmation of single crystallinity and orientation through back-reflection Laue method. Electric transport studies reveal semiconductor-like nature of Gd0.5Sr0.5MnO3 until the lowest temperature achieved. This is due to charge localization process which occurs concurrently with decrease in temperature. Gd0.5Sr0.5MnO3 exhibits charge-ordered insulator (COI) phase below 90 K (ac-cording to an earlier report). It is found that under application of magnetic field above a critical value, charge ordering melts and the phase transforms to ferromagnetic metallic (FMM) phase. This transformation is first-order in nature with associated CMR (109%). The first-order phase transition (FOPT) occurs between competing COI and FMM phases and manifests as hysteresis across the FOPT. Strontium doping at A-site induces a large size mismatch at A-site resulting in high quenched disorder in Gd0.5Sr0.5MnO3. The disorder plays a significant role in CMR as well as glass-like dynamics within the low-temperature magnetic phase. ac susceptibility studies and dynamic scaling analysis reveal very slow dynamics inside the low-temperature magnetic phase (below 32 K). According to an earlier report, spontaneous electric polarization and magnetoelectric effect were pronounced near FOPT (at 4.5 K and 100 kOe) between COI and FMM phases. It is prudent to investigate FOPT across COI and FMM phases in Gd0.5Sr0.5MnO3 to understand complex magnetic phases present. Thermodynamic limits of the FOPT (in magnetic field - temperature (H-T) plane), such as supercooling and superheating, are experimentally determined from magnetization and magnetotransport measurements. Interestingly, thermomagnetic anomalies such as open hysteresis loops are observed while traversing the FOPT isothermally or isomagnetically in the H-T plane. These anomalies point towards incomplete phase transformation while crossing the FOPT. Phenomenological model of kinetic arrest is invoked to understand these anomalies. The model put for-ward the idea that while cooling across the FOPT, extraction of specific heat is easier than that of latent heat. In other words, phase transformation across FOPT is thermodynamically allowed but kinetics becomes very slow and phase transformation does not occur at the conventional experimental time scale. Magnetization relaxation measurements (at 89 kOe) with field-cooled magnetization protocol reveal that the relaxation time constant rst decreases with temperature and later, increases non-monotonically below 30 K. This qualita-tive behavior indicates glass-like arrest of the FOPT. Further, thermal cycling studies of zero field-cooled (ZFC) and eld-cooled (FC) magnetization indicate that a low temperature phase prepared with ZFC and FC protocols (at 89 kOe) is not at equilibrium. This confirms the kinetic arrest of FOPT and formation of magnetic phase similar to glass. Chapter:5 Chapter-5 deals with the investigation of the effect of an electric field on charge ordered phase in Gd0.5Sr0.5MnO3 single crystals. As discussed in the previous chapter, application of magnetic field above a critical value collapses the charge ordered phase which transforms to FMM phase. In this view, it is interesting to investigate effect of electric field on the charge ordering. There are various reports on doped manganites such as Pr1-xCaxMnO3 (x = 0:3 to 0:4) that claim melting of charge ordering under application of electric field (or current) above a critical value. In this thesis work, current - voltage (I - V) characteristics of Gd0.5Sr0.5MnO3 are studied at various constant temperatures. Preliminary measurements show that the I-V characteristics are highly non-linear and are accompanied by the onset of negative differential resistance (NDR) above a critical current value. However, we suspect a major contribution of Joule heating in realization of the NDR. Continual I - V loop measurements for five loops revealed thermal drag and that the onset of NDR shifts systematically towards high current values until it disappeared in the current window. Two strategies were employed to investigate the role of Joule heating in realization of NDR: 1) monitoring the sample surface temperature during electric transport measurement and 2) reducing of the Joule heating in a controlled manner by using pulsed current I - V measuremenets. By tuning the duty cycle of the current pulses (or in other words, by controlling the Joule heating in the sample), it was feasible to shift the onset of NDR to any desired value of the current. At low magnitude of the duty cycle in the current range upto 40 mA, the NDR phenomenon did not occur. These experiments concluded that the NDR in Gd0.5Sr0.5MnO3 is a consequence of the Joule heating. Chapter:6 `Chapter-6 deals with the thermal and magnetocaloric properties of Gd0.5Sr0.5MnO3 oriented single crystals. Magnetocaloric properties of Gd0.5Sr0.5MnO3 have been studied using magnetic and magnetothermal measurements. Tempera-ture variation of ∆SM is estimated for magnetic field change of 0 - 70 kOe. The eld 70 kOe is well below the critical magnetic eld required for FOPT between COI and FMM phases. Magnetzation - field (M-H) loop shows minimal hysteresis for measurements up to 70 kOe. The minimal hysteresis behavior al-lows one to make fairly accurate estimation of magnetocaloric properties. ∆Tad was separately estimated from specific heat measurements at different magnetic fields. Specific heat studies show the presence of Schottky-like anomaly at low temperature. Chapter:7 Finally, Chapter-7 summarizes various experimental results, analyses and conclusions. A broad outlook of the work in general with future scope of research in this area are outlined in this chapter.

