Rozprawy doktorskie na temat „Magnetic Properties - Exotic Transition Metal Oxides”

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.

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.

The discovery of new composites by integrating materials of different physical properties with optimal control is of immense interest to researchers at present. Today, there are several composites being used for several applications. The list of composites and their applications is endless from toys to space applications in today's life, as it is a very broad area of research. Composites are made up of the combinations of two or more materials in which one of the materials, so called reinforcing phase in the form of fibers, tubes and particles which are incorporated in the other called matrix phase. The main functions of the matrix are to transfer the stresses between the reinforcing fibers or particles, and to protect them from the mechanical and environmental damage. This enhances their mechanical properties like strength and stiffness. A composite is therefore one synergistic combination of two or more phases which is superior to their individual phases due to more physical and chemical properties. Ceramic composites have successfully replaced many traditional ceramics and metals in several applications due to their light weight and high strength, high tensile strength at elevated temperatures, high creep strength and toughness. Typically, composites can be properly designed and manufactured, by the appropriate combination of the strength of the reinforcing phase and the toughness of the matrix. Such ceramic composites can be more capable to give the desirable properties, which is not possible with a single conventional ceramic. Polymer matrix composites and metal matrix composites have a large number of applications in many fields. However, there are certain issues such as homogeneity of fillers (particles or fibers), recycling, lack of stability, low mechanical and thermal strength, very high coefficient of thermal expansion, etc. The disadvantages of these composites are the difficulty in the production of fiberreinforced composites and their increased labor cost. Ceramic matrix composites are more significant over single phase ceramics, metals matrix and polymer matrix composites in some applications due to their high fracture toughness and high resistant to thermal shocks. They are used in the field of automotive industry, renewable or alternative energies, healthcare, electronics and telecommunications, aerospace, gas sensors and in many high temperature applications. These are based on the combination of physical properties and are referred to as bio-ceramics, electroceramics, magneto˗ceramics, opto˗ceramic, multiferroics and catalysts, etc. Several materials like carbon, graphene and metal oxides have been used to produce composites with different combinations to get superior physical properties. In the present work, the conducting and magnetic metal oxide mixtures were preparation and examined as metal oxide composites. The electrically conductive lanthanum nickelate (LNO) was prepared as the conducting matrix. Ferrites of spinel cubic structure like CoFe2O4, Ni Fe2O4 and barium hexaferrite BaFe12O19 were prepared as magnetic phase. The synthesis, structural, morphological and compositional studies of lanthanum nickelate (LaNiO3) and Co, Ni, Ba ferrites were carried out. The electrical conductivity of LaNiO3 and the magnetic properties of ferrites were investigated at room temperature. Three nanocomposite systems of LaNiO3 with different ferrites were prepared. All composites were investigated for their structural, morphological and compositional studies. The electrical and magnetic properties of composites were investigated. The study of these composites was further extended for electromagnetic interference (EMI) shielding to test shielding effectiveness. The main results on electrical conductivity, magnetic properties and EMI shielding of nanocomposites are briefly summarized. In the thesis, major findings on this work are discussed. The composites of conducting and magnetic metal oxides have not yet been studied and reported. The electrical and magnetic properties of composite materials can be finely tuned by varying concentration of reinforcing phase into conducting matrix and these materials are explored in order to search possible application. This work mainly focuses on the i) Preparation of new composites using conductive and magnetic metal oxides, ii) Tuning of electrical conductivity and magnetic properties, iii) To study composites for EMI shielding, iv) To check the possibilities of applications in the field of electronics and v) To explore the surface and interface physics of hetero structure. Electromagnetic interference (EMI) is a fast growing problem in the modern era of electronics, telecommunication and in various instruments. It has become a critical area to be considered in electronic design and packaging. The increasing usage of large number of electronic devices and the need of increasing processor frequencies, the environment is becoming noisy due to the increasing electromagnetic fields. Therefore, it is necessary to prevent the unwanted EM waves with the adequate EMI shielding. The desire of high performance shields with the reduction in size, weight and price had been a great interest of researchers to discover new materials as suitable candidate for electronic housing. Presently, there are several types of housings and EMI shields are made up of polymer composites and thin metal or metal-alloy sheets work to protect devices from electromagnetic waves. As a part of present thesis work, we have studied all above composites for the EMI shielding measurements in the frequency range of 8 to 18 GHz. It is observed that each composite system showed a very high shielding effectiveness.

