Transition metal oxides exhibit potential in various application fields due to the special d-electrons. Solid state chemistry focuses on discovering the structure-property relationships. The work in this thesis mainly discusses compounds with hexagonal or brownmillerite-type structure and their practical properties. Hexagonal YIn[subscript 1-x]Fe[subscript x]O��� (x = 0-0.3, 0.7-1.0) phases have been prepared and characterized. All phases appear to have the ferroelectric structure known for YInO���. The color of the phases changes from yellow to orange to dark red with increasing Fe content. Magnetic measurements confirm high-spin Fe����� for all phases. Similarly, solid solution YAl[subscript 1-x]Fe[subscript x]O��� (x = 0-0.4, 0.7-1.0) phases were successfully synthesized through the sol-gel method. The Al-rich compounds present paraelectric YAlO��� structure while the Fe-rich side samples exhibit YFeO��� structure. The color of the compounds appear to be yellow with small Fe content and change to brown which has higher Fe content. Brownmillerite-type oxides Ba���In[subscript 2-x]Mn[subscript x]O[subscript 5+x] (x = 0.1-0.7) have been prepared and characterized. Magnetic measurements confirm that Mn in as prepared samples is substituting as Mn������ for all values of x with observed paramagnetic spin-only moments close to values expected for two unpaired electrons. Neutron diffraction structure refinements show Mn������ occupies tetrahedral sites for orthorhombic (x = 0.1) and tetragonal (x = 0.2) phases. For Mn ��� 0.3 samples, neutron refinements show the phases are cubic with disordered cations and oxygen vacancies. The colors of the phases change from light yellow (x = 0) to intense turquoise (x =0.1), to green (x = 0.2, 0.3) or dark green (x ��� 0.4). Solid solution Ba���In[subscript 2-x]Fe[subscript x]O[subscript 5+y] (x = 0.1-1.5) also exhibit brownmillerite-type structure. The color of the compounds appear to be green with small Fe content and change to black with higher Fe content (x ��� 0.3). Magnetic measurements and M��ssbauer spectroscopy conclude the mixed valence of Fe�����/Fe������ for all the phases. Nonstoichiometry compound YCu���.���Ti���.���O[subscript 3-��] has been prepared and characterized. Structure study indicates that oxygen vacancy is favored under the synthesis condition. This change in oxygen content was further studied in the Mn-doped system. And the effect of stoichiometric difference in the Mn-doped samples was not as obvious as the initial compound. The disorder in the cation site enhanced the tolerance of the structure in the aspect of oxygen content. The hexagonal phases LnCu���.���Ti���.���O��� (Ln = Y, Tb-Lu) phases were prepared by the traditional solid state reactions. The prepared compounds were reduced at high temperature in the reduction atmosphere created by the H���/N��� gas mixture. Study on the structure and properties changes by reduction was conducted by X-ray diffraction, optical measurement, magnetic measurement and thermalgravimetric analysis. And we observed some evidence of the presence of Cu��� in the reduced phase by these characterization methods. Solid solution YMn[subscript x]Ti[subscript y]O[subscript 3-��] (y = 0.1-0.4) was successfully prepared through conventional solid state approach. All the samples showed hexagonal structure. But the structure transition from ferroelectric P6���cm to paraelectric P6���/mmc occurred when Ti amount is higher than 0.2. Based on the neutron diffraction refinement, the lattice expanded in the ab plane but contracted along the c axis direction. / Graduation date: 2013

Transition metal oxides have proven to be a fertile research area for condensed matter physicists due to the fascinating array of superconducting, magnetic and electronic properties they exhibit. A particular resurgence of intense activity in investigating the properties of these systems followed the discovery of high temperature superconductivity in the cuprates, colossal magnetoresistance in the manganites, ferroelectricity in the cobaltites and simultaneous ferroelectric and ferromagnetic ordering in the manganites. These diverse properties of transition metal compounds arise due to the presence of strong electron-electron interactions within the transition element 3d states. Indeed, it is the competition between the localizing effects of such interactions and the comparable hopping strengths driving the system towards delocalization, that is responsible for these wide spectrum of interesting properties. In terms of theoretical and fundamental issues, electronic structure of transition metal oxides play a most important role, providing a testing ground for new many-body theoretical approaches treating the correlation problem at various levels of approximations. In addition to this rich spectrum of properties, metal-insulator transitions often occur and can even be coincident with structural or magnetic changes due to the strong coupling between charge, magnetic and lattice degrees of freedom. However, in spite of the immense activities in this area, the underlying phenomena is not yet completely understood. A careful investigation of the electronic structure of these systems will help in the microscopic