Low-temperature magnetic force microscopy was used to study the phase diagram of a La1/3Pr1/3Ca1/3MnO3 thin film grown on a (110) NdGaO3 (NGO) substrate by pulsed laser deposition. Traditionally, one can observe the phase change at the nanoscale level as the sample is cooled from room temperature through the transition temperature to liquid nitrogen temperatures, but in this case a fixed voltage ranging from 0 V to 31 V was applied before each cooling cycle. From in and ex situ transport measurements, it is observed that the temperature of the peak of the transition increases with applied field; however, the MFM images show that the magnetic transition begins at a lower temperature with the same increase in field. Thus, this dissertation shows that a new voltage control exists for the phase transition in certain manganites. / text

Pagrindiniai bėgių tipo elektromagnetinių svaidyklių technologijos uždaviniai yra susiję su daugybe fizikinių reikinių, vykstančių sviedinio kontaktų sąlyčio su bėgiais riboje. Todėl elektromagnetinių procesų, atsirandančių dėl didelių elektros srovės tankių ir slydimo greičių tyrimas yra svarbus šios srities uždavinys. Dėl magnetinės difuzijos ir greito sviedinio judėjimo, srovė koncentruojasi galinėje kontakto dalyje, kuri dėl stipraus Joule šilimo greitai susidėvi, o tai apriboja svaidyklės efektyvumą. Disertacijoje pateikiami magnetinio lauko difuzijos tyrimai bėgių tipo svaidyklėse panaudojant specialius jutiklius magnetinių laukų matavimui. Šie nauji jutikliai, pagaminti iš plonų La0,83Sr0,17MnO3 sluoksnių, pasižyminčių milžiniškos magnetovaržos (MM) reiškiniu (MM-B-skaliariniai jutikliai), buvo pritaikyti svaidyklėse, veikiančiose statiniame ir dinaminiame režime, esant dideliems elektromagnetinių triukšmų lygiams ir mechaniniams įtempiams. Darbo metu buvo nustatyta, jog šiais jutikliais galima išmatuoti stipraus magnetinio lauko impulso amplitudę, kai nėra žinoma šių laukų kryptis. Buvo ištirti nevienalyčių magnetinių laukų pasiskirstymai bėgiuose, atsirandantys dėl artumo efekto bei greičio skinefekto, sviediniui judant greičiau nei 1500m/s. Bandymai su įtvirtintu daugelio šepetėlių konstrukcijos sviediniu parodė, kad priekiniai šepetėliai, dėl nepakankamo Lorenco jėgos sukuriamo slėgio, gali pararasti elektrinį kontaktą su bėgiais. / The development of rails and armatures which ensure a sliding solid-to-solid contact during the whole projectile acceleration is a great challenge in the field of railgun technology. Multifaceted physics exists at the sliding contact interface: The current concentrates at the rear of the interface due to magnetic diffusion processes and the fast armature movement. Consequently, Joule heating leads to enhanced wear in this region. In this dissertation, magnetic diffusion in railguns is investigated by means of measuring magnetic fields with CMR-B-scalar sensors at static and dynamic experimental conditions. These novel sensors, based on La0.83Sr0.17MnO3 thin films exhibiting colossal magnetoresistance were adapted for the use at railguns. It was found that these sensors are effective tools to measure the magnitude of high pulsed magnetic fields independent of the field orientation. Magnetic field distributions influenced by proximity and velocity skin effect could be measured in the harsh railgun environment. The obtained results allowed to estimate the skin depth in the rails at the sliding interface of a fast moving armature (>1500m/s). Furthermore experiments with fixed multiple brush armatures showed that front brushes can have contact problems in case of missing contact pressure.