Perovskites with the general chemical formula ABX3 can be categorized into oxides and halides depending on the nature of the X anion. 3D organic-inorganic halide perovskites are extensively studied in the context of solar cell and photo- and electro-luminescence applications due to their outstanding optoelectronic properties, while oxide perovskites have attracted a great deal of attention for their many interesting physical properties such as structural, electrical, magnetic, and magnetocaloric effects. More recently, 2D layered organic-inorganic hybrid (OIH) materials have emerged as a new class of materials with rich optoelectronic properties. They exhibit proven advantages over their 3D counterparts due to their large structural diversity and improved environmental stability against heat and moisture. 2D OIH materials have exhibited many interesting physical properties such as high exciton binding energies, intense photoluminescence, ferroelectricity, and chiro-optical properties. While the Lead-based hybrid perovskites have been very well studied for their spectacular optoelectronic properties, lately there have been attempts to design new materials such as Cd2+, Cu2+, Sn2+ -based hybrid halide materials which offer a new playground in the field of photovoltaic research. The work reported in this thesis explores the ferroelectric properties of Cd2+ - and Cu2+ -based halide materials and discusses the possible microscopic mechanisms for the origin of ferroelectricity in these materials. Further, using experimental and theoretical inputs, it is shown that the Cu2+ -based hybrid materials have outstanding chiro-optical properties. In addition, we also explore the interesting magnetic properties of a few transition metal compounds, exhibiting inverse magnetocaloric effect as well as Griffiths phase in some temperature ranges. Chapter 1 briefly introduces various concepts relevant to the investigations reported in subsequent chapters of this thesis. The present status of the research in the field of 2D organic- inorganic hybrid materials with an emphasis on various exciting properties such as ferroelectricity, bandgap modulation, and chiro-optical properties has been discussed. This chapter also presents discussions relevant to the family of oxide perovskites with reference to magnetic properties from fundamental and technological standpoints. Chapter 2 describes different experimental and theoretical methods that were employed to carry out the studies presented in this thesis. Chapter 3 presents a detailed study of the successive structural phase transitions of BA2CdCl4. These results establish that these structural phase transitions are associated with intrinsic ferroelectric transitions, from room temperature paraelectric to intermediate temperature ferroelectric, followed by another low-temperature ferroelectric phase. It was widely believed that ferroelectricity in these 2D organic-inorganic hybrid materials originated due to the order-disorder transition of molecular dipoles associated with the organic spacer cations. However, results in this thesis show that there are dipoles also associated with the inorganic components due to local structural distortions. The thesis presents a combination of experimental and theoretical results suggesting that the dipoles associated with the organic spacers and the dipoles originating from these local structural distortions within the inorganic units both play significant roles in the observed ferroelectricity of BA2CdCl4. Chapter 4 deals with the chiro-optical properties of quasi 2D (R-/S-MBA)2CuBr4 hybrid material. We discussed the role of chiral organic amine cations on the optical properties of these hybrid materials. Few Lead based compounds and one copper-based sample show giant chiro-optical properties but these are chirally active only for wavelengths < 490nm, limited by their large bandgaps. (R-/S-MBA)2CuBr4 shows a relatively large chiro-optical property in the orange-red part of the visible spectrum. Structural analyses of these compounds show that these are made of alternating layers of the chiral organic units and an inorganic layer of isolated CuBr4 units. Such isolated inorganic units distinguish this class of compounds from the more intensely investigated hybrid lead halide systems where the basic PbX6 (X = Cl, Br, or I) units are linked together by corner sharing of the halide ions, making them intrinsically 2D systems. The present Cu-based system would have qualified as a 0D system but for the Cu-Br….Br-Cu interactions that allow the separated CuBr4 units to interact, making the system a quasi-2D system. This subtle structural aspect plays an important role in giving this system remarkable chiro-optical properties. The semi-isolation of the CuBr4 units allows them to be rotated along the 21 screw-axis by the chiral organic units via strong hydrogen bonding, thereby imparting the giant chirality to the entire hybrid system. Simultaneously, the connectivity of the CuBr4 units via Br…Br interactions imparts a quasi-2D character helping to achieve a broadband absorption, thereby extending the chiro-optical properties to longer wavelengths. Chapter 5 discusses tuning of the bandgap while retaining ferroelectricity through halide substitution in Cu2+ -based chiral 2D hybrid materials to obtain small bandgap ferroelectric materials. Search for such small bandgap ferroelectric materials has been popular in the literature, not only because most ferroelectrics tend to have large bandgaps but also because of their obvious applications in solar photovoltaics. The ferroelectric Lead-based hybrid perovskites have been very well studied for their rich optoelectronic properties that are relevant in photovoltaic applications, but all are having a relatively higher bandgap. We have lowered the ligand to metal charge transfer bandgap from ~2.53 to 2.09 eV while retaining ferroelectricity in a copper chloride based low-dimensional hybrid material through partial substitution of chlorine with bromine. Our results show that a complete substitution of Cl- by