understanding of these and photoelectron spectroscopy has been established as the most powerful tool for investigating the electronic structures of these systems. In this thesis we investigate the electronic structures of some of these transition metal oxides and the metal-insulator transition as a function of electron correlation strength and doping of charge carriers by means of photoelectron spectroscopy; we analyze the experimental results using various theoretical approaches, in order to obtain detailed and quantitative understandings. This thesis is organized into seven chapters. Chapter 1 is a general introduction to the various concepts discussed in this thesis. Here we briefly describe the various mechanisms and theoretical formalisms used for understanding the metal-insulator transitions in strongly correlated systems and the evolution of the electronic structure across the transition. The experimental and the calculational techniques used in this thesis is described in Chapter 2. This includes different sample synthesis techniques and the characterization tools used in the present study. Photoelectron spectroscopic techniques used for probing the electronic structure of various systems are also discussed in this chapter. In Chapter 3, we discuss the coexistence of ferromagnetism and superconductivity in ruthenocuprates by looking at the electronic structures of RuSr2Eu1.5Ce0.5Cu2O10 which is a ferromagnetic superconductor having the ferromagnetic TC ~ 100 K and a superconducting transition of ~ 30 K compared with RuSr2EuCeCu2O10 which is a ferromagnetic (TC ~ 150 K) insulator in conjunction with two reference systems, RuSr2GdO6and Sr2RuO4. The coexistence of ferromagnetic order with superconductivity below the superconducting temperature is an interesting issue since the pair-breaking due to magnetic interactions is not significant in these cases. Extensive photoelectron spectroscopic measurements were performed on these systems and our results show that Eu and Ce in both the ruthenocuprates exists in 3+ and 4+ states, respectively. Also the analysis of the Ru 3d and 3p core levels suggests that Ru remains in the pentavalent state in both the cases. The constancy of Ru valency with doping of charge carriers that bring about an insulator to metal transition and the superconducting state suggests that the electronic structure and transport properties of these compounds are not governed by the Ru-O plane, but by the Cu-O plane, much as in the case of other high TC cuprates. Analysis of the Cu 2p core level spectra in terms of a cluster model, including configuration interaction and multiplet interactions between Cu 3d and 2p as well as that within the Cu 3d states, establish a close similarity of the basic electronic structure of these ruthenocuprates to those of other high TC cuprates. Here the charge transfer energy, Δ << Udd,Cu 3d multiplet-averaged Coulomb repulsion energy, establishing the compounds to be deep in the charge transfer regime. Continuing with the ruthenocuprate systems in Chapter 4, we look at the electronic structure of hole doped La2CuRuO6systems using various photoemission techniques. It was expected that since the substitution of La3+by Sr2+changes the d electron count, the system will undergo a metal to insulator transition, but the transport properties show that all of them remain semiconducting through out the lowest temperature of measurement. A careful analysis of the Ru 3d and 3p core level spectra shows that Ru exists in Ru 4+state in La2CuRuO6and goes towards Ru 5+state with hole doping. This suggests that the doped holes affects the electronic structure of the Ru levels in these systems. A spectral decomposition of the Ru 3d core level suggests the existence of a spin orbit split doublet having two peaks, a main core level peak and a satellite peak at the higher binding energy side of the main peak and the intensity ratio of the satellite peak to the main peak increases with the insulating nature of the compounds as reported for other Ru 4d strongly correlated systems. This observation is also consistent with the transport properties. Cu 2p core level spectra also shows variations in the satellite-to-main peak Cu 2p intensities suggesting that the electronic structure of the Cu levels are also getting affected with Sr doping. Valence band spectral features near the Fermi level shows that the spectral weight is highest for La2CuRuO6and depletes slowly with Sr doping consistent with the expected d electron count as suggested by the Ru valencies. In Chapter 5 and Chapter 6 we discuss the electronic structure investigations of two early transition metal oxide series, namely Ca1−xSrxVO3and Ce1−xSrxTiO3. Surface sensitivity dependence of photoemission experiments has been explored to show that the surface and the bulk electronic structures of Ca1−xSrxVO3system is different. Photoemission spectra of this system using synchrotron radiation reveal a hither to unnoticed polarization dependence of the photoemission matrix elements for the surface component leading to substantial underestimation. Extracted bulk spectra from experimentally determined electron escape depth and underestimation of surface contributions resolve the puzzling issues that