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: Ln0.50Ca0.50MnO3 (Ln=REE). Nous avons focalisé notre attention sur l'ordre de charge (CO) et l'ordre orbital (OO) présents dans les manganites. Nous avons dérivé un modèle microscopique de structure nucléaire et magnétique à partir de la diffraction neutronique sur les poudres (NPD) 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, induisant des changements structurels de coopération. En contraste avec l'ordre 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+). Ceci 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 et magnétiques, plusieurs échantillons ont été caractérisés. Les résultats de NPD sont une étape fondamentale vers la compréhension de la relation entre les propriétés structurales et magnétique et sont une source de motivation pour l'étude de la structure magnétique et des phénomènes de CO/OO par des simulations ab-initio. L'effet Jahn-Teller, les interactions de double- et super-échange, et le modèle de Zener seront introduits. Les propriétés magnétiques macroscopiques ont été mesurées en fonction de la température á l'aide d'un SQUID. La technique microscopique principale utilisée pour cette étude a été la NPD. Les instruments utilisés à l'institut Laue Langevin à Grenoble, D20 et D1A, et la méthode de Rietveld utilisée pour affiner les données expérimentales et en extraire les informations structurales seront décrits. Les résultats expérimentaux correspondant à l'étude systématique sur les échantillons de manganites de Ln0.50Ca0.50MnO3 (Ln = Pr, Nd, Tb, Dy, Ho, Tm, Yb and Lu) et une description de leur préparation seront présentés. La susceptibilité magnétique mesurée jusqu'à 530 K présente un pic large à températures élevées correspondant à la température de CO (TCO). Nous définissons la nature des corrélations magnétiques au-dessus et en-dessous de TCO dans le cadre du modèle des polarons de Zener. Nous présentons la structure nucléaire et magnétique pour tous les échantillons en fonction de la température et les déformations dues au dopage et à l'effet du rayon ionique. Tous nos systèmes ont une configuration magnétique de type pseudo-CE à la plus basse température correspondant à un état de type CE avec un effet de canting. Nous décrirons les calculs ab-initio pour modéliser notre série : le programme VASP, utilisé pour les calculs, la théorie DFT, les approximations faite, comme le fonctionnelle d'échange-corrélation (GGA-PBE), la correction d'Hubbard (GGA+U) seront présentée. Les calculs ont été effectués pour confirmer les résultats expérimentaux et pour accéder à d'autres quantités significatives comme la densité d'états électroniques. Les simulations ont été effectuées avec la DFT spin-polarisée, le GGA-PBE, et la GGA+U, pour considérer la corrélation électronique forte. Nous avons choisi deux systèmes purs: CaMnO3 et NdMnO3. Deux systèmes semi-dopés (Ln=Nd et Lu), ont été considérés, parce que les composés avec le La et le Pr ont été déjà étudié (Picozzi, Anisimov), et nous suivons la série avec Ln=Nd, et l’outre 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. / The aim of this work was to realize a systematic study of the electronic and magnetic structure of Ca half-doped manganite family: Ln0.50Ca0.50MnO3 (Ln=REE). In particular, we focused our attention on charge ordering (CO) and orbital ordering (OO) phenomena present in manganites. We derived a microscopic model of nuclear and magnetic structure using both neutron powder diffraction (NPD) techniques and ab-initio calculations in order to compare experimental results and numerical models and to understand the role of chemical and magnetic in-homogeneity in our systems. The change of the electronic and spin state of the transition metal by doping, corresponds to a structural modification of the coordination polyhedron geometry of the atoms around the transition metal, inducing cooperative structural changes. In contrast with this order induced by doping, an intrinsic chemical disorder is associated with the doping element on the A site of the perovskite (general formula ABO3) on which the trivalent (RE3+) and divalent ions (Ca2+) reside. This disorder is due to the difference of the ionic radius and chemical affinity between the ions. In order to study systematically the effect of the A site substitution and the relation between the structural properties and the macroscopic magnetic properties, several samples have been synthesized and characterized by macroscopic magnetic measurements. Neutron diffraction is a fundamental