Br- leads to a bandgap of ~1.62 eV with a loss of the ferroelectric state in the pure bromide material. Chapter 6 discusses the low temperature magnetic state along with the main ferromagnetic ordering at ~200 K and the magnetocaloric effect of double perovskite, Nd2NiMnO6. An earlier study on this compound established that for any applied magnetic field lower than 3 T, these samples show a downturn in M(T), and any field higher than 3 T, shows an upturn in M(T) for the temperature range below 100 K. This has been interpreted as Nd moments experience an effective ~3 T internal magnetic field due to the presence of the ordered Ni-Mn ferromagnetic sublattices. This indicates that the low temperature magnetic state of this compound is easily influenced by an externally applied magnetic field in the tune of 3 T, suggesting possible interesting magnetocaloric effects in this material. This chapter presents a detailed study of the magnetocaloric properties of Nd2NiMnO6. Interestingly, it shows a significant inverse magnetocaloric effect (IMCE) at low temperatures (T < 50 K) together with a significant conventional magnetocaloric effect (CMCE) at the ferromagnetic ordering temperature (Tc ~200 K). IMCE and CMCE correspond to the antiferromagnetic arrangement of Nd and Ni–Mn sublattices and ferromagnetic ordering of Ni–Mn sublattices, respectively. Nd2NiMnO6 with its second order phase transition follows the universal behavior of magnetic entropy change, ΔSM(T); it also shows a power law dependency on the magnetic field as ΔSM ∝ 𝐻𝜂. Chapter 7 deals with Griffiths phase-like magnetic anomalies in disordered La0.85Sr0.15CoO3 induced by chemical doping. In earlier studies on doped LaCoO3 with doping concentration higher than the percolation threshold (18%) shows non-Griffiths phase-like behavior. But no reports are available for the composition just below the percolation limit in this context. So, we chose the 15% doping concentration and explored its magnetic properties carefully. Our results establish that this composition shows typical Griffiths phase-like behavior in the intermediate temperature range and followed by spin glass behavior below 60 K. The existence of nanoscale ferromagnetic clusters below 240 K contributes to the total magnetization of the system for low applied magnetic fields resulting in a downturn of the χ-1 vs. T plot. The extent of this downturn is strongly suppressed by increasing the dc applied magnetic field, a typical signature of Griffiths phase. In the appendix, we present results of investigating Cs- and Na-doped WO3 exhibiting strong absorption in the near-infrared (NIR) and transmittivity in the visible range. Despite several publications, there is a lack of agreement in the community on the origin of this strong optical absorption with competing claims of polaronic and plasmonic origins. We address this controversy by first investigating bulk samples that are relatively free of complications arising from any shape anisotropy; we show by combining experimental and theoretical results that all spectral features in both bulk and nanoparticle samples are consistent with plasmonic excitations, without any need to invoke a polaronic mechanism. Doped WO3 exhibits strong optical absorption primarily due to surface plasmon resonances in colloidal nanoparticles, while their bulk counterparts are dominated by bulk plasmonic features. Investigating systems with different crystal structures and charge doping levels, we established that the complex spectral features of these plasmonic absorption bands for both bulk and nano samples are dominated by the underlying structure-dependent anisotropic electronic properties that determine the plasmonic features

The combined electronic, optic and magnetic properties of transition metal (TM) implanted ferromagnetic TiO2 is of interest for spintronic applications. The nature of the observed abundant ferromagnetism in such materials has been investigated for more than one and a half decades, yet still no clear explanation for its appearance can be given. In this thesis, the origin of the ferromagnetic order in TM:TiO2 systems is studied by investigating the interplay between structural order, defects and incorporation of implanted ions within the host lattice. The defect properties of the host TiO2 are altered by preparing different microstructures of TiO2 (e.g. amorphous, polycrystalline anatase and epitaxial anatase). The difference in microstructure is also found to influence the incorporation of the implanted ions with the host lattice. The crystallographic incorporation of the implanted TM atoms is found only in crystalline films. Moreover, it is observed that the suppression of the dopant related secondary phases can also be achieved by changing the microstructure. The obtained experimental results are compared with the existing theoretical frameworks, while the most relevant one describing our findings is elucidated. Based on this discussion, we propose an ideal microstructural candidate for a dilute magnetic oxide material based on our results.:1 Introduction 2 1.1 Spintronics 2 1.2 Dilute magnetic oxides 5 2 Fundamentals 13 2.1 Introduction 13 2.2 Magnetism in diluted magnetic oxides 13 2.2.1 Possible locations of dopant 3d ions in an oxide matrix 14 2.2.2 Mean field model 15 2.2.3 Bound magnetic polaron model 16 2.2.4 Charge transfer model 17 2.2.5 Ferromagnetism in undoped oxides 19 2.2.6 Extrinsic sources of ferromagnetism 20 2.3 Motivation 21 3 Experimental 24 3.1 Sample preparation 24 3.1.1 DC magnetron sputtering 24 3.1.2 Ion implantation 27 3.2 X-ray methods 29 3.2.1 X-ray diffraction 29 3.2.2 X-ray absorption 31 3.2.2.1 Synchrotron radiation 31 3.2.2.2 Physics of X-ray absorption 31 3.3 SQUID magnetometry 35 3.3.1 Avoiding magnetic contamination 37 3.4 Positron annihilation spectroscopy 38 4 TM Implantation into Different TiO2 Structures (TM = Co, Mn, V) 43 4.1 Experiments 43 4.2 Co+ implantation: from diluted paramagnetism to superparamagnetic clusters 45 4.2.1 Cluster-free Co+-implanted TiO2 thin films 45 4.2.1.1 Experiments 