arose due to the recent diverse interpretations of the electronic structure in Ca1−xSrxVO3. Keeping in mind the above-mentioned caveat, the present results still clearly establish that the linear polarization of synchrotron radiation plays a key role in determining the spectral lineshape in these systems. The experimentally-determined bulk spectra provide an understanding of the electronic structure in Ca1−xSrxVO3, consistent with experimental γ values, calculated change in the d-bandwidth and the geometrical/structural trends across the series, thereby resolving the puzzle concerning the structure-property relationship in this interesting class of compounds. In Chapter 6 we discuss the issues of metal-insulator transition close to the d0limit as well as the evolution of the electronic structure of a strongly correlated system as a function of electron occupancy, by investigating the family of Ce1−xSrxTiO3compounds by recording core level as well as valence band photoemission spectra using lab source as well as synchrotron radiations. Core level Ce 3d spectra from Ce1−xSrxTiO3samples establish a trivalent state of Ce in these compounds for all values of x confirming that charge doping in the present system does not alter the electronic structure of Ce. Hence the change in valency due to Sr substitution and thus, the carrier number, takes place only in the Ti 3d-O 2p manifold. We also carried out extensive VUV photoemission experiments on these samples with the photon energy varying between 26-122 eV. From the difference spectrum obtained by subtracting the off-resonance spectrum from the on-resonance one, we obtain the Ce 4f spectral signature; thus obtained Ce 4f spectrum which has a peak at about 3 eV binding energy and shows no intensity at EF even for the metallic samples, consistent with a Ce3+state. In order to study the states near EF responsible for the metal-insulator transition in these compounds, we recorded the valence band spectra at the Ce 4f off-resonance condition so that the coherent and the incoherent spectral features arising from the Ti 3d states could be clearly resolved, allowing us to investigate the metal insulator transition in the Ce1−xSrxTiO3system as a function of Sr or hole doping. The experimental spectra of the metallic compounds exhibit an intensity of the incoherent feature considerably larger than that predicted by theory. This discrepancy is possibly due to a difference in the surface and the bulk electronic structures of these compounds. Chapter 7 is divided into two parts. In the first part we discuss the extended x-ray absorption fine structure (EXAFS) studies performed on two transition metal oxide series, La1−xSrxCoO3and La1−xSrxFeO3to look at the local structure distortions happening around the transition metal ions and its role in bringing out metal to insulator transitions in transition metal oxide systems. Here we chose to investigate these two systems since La1−xSrxCoO3undergoes an insulator to metal transition for x ∼ 0.15 and La1−xSrxFeO3remains insulating for the entire range of doping. The static mean square relative displacement, which we believe to be a representation of the disorder present in the system, extracted by fitting the experimental data by a correlated Einstein model, as a function of composition in La1−xSrxCoO3saturates beyond the critical composition where as the disorder parameter continues to increase through out the entire doping range in the case of La1−xSrxFeO3where metal-insulator transition is absent. In the second part of Chapter 7 we discuss the x-ray absorption near edge structure (XANES) studies performed on the above mentioned series of systems. Co K-edge XANES spectra of La1−xSrxCoO3show that there is a systematic shift of the main absorption peak with hole doping suggesting that the Co valency changes systematically with Sr doping. Also, the pre-edge feature of LaCoO3shows the transitions to t2g level clearly showing that Co3+in LaCoO3is not in a pure low spin (t6 2g) state. The Fe K-edge XANES spectra of La1−xSrxFeO3also exhibit a systematic shift to the higher energy side with increase in Sr content, indicating an increase in the Fe valence. Also from the La L3edge analysis, it can be concluded that the oxygen environment around La and the electronic configuration of La are systematically changing with Sr doping.

High dielectric constant materials have tremendous impact on miniaturization of devices that are used in various applications like wireless communication systems, microelectronics, global positioning systems, etc. To store electric charge in a very small space necessarily needs a capacitor with very high dielectric constant. Thus, these materials are very important in fabricating capacitors, or metal oxide semiconductor filed effect transistor (MOSFET). Among the existing commercially available devices, silicon-based microelectronic devices are commonly used based on the moderately stable dielectric constants of silicon with low losses and minimal temperature and frequency dependence. However, now-a-days, the perovskite based transition metal oxides have drawn attention that have the ability to fulfill all the requirements for being a good dielectric material in all the industrial