step towards understanding the relation between the structural and macroscopic properties. The resulting structures represent a good starting point for ab-initio calculations in the study of magnetic structure and CO/OO phenomena. Important concepts and models are described: Jahn-Teller effect, double and super-exchange interaction and the Zener polarons model. The macroscopic magnetic properties have been measured versus temperature by using a SQUID magnetometer. The principal microscopic technique used for this thesis was NPD. The technique and the layout of the instruments - D20 and D1A at the Institute Laue Langevin, Grenoble - are described together with details of the Rietveld method used to refine the diffraction data. Experimental results from the systematic study of the Ln0.50Ca0.50MnO3 (Ln = Pr, Nd, Tb, Dy, Ho, Tm, Yb and Lu) manganites, are presented, with a description of sample preparation. The magnetic susceptibility, measured up to 530 K for our samples, presents a broad peak at high temperatures corresponding to the onset of the CO (TCO). From these results we define the nature of the magnetic correlations above and below TCO in the framework of the Zener polarons model. We also present the details of the nuclear and magnetic structure for all samples versus temperature, analyzing the distortions due to the doping and the effect of the ionic radius of the cations. All our samples have a pseudo-CE magnetic configuration at the lowest temperatures, corresponding to a CE-type ground state with canting. We describe the ab-initio method using the density functional theory (DFT), that have been used to model the Ln0.50Ca0.50MnO3 series. We present DFT and we discuss the most important features (spin polarization), approximations (pseudo-potentials and exchange-correlation functional) and (Hubbard) corrections used in this work, including a presentation of the VASP code used for the DFT calculations, with the corresponding input files. These calculations have been performed to confirm our experimental results and to access other significant quantities such as the electronic density of states. The computational approach has been tested on two pure systems: CaMnO3 and NdMnO3. Two half-doped systems have been chosen with Ln=Nd and Lu. The first because the La and Pr compounds were already studied (Picozzi, Anisimov) so we continued the series with the Nd system, and the Lu has the smallest ionic radius in the lanthanides series, its 4f shell is full and Lu is therefore comparable with La.

No description available.

Este trabalho contempla um estudo sistemático das propriedades elétricas de óxidos cerâmicos a base de manganês. Ênfase foi dada a sistemas onde uma correlação forte entre os graus de liberdade de carga, spin e rede com ordenamento orbital resultam em um estado fundamental heterogêneo, devido a uma separação de fases. Com esse objetivo, foram preparadas amostras policristalinas e monocristalinas de Nd0.5Ca0.5Mn1-xCrxO3, 0.0 x 0.07. A caracterização destas amostras, via medidas de transporte elétrico (T) e de susceptibilidade magnética (T), revelou a ocorrência de uma temperatura de ordenamento de carga CO em TCO 250 K e que uma substituição pequena de Mn por Cr resulta na supressão desse estado CO, induzindo uma transição de fase do tipo metal-isolante (MI) no sistema. Concomitantemente a esta transição MI observa-se uma transição de fase do estado paramagnético PA isolante para um estado ferromagnético FM metálico em TMI ~ TC ~ 140 K. A análise combinada dos resultados experimentais de resistividade elétrica (T,H), magnetização (T) e de espectroscopia de impedância Z(,T) revelaram uma coexistência e competição entre fases na determinação do estado fundamental dessas manganitas. Tal competição foi observada ocorrer em uma larga faixa de temperatura, ou seja, abaixo da temperatura TCO 250 K até a mais baixa temperatura estudada de 1.4 K. Os dados também permitiram concluir que a natureza do estado fundamental desses materiais compreende de uma mistura de fases isolantes entre as temperaturas TCO 250 K e TMI ~ TC ~ 140 K. Por outro lado, e abaixo de TMI, o estado fundamental do sistema pode ser visualizado como sendo composto de uma fina mistura de duas fases: uma com ordenamento de carga e orbital (CO/OO) e de caráter isolante e uma outra ordenada ferromagneticamente FM e com características metálicas. A natureza deste estado fundamental heterogêneo foi confirmada através de medidas de relaxação da resistência elétrica (T,t) obtidas nas duas regiões de