45 4.2.1.2 Structural properties 46 4.2.1.3 Implantation-induced structural defects 47 4.2.1.4 Magnetic properties 50 4.2.1.5 Local environment of implanted Co atoms 51 4.2.1.6 Summary 52 4.2.2 Revealing nano-clusters within Co+-implanted TiO2 thin films 54 4.2.2.1 Experiments 54 4.2.2.2 Structural properties 54 4.2.2.3 Magnetic properties 57 4.2.2.4 Local environment of implanted Co atoms 59 4.2.2.5 Summary 61 4.3 Mn+ implantation: from a non-magnet to a ferromagnet 63 4.3.1 Experiments 63 4.3.2 Relation between lattice damage and defects 64 4.3.3 Electrical transport properties 65 4.3.4 Local environment of implanted Mn atoms 66 4.3.5 Magnetic properties 67 4.3.6 Summary 69 4.4 V+-implanted TiO2 thin films 71 4.4.1 Experiments 71 4.4.2 Magnetic properties 71 4.4.3 Summary 74 5 The effect of the open volume defects on the magnetic properties of V:TiO2 films prepared by doping during deposition 76 5.1 Experiments 76 5.2 Structural Properties 77 5.3 Investigation of the open volume defects 78 5.4 Magnetic, optical and electrical properties 79 5.5 Summary 81 6 Conclusions 84 6.1 Defects in TiO2 84 6.2 Formation of secondary phases 85 6.3 Evolution of the ferromagnetism in different microstructures of TiO2 87 7 Acknowledgments 91
Die kombinierten elektrischen, optischen und ferromagnetischen Eigenschaften von TiO2, welches mit einem Übergangsmetall (TM) dotiert wurde, sind für Anwendungen in der Spintronik von hoher Bedeutung. Obwohl dieses Material seit mehr als anderthalb Jahrzehnten untersucht wird, kann derzeit noch keine eindeutige Erklärung für den beobachteten Ferromagnetismus gegeben werden. In dieser Arbeit wird die Ursache für die ferromagnetische Ordnung in TM:TiO2-Systemen untersucht, indem der Zusammenhang von struktureller Ordnung, Defekten und der Einlagerung der implantierten Ionen im Wirtsgitter analysiert wird. Durch die Verwendung unterschiedlicher Mikrostrukturen (z.B. amorphes, polykristalliner Anatas und epitaktischer Anatas) wurden auch die Defekteigenschaften des Wirts-Titanoxid variiert. Dabei zeigte sich ein Einfluss der unterschiedlichen Mikrostrukturen auf die Einlagerung der implantierten Atome in das Wirtsgitter. So konnte die Substitution von Ti-Atomen durch Atome des dotierten Übergangsmetalls nur in kristallinen Filmen beobachtet werden. Weiterhin wurde herausgefunden, dass die vom Dotanden hervorgerufenen Sekundärphasen durch die initiale Mikrostruktur unterdrückt werden können. Die experimentellen Ergebnisse wurden mit aktuellen Theorien verglichen. Zusammenfassend wird ein Überblick über die wichtigsten Ergebnisse gegeben, auf Basis welcher eine optimale Mikrostruktur für ein verdünntes magnetisches Oxid vorgeschlagen wird.:1 Introduction 2 1.1 Spintronics 2 1.2 Dilute magnetic oxides 5 2 Fundamentals 13 2.1 Introduction 13 2.2 Magnetism in diluted magnetic oxides 13 2.2.1 Possible locations of dopant 3d ions in an oxide matrix 14 2.2.2 Mean field model 15 2.2.3 Bound magnetic polaron model 16 2.2.4 Charge transfer model 17 2.2.5 Ferromagnetism in undoped oxides 19 2.2.6 Extrinsic sources of ferromagnetism 20 2.3 Motivation 21 3 Experimental 24 3.1 Sample preparation 24 3.1.1 DC magnetron sputtering 24 3.1.2 Ion implantation 27 3.2 X-ray methods 29 3.2.1 X-ray diffraction 29 3.2.2 X-ray absorption 31 3.2.2.1 Synchrotron radiation 31 3.2.2.2 Physics of X-ray absorption 31 3.3 SQUID magnetometry 35 3.3.1 Avoiding magnetic contamination 37 3.4 Positron annihilation spectroscopy 38 4 TM Implantation into Different TiO2 Structures (TM = Co, Mn, V) 43 4.1 Experiments 43 4.2 Co+ implantation: from diluted paramagnetism to superparamagnetic clusters 45 4.2.1 Cluster-free Co+-implanted TiO2 thin films 45 4.2.1.1 Experiments 45 4.2.1.2 Structural properties 46 4.2.1.3 Implantation-induced structural defects 47 4.2.1.4 Magnetic properties 50 4.2.1.5 Local environment of implanted Co atoms 51 4.2.1.6 Summary 52 4.2.2 Revealing nano-clusters within Co+-implanted TiO2 thin films 54 4.2.2.1 Experiments 54 4.2.2.2 Structural properties 54 4.2.2.3 Magnetic properties 57 4.2.2.4 Local environment of implanted Co atoms 59 4.2.2.5 Summary 61 4.3 Mn+ implantation: from a non-magnet to a ferromagnet 63 4.3.1 Experiments 63 4.3.2 Relation between lattice damage and defects 64 4.3.3 Electrical transport properties 65 4.3.4 Local environment of implanted Mn atoms 66 4.3.5 Magnetic properties 67 4.3.6 Summary 69 4.4 V+-implanted TiO2 thin films 71 4.4.1 Experiments 71 4.4.2 Magnetic properties 71 4.4.3 Summary 74 5 The effect of the open volume defects on the magnetic properties of V:TiO2 films prepared by doping during deposition 76 5.1 Experiments 76 5.2 Structural Properties 77 5.3 Investigation of the open volume defects 78 5.4 Magnetic, optical and electrical properties 79 5.5 Summary 81 6 Conclusions 84 6.1 Defects in TiO2 84 6.2 Formation of secondary phases 85 6.3 Evolution of the ferromagnetism in different microstructures of TiO2 87 7 Acknowledgments 91

Electronic structure of transition metal oxides has been a subject of intense research since decades due to the wide spectrum of properties that they exhibit, like high temperature superconductivity, metal-insulator transitions (MIT), phase separation etc. Among these, colossal magnetoresistance (CMR), i.e. a sharp drop in the electrical resistance by the application of an external magnetic field, is a property of fundamental and technological importance. In the present study we investigate several of these interesting properties ranging from colossal magnetoresistance, metal-insulator transitions and phase separation phenomena on a wide range of magnetoresistive systems. All these properties originate in transition metal oxides due to a competition between the strong inter-atomic Coulomb interaction strength within the transition metal d electrons and a large hopping interaction strength between the metal d and oxygen 2p states. In this thesis we report the investigation of the electronic and magnetic structures of some magnetoresistive oxides, including various double perovskites and manganites, using various high energy