applications. In this thesis we have studied a few selected perovskite based transition metal oxide systems in terms of their dielectric and magnetic behaviour. In Chapter 1, we have have given brief introductions about the some application of dielectric materials and the origin of dielectric and magnetic properties in the materials. We have also discussed about the polarisation in the dielectric materials to understand it’s frequency dependence and also to formalise different relaxation behaviour with the help of physical and mathematical explanation. In Chapter 2, we describe the various methodologies adopted in this thesis. In Chapter 3, we have studied the dielectric behaviour of Nd2NiMnO6, a rare earth based double perovskite ferromagnetic insulator. We successfully synthesised and characterised the compounds, settled the valency issues with the help of temperature dependent XAS of the transition metal atom in contrast to the existing controversy available in literature. We have found that this material shows relaxor kind behaviour with a colossal dielectric constant value. We have studied in details the origin of the colossal dielectric constant and the relaxation behaviour along with the a.c and d.c. transport properties. We have shown the origin of the ferromagnetism (TC ∼ 200 K) with a low temperature antiferromagnetic ordering (TN ∼ 55 K) with the help of detailed studies of temperature dependent d.c., a.c. magnetism and their XMCD. We have also investigated the isothermal variation of magnetodielectric and magnetoresistance behaviour as a function of magnetic field and their origin. In Chapter 4,we study the effect of cation anti-site disorder on the magnetic, dielectric and transport properties of another rare earth based ferromagnetic double perovskite insulator La2NiMnO6 by controlling different extent of anti-site disordered. We have confirmed the valency of the transition metal cations using XAS technique and followed by shown, different types of magnetic interaction between the transition metal cations using d.c magnetic, quantitative XMCD analysis and the origin of large dielectric response, a.c. transport & dielectric relaxation using temperature variation dielectric measurement as an experimental evidence in contrast of our previous speculation published in literature. We further have studied, the coupling between the magnetic and electric spin through isothermal magnetodielectric measurement. In Chapter 5, we have successfully synthesised and characterised a solid solution of YMnxIn1−xO3 series via different mol % of In doping in the multiferroic YMnO3 system. YMnO3 is a well known multiferroic material studied rigorously during past few decades. We have seen, YMnO3 which has a antiferromagnetic ordering temperature of ∼ 75 K suppressed with increasing the dopant concentration In. We have figured out the effect of In doping in the suppression of multiferroic phase and extended it to the dielectric properties. We have found that, the temperature dependence of dielectric constant shows an anomaly at the magnetic ordering temperature and studied magnetodielectric coupling. We have also investigated the temperature variation of dielectric relaxation and a.c. transport behaviour as a function of composition. In Chapter 6, we have identified the phase seperation and proposed a phase diagram as function of Gd doping in the Ho2−xGdxCuTiO6 double perovskite, where two end member, namely Ho2CuTiO6 and Gd2CuTiO6 are found to be in two different crystallographic phase as, hexagonal (P63cm) and orthorhombic (Pnmm), respectively. We have characterised the valency of the transition metal cations using XAS.We have seen very less temperature and frequency dependence of dielectric constant in hexagonal phase in compare to the orthorhombic phase and tried to figuring out from experimental analysis by performing temperature dependence dielectric const measurement. We also have shown the effect of doping in the origin of dielectric relaxation, a.c transport and magnetic behaviour of this system. In Chapter 7, we have synthesised and characterised successfully two different rare earth based layered perovskite La3Cu2VO9 and La4Cu3MoO12 compounds are of centrosymmetric space group. We have figured it of the valency of the different atoms present in the compound using XAS. We also do have observed the good temperature stability of dielectric constant of these materials and explored origin of mechanism in the dielectric relaxation, a.c. transport property by performing the temperature dependance dielectric measurement. The magnetic structure also have shown with the help of d.d. magnetic measurements. In Appendix A, we have seen the very stable dielectric constant constant from very low to above room temperature of the 2D nano PbS. The frequency stability of dielectric constant is also remarkable in compare to bulk PbS values available in literature. We have explored the origin of the conductivity and relaxation mechanism performing dielectric constant measurement. In conclusion, we investigate, in this thesis, dielectric properties of different transition metal oxides system and the mechanism of dielectric relaxation, a.c, d.c transport and their origin of magnetic response.