temperatura acima citadas. Os dados de (T,t) ainda permitiram concluir que o estado fundamental desses materiais além de heterogêneo é dinâmico, como esperado em um cenário de separação de fases. Uma outra característica desse estado heterogêneo, notadamente abaixo de TMI, é que o mesmo responde de forma não convencional a estímulos diversos, incluindo grandes excitações de corrente elétrica aplicada I. Nesse contexto, a natureza heterogênea do estado CO para T < TCO, bem como da coexistência de fases CO e FM em T < TMI foi provada via um estudo sistemático das propriedades de transporte e magnetização usando diferentes intensidades de corrente elétrica aplicada em medidas de (T,I), M(T,I) e através de curvas características V-I. A observação de fenômenos não lineares, principalmente em curvas características V-I, indicou que os mesmos são precursores de transições de fase abruptas, quando altas densidades de corrente são aplicadas nos materiais. Os dados também permitiram concluir que a corrente elétrica não é distribuída homogeneamente neste estado fundamental heterogêneo. Isto implica em uma localização de corrente e conseqüente efeito Joule dentro do material. A dissipação devido ao efeito Joule é responsável por um auto-aquecimento do material e pode ser suficiente para induzir transições de fase devido ao aumento de temperatura da amostra. A aplicação de um modelo simples de dissipação de calor aplicado aos dados experimentais indicam que o fenômeno de localização de corrente e efeito Joule são fundamentais para o entendimento de transições de fase induzidas por corrente elétrica nessas manganitas. / A systematic study of the electrical properties in doped manganese oxides is presented. Special attention was given to compositions where the strong correlation between charge, spin, and lattice degrees of freedom with orbital ordering resulting in a heterogeneous ground state leads to phase separation. To do this work, polycrystalline and monocrystalline Nd0,5Ca0,5Mn1-xCrxO3, 0,0 x 0,07 samples were prepared. The results obtained through electrical transport (T) and, magnetic susceptibility (T) have revealed the occurrence of charge ordering at TCO 250 K. A small partial substitution of Mn by Cr results in a suppression of the long range charge ordering state and induces both a magnetic from paramagnetic PA to ferromagnetic FM and a electronic from insulating to metallic phase transition at TMI ~ TC ~ 140 K. A combined analysis of the experimental results performed through (T,H), (T), and impedance spectroscopy Z(,T) revealed the coexistence of competing phases in the ground state of these manganites. Such a competition has been found in a large temperature range, from TCO 250 down to 1,4 K. In addition, it is suggested that the ground state comprises a delicate mixture of insulating phases between TCO 250 K e TMI ~ TC ~ 140 K. On the other hand, below TMI, the ground state can be visualized as comprised of two phases: (1) insulating charge orbital ordering (CO/OO) and (2) ferromagnetic metallic phases. The nature of this heterogeneous ground state was confirmed through relaxation measurements (T,t) performed in both temperature intervals cited above. The data indicated that besides to be heterogeneous this ground state is dynamical, as expected in the phase separation scenario. Moreover, this ground state responds in an unconventional fashion when the system is stimulated by electrical current, notably below TMI. Within this context, the heterogeneous nature of the CO state for T < TCO, and the coexistence of CO and FM phases for T < TMI, were studied through magnetic and electrical measurements using electrical current of different magnitude (T,I), M(T,I) and characteristic V-I curves. The non-linear phenomena are precursors of the very sharp transition when high electrical current density is applied. The data also allows to conclude that the electrical current is not homogeneously distributed throughout the sample in this ground state. Differently, the electrical current is localized in thin channels bringing about a large self-heating Joule effect. We argue that the dissipation due to Joule effect is responsible for the self-heating which in turn is large enough to induce phase transition due to the temperature raise. The application of a simple heat dissipation model to the experimental data reveals that both the electrical current localization phenomenon and the Joule effect are very important to the understanding of the current-induced phase transition in these manganites.