spectroscopies in conjunction with various theoretical approaches. The samples for the present experimental investigation were prepared by different synthetic routes, such as solid state reaction, nitrate method, d.c arc melting and float zone method, and were characterized by x-ray diffraction, four probe resistivity, magnetic susceptibility, optical absorption and energy dispersive analysis of x-rays while some of the samples were supplied by our collaborators. Various spectroscopic techniques like x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) , bremsstrahlung isochromat spectroscopy (BIS), x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism spectroscopy (XMCD) , electron energy loss spectroscopy (EELS), spatially resolved photoelectron spectroscopy and M¨ ossbauer spectroscopy were used to probe the samples. Theoretical methods include configuration interaction cluster approach to fit the XAS and XMCD spectra while ab initio band structure calculations along with the least-square fitting procedure was used to fit some of the valence and conduction bands. Following a general introduction in Chapter 1, the details of various experimental and theoretical techniques are discussed in Chapter 2 of this thesis. Recently, a double perovskite, Sr2FeMoO6, belonging to a general family of halfmetallic ferromagnetic oxides, has shown a spectacularly large magnetoresistance even at the room temperature and at relatively small applied magnetic fields compared to the extensively investigated class of magnetoresistive manganites. Physical properties of this compound is strongly influenced by the Fe -Mo ordering. We hence synthesized Sr2FeMoO6 sample, both with high and low degree of Fe/Mo ordering. Spectroscopic investigations of these samples suggest the presence of Fe rich and Mo rich domains of the type Sr2Fe1+xMo1−xO6 in disordered Sr2FeMoO6 at times. This prompted us to prepare bulk samples of Sr2Fe1+xMo1−xO6. In Chapter 3 we address various issues related to Fe/Mo ordering like saturation magnetization, variation of TC, and CMR as well as oxidation state of Fe and Mo in Sr2FeMoO6using this new series, ”Sr2Fe1+xMo1−xO6” as it offers a better control on the Fe/Mo bonds by controlling x. On the basis of the electron spectroscopic studies in conjunction with a configuration interaction cluster calculation model coupled with the conduction band, we claim that Fe remains in 3+oxidation state throughout the series, where as Mo changes its valency to maintain the charge neutrality. An analysis of the magnetic momentas a function of x suggests that Fe at the ”wrong” crystallographic site is coupled anti-parallel to the Fe moments at the ”correct” site. Additionally, Mo depolarizes to the extend proportional to the number of Mo sites in the near-neighbor co-ordination shell. Continuing with the double perovskites in Chapter 4 we investigate the electronic and magnetic structure of Sr2FeMoO6, Ca2FeMoO6 and Ba2FeMoO6using XAS and XMCD studies. We find that the conventional XAS and XMCD calculations based on configuration interaction of a typical fragment, FeO6in this case, is insufficient to reproduce the experimental spectrum as the compounds considered here are metallic. In order to include the non local charge transfer, we coupled FeO6 octahedra to a conduction band which mimics the Mo band. Within this model we obtained a good fit to the experimental spectrum. Chapter 5 deals with another series of double perovskite (Sr1−yCay)2FeReO6which exhibits a rich phase diagram since it undergoes a metal insulator transition (MIT) with composition at low temperatures. This system becomes more interesting due to the presence of a temperature driven MIT for higher y compositions. We find that the MIT is not related to the change in valency of Fe and Re. Analysis of the near Fermi edge valence band spectra suggests opening up of a soft gap. The main reason for MIT in this system is most likely the presence of strong electron-electron correlation between multiple electrons at the Re site, which is caused by the mismatch of the Re ionic radius and change in the crystal structure across MIT. Another issue which has been extensively investigated in this thesis is phase separation in manganites presented in Chapter 6. We use a spatially resolved, direct spectroscopic probe for electronic structure with an additional unique sensitivity to chemical compositions, to investigate high quality single crystal samples of La1/4Pr3/8Ca3/8MnO3 in the first section. This unique probe establishes the formation of distinct insulating domains embedded in the metallic host at low temperatures, significantly in the absence of any perceptible chemical inhomogeneity, with the domain-size at least an order of magnitude larger than the previous largest estimate. We also provide compelling evidence of memory effects in such domain formation and morphology, suggesting an intimate connection between these electronic domains and long-range strains, often thought to be an important ingredient in the physics of doped manganites. In second part of this chapter we discuss another system namely Eu0.5Y0.5MnO3 which undergoes a chemical phase separation forming alternate stripes of Eu rich (Y deficient) orthorhombic phase and Y rich (Eu deficient) hexagonal phases. These stripes are amazingly straight and run parallel over millimeters. One more system that we investigated is a mixture of ferromagnetic La5/8Sr3/8MnO3and insulating ferroelectric LuMnO3 taken in ratio 3:7, here too the attempt to make a single crystal resulted into a chemical phase separation forming strips of metallic La5/8Sr3/8MnO3and insulating LuMnO3 throughout the sample surface. Preliminary studies suggests that strain between the chemically and crystallographically different species may result into such interesting morphology. In Chapter 7 we study pseudo-one dimensional compounds Sr3CuIrO6 and Sr3ZnIrO6 using photo electron spectroscopy. The experimental results were fitted using band structure calculations with Full Potential Linearized Augmented Plane Wave (FP-LAPW) method.