There was a fame time for silicon in condensed matter physics, then the graphene era came and now topological insulators are gaining lot of attention, but magnetism in condensed matter physics has remained always fascinating starting from the ancient days up to now and it will remain as one of the core topic in basic or applied physics. The improvement in the modern techniques allows one to explore magnetism in different length scales as well as in different time scales. As an effect of the improvement in experimental techniques, different magnetic anomalies are unearthed. As a result theories are getting refined and the area of magnetism progresses. From the material point of view, oxides carry the most diverse nature in condensed matter starting from high temperature superconductivity (HTS), colossal magnetoresistance, metal insulator transition etc. to ferromagnetism (FM), anti-ferromagnetism (AFM), spin glass (SG) and so on. Among this list, SG and HTS are one of the least understood topics in magnetism till today. A large research community is involved in understanding the underlying physics behind these two, especially in transition metal oxides. It has drawn attention not only due to fundamental aspects but also due to various applications in day to day life. This thesis is an attempt to understand these two phenomena in transition metal oxides. As the title of this thesis suggest, it is all about magnetic and magneto-transport properties of certain transition metal oxide (crystalline) addressing the interplay between two competing order parameters to understand the underlying physics behind it from an experimental point of view. We have studied two different kinds of competing interactions: (i) the FM/AFM interplay either in bulk or at the interface of the two layers in thin films; (ii) the interplay between FM and superconductivity (SC) in superconductor (S)/ferromagnet (F) heterostructures. Basically both of these two kinds lead to non-equilibrium phenomena in these oxides. One of such competition is between FM and AFM leading to slow dynamics (glassy physics). Disorder and frustrations are the key ingredients for such slow dynamics. The spin frustration arises either due to geometry or due to competing interactions. For example, in a triangular antiferromagnet due to the triangular geometry spins gets frustrated. Now, if it prevails spin disorder as well then it satisfies both the criteria for a spin glass and hence it gives birth to glassiness. Another kind of competition is the interplay among SC and FM. It is known that SC and FM are two antagonistic quantum phenomena thus in a single material SC (singlet pairing) and FM does not co-exist. However one can realize this by making F/S heterostructures and observe the battle between these two competing order parameters. The spin polarized quasiparticle injection from F creates non equilibrium spin density inside S and thereby suppressing the order parameter of S. Also by choosing an appropriate ferromagnet the vortex motion inside S can be arrested to certain extent which can enhance the critical current density of S. Thus FM/SC interplay has become an alternative way to look at the high temperature superconductivity. This thesis is categorized into nine chapters. The summary of each chapter is as follows: Chapter: 1 contains certain concepts of magnetism and superconductivity which is useful to understand the topics and experiments described in this thesis. Chapter: 2 gives the underlying principles of the various experimental techniques used in this thesis. Chapter: 3 describes the magnetic properties of successfully synthesized five compositions of LixNi(2-x)O2 (0.67<x<0.99) which has five distinct ground states namely antiferromagnet (AF), spin glass (SG), cluster glass (CG), re-entrant spin glass (RSG) and ferromagnet (FM). The SG and CG ground state has been well described by the frequency dependent peak shift. From the power-law divergence of critical slowing down the estimated value of relaxation time indicates the presence of interacting macro spins (spin cluster) rather than individual spins in certain LixNi(2-x)O2 samples possessing CG ground state which is also supported by the Arrhenius law. The shift in the spin freezing temperature with the application of dc field obeys Almeida-Thouless line. It also exhibits memory effect which is generic to the slow dynamics. The remnant magnetization relaxation follows logarithmic decay. Interestingly, the sample having RSG ground state shows memory effect up-to 50K and behaves like a FM above that temperature. FC-ZFC M(T) curve shows a splitting at the ordering temperature. The critical analysis across the ferromagnetic-paramagnetic phase transition yields a self-consistent γ, β and δ value and the spin-spin interaction in this material follows long range mean field model. The critical exponents obey Widom scaling law: δ = 1 + γ β −1. The universality class of the scaling relations is also verified where the scaled m and scaled h collapses into two branches. Finally the magnetic phase diagram illustrates a vivid picture of the gradual evolution of ferromagnetism in LixNi(2-x)O2 through a glassy state. As a concluding remark, we think, the present study of glassy physics in magnetic insulator/semiconductor sets an example to compare them with the conventional metallic spin glass system. Chapter: 4 exhibits the results of the structural, magnetic and transport measurements to elucidate some of the most striking unusual physical responses of bulk SrRuO3. Two set of polycrystalline SrRuO3 samples with unique ordering temperature have been synthesized. In one case, we have taken the stoichiometric weight ratio of precursors that eventually resulted in Ru-deficient SrRuO3(SROD). In the other case, we have taken extra 2% wt. RuO2 deliberately to form stoichiometric SrRuO3(SRO). Both the samples are found to crystallize in orthorhombic crystal structure with Pnma space group. The low temperature magnetization is found to be well described by the Bloch T3/2 law and the magnetization near Tc is found to follow the scaling law; M~(Tc-T)β with β=0.35 and β=0.30 for SRO and SROD respectively, apparently showing the 3D Ising behaviour. This aspect will be elaborated in the next chapter. The magnetic ac susceptibility study exhibits a broad hump far below the ferromagnetic ordering temperature and the frequency dependence of this hump position exhibits the characteristics of multiple relaxations. Most strikingly, we notice a low temperature glassy magnetic behaviour clearly demonstrated by the time dependent memory effect. This is very surprising and unlikely to happen in systems, which have itinerant ferromagnetic character. However, we conjecture that slow domain growth and spin canting could be the cause for such effect. The transport study evidences a crossover from Fermi liquid (FL) to non-Fermi liquid (NFL) behaviour around 40 K and a slope change in dρ/dT vs. T plot in the vicinity of that temperature. Astonishingly, we observe two distinct dips (one around ferromagnetic ordering temperature and the other far below the ferromagnetic ordering temperature) in the temperature dependent MR response. In addition, we also observe the signature of an unusual dip in the temperature dependent coercive field towards low temperature side. The emergence of such unusual magnetic and transport response is strongly believed to be connected with hidden magnetic interactions. Our effort on neutron diffraction study has been able to trace the cause of such cryptic magnetic interaction. The findings of neutron diffraction study evidence the change in the unit cell lattice parameters around 75 K and that could be the central cause behind such anomalous low temperature magnetic responses. It also demonstrates that the octahedral tilt freezes around the FM transition and has a minimum around the low temperature glass transition temperature. Most remarkably we observe a decline in the total integrated magnetic intensity towards the low temperature side indicating the presence of antiferromagnetic like interaction in SrRuO3. Chapter: 5 resolves the ambiguity in determining the crritical exponents in SrRuO3. Most remarkably, the application of scaling law in the FC magnetization leads a systematic change in the values of critical exponent with the measuring field in SRO. The β value changes from 0 to o.44 to to 0.29 (corresponds to mean field to Ising) with the increase in the measurement field from 10 to 2500 Oe. However, the H→0 extrapolation fields β=0.5. In order to substantiate the actual nature, the critical behavior is studied across the phase transition from the M-H isotherms. The critical analysis yields a self-consistent β, γ and δ values and the spin-spin interaction follows long range mean field δ=γ β model 1+. The critical exponents also obey Widom scaling law: δ = 1 + γ β-1 The universality class of the scaling relations is verified where the scaled m and scaled h collapses into two branches. We have also found that Ru deficiency does not affect the nature of the spin-spin interaction (though ferromagnetism gets reduced). Further the directional dependence of the critical exponent reflects the isotropic nature of the magnetic interaction. In other words the spin-spin interaction found to be: i) three dimensional, ii) long range, iii) mean field type and iv) isotropic in SrRuO3. We have also found magnetocaloric effect (calculated from the M-H isotherms) that across the phase transition. The specific heat measurements find sharp jump at the ferromagnetic transition due to the magnetic contribution of the specific heat. Chapter: 6 describes the magnetism at the SrRuO3 (SRO)/LaAlO3 (LAO) interface where SRO is an itinerant ferromagnet (FM) and LAO is non-magnetic (NM) (rather diamagnetic). Most surprisingly SRO/LAO exhibits pronounced exchange bias (EB) effect realized by observing a shift in the field cooled M-H hysteresis. Further investigation results an increasing trend of the strength of the EB with the decreases in the thickness of ferromagnetic layer. This system also displays the training effect which essentially confirms that this effect is due to EB. EB arises due to the uncompensated spins at the FM/AFM interface hence the EB effect in SRO/LAO system is unconventional. However, the origin of such AFM interaction (responsible for EB effect in FM/NM system) at SRO/LAO interface is realized and explained through the temperature dependence of the EB effect. Further, we have extensively investigated EB effect in other analogous ferromagnets, FM/FM bilayers and FM/FM superlattices. We found that La0.7Sr0.3MnO3 (LSMO) grown on LAO exhibits the signature of EB. In contrast to that La0.5Sr0.5CoO3 (LSCO) does not show any signature of EB. All the bilayers (LSMO/SRO, LSMO/LSCO and LSCO/SRO) exhibit EB and have similar kind of temperature dependence. In order to gain more insight we have grown a (LSMO/SRO)8 superlattice and observed a complex magnetic behaviour. It exhibits partial inverted magnetic hysteresis. But the system shows EB effect characterized by the shift in the FC hysteresis and training effect. All these observations essentially demonstrate that the magnetic nature of various ferromagnetisms at the interfaces can be changed by choosing a proper partner (acts like adding perturbations into one of those system which lies close to the instability region). Chapter: 7 presents the magneto-transport properties of three SRO films grown on LAO (100) of thicknesses of 12, 24 and 48 nm are studied extensively. For a one to one comparison one of the sample is also grown on STO(100). The coercivity vs. temperature in SRO(48 nm)/LAO exhibits a plateau at ~40 K. The dR/dT exhibits the low temperature hump in all the samples which very much