Electronic structure of transition metal oxides has been a subject of intense research since decades due to the wide spectrum of properties that they exhibit, like high temperature superconductivity, metal-insulator transitions (MIT), phase separation etc. Among these, colossal magnetoresistance (CMR), i.e. a sharp drop in the electrical resistance by the application of an external magnetic field, is a property of fundamental and technological importance. In the present study we investigate several of these interesting properties ranging from colossal magnetoresistance, metal-insulator transitions and phase separation phenomena on a wide range of magnetoresistive systems. All these properties originate in transition metal oxides due to a competition between the strong inter-atomic Coulomb interaction strength within the transition metal d electrons and a large hopping interaction strength between the metal d and oxygen 2p states. In this thesis we report the investigation of the electronic and magnetic structures of some magnetoresistive oxides, including various double perovskites and manganites, using various high energy spectroscopies in conjunction with various theoretical approaches. The samples for the present experimental investigation were prepared by different synthetic routes, such as solid state reaction, nitrate method, d.c arc melting and float zone method, and were characterized by x-ray diffraction, four probe resistivity, magnetic susceptibility, optical absorption and energy dispersive analysis of x-rays while some of the samples were supplied by our collaborators. Various spectroscopic techniques like x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS) , bremsstrahlung isochromat spectroscopy (BIS), x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism spectroscopy (XMCD) , electron energy loss spectroscopy (EELS), spatially resolved photoelectron spectroscopy and M¨ ossbauer spectroscopy were used to probe the samples. Theoretical methods include configuration interaction cluster approach to fit the XAS and XMCD spectra while ab initio band structure calculations along with the least-square fitting procedure was used to fit some of the valence and conduction bands. Following a general introduction in Chapter 1, the details of various experimental and theoretical techniques are discussed in Chapter 2 of this thesis. Recently, a double perovskite, Sr2FeMoO6, belonging to a general family of halfmetallic ferromagnetic oxides, has shown a spectacularly large magnetoresistance even at the room temperature and at relatively small applied magnetic fields compared to the extensively investigated class of magnetoresistive manganites. Physical properties of this compound is strongly influenced by the Fe -Mo ordering. We hence synthesized Sr2FeMoO6 sample, both with high and low degree of Fe/Mo ordering. Spectroscopic investigations of these samples suggest the presence of Fe rich and Mo rich domains of the type Sr2Fe1+xMo1−xO6 in disordered Sr2FeMoO6 at times. This prompted us to prepare bulk samples of Sr2Fe1+xMo1−xO6. In Chapter 3 we address various issues related to Fe/Mo ordering like saturation magnetization, variation of TC, and CMR as well as oxidation state of Fe and Mo in Sr2FeMoO6using this new series, ”Sr2Fe1+xMo1−xO6” as it offers a better control on the Fe/Mo bonds by controlling x. On the basis of the electron spectroscopic studies in conjunction with a configuration interaction cluster calculation model coupled with the conduction band, we claim that Fe remains in 3+oxidation state throughout the series, where as Mo changes its valency to maintain the charge neutrality. An analysis of the magnetic momentas a function of x suggests that Fe at the ”wrong” crystallographic site is coupled anti-parallel to the Fe moments at the ”correct” site. Additionally, Mo depolarizes to the extend proportional to the number of Mo sites in the near-neighbor co-ordination shell. Continuing with the double perovskites in Chapter 4 we investigate the electronic and magnetic structure of Sr2FeMoO6, Ca2FeMoO6 and Ba2FeMoO6using XAS and XMCD studies. We find that the conventional XAS and XMCD calculations based on configuration interaction of a typical fragment, FeO6in this case, is insufficient to reproduce the experimental spectrum as the compounds considered here are metallic. In order to include the non local charge transfer, we coupled FeO6 octahedra to a conduction band which mimics the Mo band. Within this model we obtained a good fit to the experimental spectrum. Chapter 5 deals with another series of double perovskite (Sr1−yCay)2FeReO6which exhibits a rich phase diagram since it undergoes a metal insulator transition (MIT) with composition at low temperatures. This system becomes more interesting due to the presence of a temperature driven MIT for higher y compositions. We find that the MIT is not related to the change in valency of Fe and Re. Analysis of the near Fermi edge valence band spectra suggests opening up of a soft gap. The main reason for MIT in this system is most likely the presence of strong electron-electron correlation between multiple electrons at the Re site, which is caused by the mismatch of the Re ionic radius and change in the crystal structure across MIT. Another issue which has been extensively investigated in this thesis is phase separation in manganites presented in Chapter 6. We use a spatially resolved, direct spectroscopic probe for electronic structure with an additional unique sensitivity to chemical compositions, to investigate high quality single crystal samples of La1/4Pr3/8Ca3/8MnO3 in the first section. This unique probe establishes the formation of distinct insulating domains embedded in the metallic host at low temperatures, significantly in the absence of any perceptible chemical inhomogeneity, with the domain-size at least an order of magnitude larger than the previous largest estimate. We also provide compelling evidence of memory effects in such domain formation and morphology, suggesting an intimate connection between these electronic domains and long-range strains, often thought to be an important ingredient in the physics of doped manganites. In second part of this chapter we discuss another system namely Eu0.5Y0.5MnO3 which undergoes a chemical phase separation forming alternate stripes of Eu rich (Y deficient) orthorhombic phase and Y rich (Eu deficient) hexagonal phases. These stripes are amazingly straight and run parallel over millimeters. One more system that we investigated is a mixture of ferromagnetic La5/8Sr3/8MnO3and insulating ferroelectric LuMnO3 taken in ratio 3:7, here too the attempt to make a single crystal resulted into a chemical phase separation forming strips of metallic La5/8Sr3/8MnO3and insulating LuMnO3 throughout the sample surface. Preliminary studies suggests that strain between the chemically and crystallographically different species may result into such interesting morphology. In Chapter 7 we study pseudo-one dimensional compounds Sr3CuIrO6 and Sr3ZnIrO6 using photo electron spectroscopy. The experimental results were fitted using band structure calculations with Full Potential Linearized Augmented Plane Wave (FP-LAPW) method.