replicates with the bulk scenario that we observed in SRO. Most strikingly the 12 nm SRO sample exhibits NFL behaviour throughout the temperature range of measurement (10-150 K). Our careful investigation reveals a cross-over from FL to NFL in all SRO thin films. The cross-over temperature increases with the increase in thickness and eventually shifts towards the bulk cross-over value. It is apt to remind that in bulk SRO we have demonstrated (by employing temperature dependent neutron diffraction) that there is a presence of antiferromagnetic like interaction at low temperature giving birth to glassiness in bulk SRO. Further, an attempt is made to understand the low temperature magneto-transport anomaly by looking into the spin fluctuation through the low frequency 1/f noise measurements. It conveys a message that there are two types of magnetic ordering present in SRO giving rise to two peaks in the temperature dependence of the relative variance. Application of magnetic field suppresses both the peaks in the relative variance. This certainly indicates that the origin of such peak is caused by the spin fluctuations and thereby it is of magnetic origin. Further we have looked into the Hall effect of a structured (Hall patterned) SRO thin film and observed regular Hall effect (RHE) as well as anomalous Hall effect (AHE) in it. Most remarkably the temperature dependence of the RHE coefficient changes its sign close to the ferromagnetic transition temperature of SRO. This implies a change of the type of the carrier as the temperature is varied. Based on these results, the carrier concentration of SRO as a function of temperature is determined. Chapter: 8 is about the magnetic and magnetotransport studies on the successfully grown high quality S/F heterostructures. The oxygen content plays a vital role in superconductivity of oxide materials thus for studying FM/SC interplay in oxides we have discussed how to achieve a high quality sample (oxygen stoichiometric). We have observed a great influence of a FM in suppressing the superconductivity in YBa2Cu3O(7-δ) (YBCO) in FM/SC heterostructures. The analysis of the out of plane M-H hysteresis reveals a significant reduction of the critical fields (HC1 and HC2) of the SC (in SRO/YBCO bilayer) which might have a great significance to understand the superconductivity in a better way (from both the perspectives: theory and experiments). Most remarkably we have found 40% enhancement of the critical current density of YBCO in SRO/YBCO bilayer. We have demonstrated that in order to see the effect of spin polarizes quasiparticle (SPQP) injection into YBCO, one should not apply more than 20mA current since Joule heating contribution wins over pair breaking effect. The SPQP injection from SRO into YBCO exhibits pair breaking effect as the TC (of the SC) shift follows I2/3 law. The resistive transitions under various applied magnetic fields and the field dependence of the activation energy confirms that the vortices are in the 2D regimes (it follows power law, U0~Hα withα=0.5) in SRO/YBCO. To get a better insight into the FM/SC interplay we have looked into two of the FM/YBCO combinations (LSCO/YBCO and LSMO/YBCO). We observe that the degree of the spin polarizations of the FMs scales with the suppression of superconductivity in YBCO which means more the spin polarization more is the suppression. We have also found out that spin polarization is not the sole parameter in suppressing superconductivity in SRO/YBCO bilayers. It also depends upon the state of magnetization of the ferromagnet. Further, we observed a significant reduction (one order) of the activation energy in LSCO/YBCO compared to SRO/YBCO which clearly indicates that the vortex dynamics might depend on other aspects as well (of the FM). It also reveals the formation of decoupled pancake vortices (pure 2D regime) in LSCO/YBCO and LSMO/YBCO bilayers whereas in case of YBCO and SRO/YBCO it is of 2D coupled type. Chapter: 9 summarizes the whole work presented in this thesis. It also discusses about few research problems which one need to look at in future.

CoordenaÃÃo de AperfeiÃoamento de NÃvel Superior

Thesis (M.Sc. (Physics)) -- University of Limpopo, 2021 / The stead-fast demand for sustainable lithium-ion batteries (LIB) with competitive electrochemical properties, safety, reduced costs, and long-life cycle, calls for intensive efforts towards the development of new battery cathode materials. The layered transition metal oxides formulated LiMO2 (M: Mn, Ni and Co) have attracted considerable attention due to their capability to optimize the discharge capacity, cycling rate, electrochemical stability and lifetime. The transition metals Mn, Ni and Co (NMC) have been reported to contribute towards enhancement of the performance of NMC based lithium-ion batteries. In this work, the electronic properties of transition metal oxides LiMO2 (M: Mn, Ni and Co) as individual crystal structures are studied using density functional theory (DFT+U) in the local density and generalized gradient approximation (LDA and GGA). The Hubbard U values together with the low spin transition metal in 3+ charge state (Mn3+, Ni3+ and Co3+) predicts the electrical conductivity of the materials. The conductivity is associated predominantly with 3d states of the transition metals (Mn, Ni and Co) and 2d character in oxygen. The LiNiO2 material is high in conductivity, while both LiMnO2 and LiCoO2 are low in electrical conductivity. All independent elastic constants satisfy the mechanical stability criterion of orthorhombic materials implying stability of the materials. However, the phonon dispersion curves display imaginary vibration along high symmetry direction for LiCoO2. The heats of formations predict that the LiNiO2 is the most thermodynamically stable material while the LiMnO2 is the least thermodynamically stable material. The derived interatomic potentials produced NiO and CoO structures with a difference of less than 1% and 9% respectively, from the experimental structures. The structures were melted at temperatures close to their experimental values from molecular dynamics. The radial distribution curves and Nano architectures presented the melting point of NiO and CoO at 2250K and 2000K respectively. All independent elastic constants satisfy the mechanical stability criterion of cubic materials implying stability of the materials. The high electrical conductivity and thermodynamic favourability LiNiO2 suggests that the material can be the most recommendable material as a cathode material and further improved through doping. This will add the overall enhancement of the electrochemical performance while stabilizing structural stability of the cathode material in high energy density Li-ion batteries. / National Research Foundation (NRF)

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

In the 4d transition metal oxides, the extension of the 4d orbitals leads to comparable and thus competitive kinetic and coulomb energies. As a result, small perturbations can induce significant changes in their physical properties, giving rise to a class of exotic phenomena that are rarely found in other materials. The ruthenates materials with readily tunable parameters open an avenue to study the strong electronic correlation in the rarely explored territory: the 4d transition metal oxides. The bilayered system, Ca3Ru2O7, belongs to the Ruddlesden-Popper series in which the physical properties are intimately linked to the lattice degrees of freedom. Ca3Ru2O7, with its quasi-2D and severe structure distortion, is believed to be placed in a unique position at which the role of orbital degrees of freedom is highlighted. The system displays strikingly different behaviors when the field is applied along different crystalline axes. A ferromagnetic (FM) state with full spin polarization is achieved for B||a-axis, but colossal magnetoresistance is realized only for B||b-axis by avoiding the ferromagnetic state. In addition, for B rotating within the ac-plane, slow and strong SdH oscillations periodic in 1/B are observed for T.1.5 K in the presence of metamagnetism. For B|| [110], oscillations are also observed but periodic in B (rather than 1/B) and persist up to 15 K. These properties together with highly unusual spin-charge-lattice coupling near the Mott transition (48 K) are driven by the orbital degrees of freedom. Complex thermodynamic properties are also observed in the other ruthenates system such as Sr4Ru3O10 and Pr3RuO7. The Sr4Ru3O10 is a triple-layered system that shows a dedicate balance between fluctuations and order. Besides the anomaly at TC=102K, anomalous behavior at low temperatures are also observed in the thermal study, indicative of an unusual magnetic order in this material. The Pr3RuO7 shows one-dimensional structure with zig-zag chain of corner sharing RuO6 octahedra running in parallel with the rows of edge-shared PrO8 pseudo-cubes. Magnetic and thermal properties studies on its single crystals indicate that the exchange interaction is strongly anisotropic. A Schottky-type anomaly at low temperature suggests that the gorderedh chain Pr ions are still sensitive to a crystal field.

Neutron scattering measurements were used to investigate the magnetic and crystal structure and magnetic excitations of three compounds characterized as low-dimensional quantum magnets. The materials are frustrated systems with low spin quantum number. The first was a powder sample of AgNiO₂. The Ni ions form a triangular lattice antiferromagnet in which, according to the published crystal structure, both the orbital order and magnetic couplings are frustrated. However, it is shown here that there was a small distortion of the crystal structure at 365 K, which is proposed to result from charge disproportionation and this relieves the orbital frustration. The magnetic structure was investigated and, below 20 K, the triangular lattice of electron-rich Ni sites was observed to order into antiferromagnetic stripes. Investigations of the magnetic excitations showed that the main dispersions were within the triangular plane, indicating a strong two-dimensionality. The dispersion was larger along the stripes than between the stripes of collinear spins. The second material investigated was CoNb₂O₆, a quasi Ising-like ferromagnet. It was studied with a magnetic field applied transverse to the Ising direction. The magnetic field introduced quantum fluctuations which drove a phase transition at a field comparable to the main exchange interaction. The phase diagram of the magnetic order was mapped outs and a transition from an ordered phase to a paramagnetic phase was identified at high field. This low-temperature high-field phase transition was further investigated by inelastic neutron scattering measurements to observe the change in the energy gap and magnetic excitation spectrum on either side of the transition. The spectrum had two components in the ordered phase and had sharp magnon modes in the paramagnetic phase. The third material was the spin-half layered antiferromagnet CuSb₂O₆. It has a square lattice of Cu²⁺ ions in which the main interaction is across only one diagonal of the square. The magnetic structure was studied by neutron scattering with a field applied along the direction of the zero-field ordered moment. A spin-flop was observed at low field and there was evidence for a high-field transition. The magnetic excitation spectrum was unusual in that it had an intense resonance at 13 meV at the magnetic Brillouin zone boundary.

Zhang Lizhi. / "July 2003." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2003. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. Ann Arbor, MI : ProQuest Information and Learning Company, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.