The physics of doped transition metal perovskite has been an area of intense research in the last few decades due to their interesting magnetic and transport properties. Various exciting phenomena such as, colossal magneto resistance, high Tc superconductivity, multiferroicity, ferroelectricity, high temperature ferromagnetism, etc., have made these systems more fascinating in terms of fundamental study as well as technological applications. There are several intrinsic material characteristics in these perovskite oxides that can impact their magnetic properties. Lattice distortion and chemical in homogeneity are two important ones. Changes in valence and ionic radius in rare earth (A- site) and transition metal (B- site) directly result in structural modification through internal pressure. Consequently, atomic distances and bond angles between the transition metals vary. This, intern, influences the nearest neighbour exchange coupling energy and magnetic interaction. A detailed investigation has been carried out on two A-site doped perovskite namely, La0.85Sr0.15CoO3 & La0.5Sr0.5CoO3 and two B-site doped perovskite, LaMn0.5Co0.5O3 & LuMn0.5Ni0.5O3 with a view to study the impact of chemical in homogeneity and lattice distortion on their respective magnetic ground states. The thesis is organized in seven chapters. A brief summary of each is given below: Chapter 1: Provides a brief introduction about the perovskite structure. Origins of lattice distortions and its effect on the magnetic properties are discussed. It includes a discussion on different types of indirect magnetic interactions involved in perovskite oxide structure. The chapter concludes with a description of spin-glass, phase separation/ cluster-glass, memory effect in glassy magnetism, critical behaviour at phase transition and specific heat in magnetic systems. Chapter 2: This chapter outlines basic principles of the experimental techniques employed for the work presented in this thesis. Chapter 3: Details macroscopic as well as microscopic investigations carried out to understand the glassy magnetic anomalies in La0.85Sr0.15CoO3 samples. The origin of phase separation (PS) has been reinvestigated. Since the magnetic behavior of La0.85Sr0.15CoO3 (LSCO15) lies in the border of spin glass (SG) and ferromagnetic (FM) region in the x-T phase diagram, it is subject to controversial debate for the last several years. While some research groups favour PS, others regard SG behaviour as the dominant phenomenon. In-depth investigation carried out to elucidate these views is outlined in this chapter in two sections. The first section deals with the glassy magnetic anomalies in single crystals of LSCO15 grown by optical floating zone method. Since the sample crystallizes from melt, it possesses good compositional homogeneity and the phase purity is confirmed by XRD pattern. Many characteristics of canonical SG systems are discernible in the magnetic study, such as, kink in field-cooling curve below Tf, frequency-dependent peak shift and the time dependent memory effect. The relaxation time in sub-pico second range (~10-13 s) is very similar to that of the typical SG systems. Time dependent transport relaxation study exhibits memory effect and the time evolution of resistance scales with magnetization and strictly adheres to the stretched exponential behaviour as commonly expected for a SG-like disordered system. However, a detailed study on transport mechanism and temperature-dependent inverse susceptibility reveals the existence of nanoscopic PS in the sample. In the second section, the origin of PS has been examined through a comprehensive study on two sets of LSCO15 polycrystalline samples prepared from the same initial mixture but subjected to different heat treatment processes. This study depicts the dependence of PS on the preparation conditions. The contrasting magnetic behaviour of PS and SG was resolved by experiments of dc magnetization, linear & non-linear ac susceptibility, neutron depolarization and field-cooled magnetic relaxation. Both samples conform to the general characteristics of a glassy behaviour: a kink in FC magnetization, frequency-dependent peak shift (Vogel–Fulcher law), dc bias-dependent peak shift in accordance with de Almeida–Thouless relation, and characteristic relaxation time in the range of 10-13/10-14 s. This is despite their internal spin structure and interaction being much different at a microscopic level. It is found that the sample processed through a proper homogenization process mimics the SG behaviour, whereas the sample prepared by the conventional method behaves like the PS phase. It is confirmed from neutron depolarization experiments that no ferromagnetic correlation exists in the SG phase of La0.85Sr0.15CoO3, a result in contrast to that of PS phase. Higher harmonic ac susceptibility measurement complements the above observation by the evidence that of 2nd order harmonics are not present in the SG phase of La0.85Sr0.15CoO3. The field-cooled magnetic relaxation study makes a distinct reference to the relaxation process and the strength of interaction between PS and SG like phases. In essence, a concerted effect is made to identify and resolve the spin-glass phase from phase-separated/ cluster-glass. This work shows that chemical in homogeneity is a key factor responsible for phase separation in La0.85Sr0.15CoO3; also intrinsic differences between PS and SG are identified that can serve as guiding tools for research in other similar magnetic oxide systems. It is concluded that the true ground state magnetic property of La0.85Sr0.15CoO3 is spin-glass in nature. Chapter 4: This chapter contains two sections. In the first part, the origin of the re-entrant spin-glass (RSG) behaviour in La0.5Sr0.5CoO3 has been investigated using the conventional magnetometer measurements. Polycrystalline samples prepared by the conventional solid-state synthesis exhibit RSG characteristics with a glassy transition at 190 K. The nature of frequency dependence of χ″(T), a pronounced memory effect and the sluggish response in dc magnetization measurement, all of which clearly indicate the re-entrant behaviour. But, once the sample is taken through a rigorous homogenization procedure of repeated grinding and annealing, its phase turns into pure ferromagnetic one. During the course of this homogenization process, the sample loses oxygen with concurrent degeneration of TC to a lower level. In order to regain the oxygen stoichiometry, it is necessary to anneal the sample in oxygen environment at 900 oC, which triggers deleterious ageing effect by which TC falls progressively with time. In the second part, the effect of oxygen stoichiometry on La0.5Sr0.5CoO3 (LSCO50) thin-films has been investigated. The highest TC reported so far for LSCO50 thin film is 250 K, which is significantly less compared to the bulk TC (262 K) of an oxygen stoichiometric compound. This work focuses on achieving the highest ferromagnetic transition temperature (TC) for LSCO50 films under optimized growth conditions. The analysis of experimental data suggests that the Curie temperature can be enhanced to 262 K, irrespective of whether or not, (a) the film on LAO or STO or (b) any induced strain occurs in the LSCO50 film. Apart from different thin-film growth parameters such as oxygen pressure and substrate temperature during the growth, and post-growth annealing temperature and oxygen pressure, the profile of the laser beam used for ablation of bulk material profile also plays an important role. The elevation of Curie temperature observed in thin-films to that close to the bulk value is believed to be a result of improved stoichiometric composition of oxygen facilitated during thin film growth. However, the strong ageing effect seen is quite close to that is observed in oxygen-annealed polycrystalline sample. Chapter 5: Of the three segments constituting this chapter, the first outlines different magnetic anomalies induced by lattice distortion in LaMn0.5Co0.5O3 (LMCO) single crystals. Single crystals of LMCO compound [(100) orientation] have been successfully grown using the optical floating zone method. Powder as well as single crystal x-ray diffraction analyses provides evidence of large strain dependent structural distortion in as-grown crystals. Spatially resolved 2-D Raman scan reveals that the strain generates a distribution of octahedral distortion in the lattice. While some are compressive in nature, others in the nearby territory relate to tensile distortion. The ac susceptibility measurement elucidates distinct changes in the ferromagnetic transition temperature (TC) in the as grown (strained) crystal. It is possible to release strain by rigorous annealing process. Which also results in a uniform TM-O octahedral deformation. Room temperature 2-D Raman spectra bears testimony to this. Upon annealing, the single crystalline order is diminuend by the atomic rearrangement. This causes tilting of the oxygen octahedra, by decreasing intra-octahedral angle θTM-O-TM, and lowering of exchange energy Jex between the magnetic ions. The transition temperature falls and the magnetic phase merges with that in the strain-free polycrystalline material. A detailed critical analysis performed in the vicinity of paramagnetic to ferromagnetic phase transition in both the samples establishes that the ground state magnetic behaviour, assigned to the strain-free LMCO crystal is of 3D Heisenberg type. But the local octahedral distortion present in the as-grown crystal causes mean field like magnetic interaction at few local sites. This serves as a key drive for the critical exponents to distance from the 3D Heisenberg model towards the mean-field type. The second part of this chapter concerns the anomalous re-entrant glassy magnetic behaviour observed in LMCO single crystals. The ac susceptibility study illustrates the low temperature anomalous glassy magnetic ordering in these crystals. The material behaves like a normal magnetic glass, (frequency-dependent peak-shift in ac susceptibility) in conformance with the phenomenological Vogel-Fulcher law, of spin flips time: ~10-4 s. However, the crystal does not respond to the external dc bias and just as well remains free from memory effect. Anomalous behaviour of this kind is rare in magnetic oxides. The magneto-dielectric effect in LMCO is discussed in the third section of this chapter. The real part of dielectric permittivity (ε′) has a colossal value of 1800 at 220 K and 10 kHz. However as the sample is cooled further, ε′ decreases slowly; followed by dielectric relaxation in the region, 120 - 150 K. Detailed analysis of the temperature dependence of the imaginary part of the dielectric permittivity (ε″) show that there is no relaxor-like phenomena in this compound. The frequency dependence of ε″ reveals that the low frequency region is dominated by Maxwell-Wagner relaxation, whereas, at high frequency, a Debye type relaxation persists. The temperature dependent full-width at half-maximum for this Debye relaxation, peaks at the corresponding TC. The temperature variation of the relaxation time has two domains of different slopes. At zero external field, ε″(ω) has a low activation energy (U = 46.4 meV) in the ferromagnetic region, compared to that in the paramagnetic (60.1 meV) phase. The boundary lies near the corresponding TC. In the presence of external applied field 5 T, U remains unchanged in the ferromagnetic region, but decreases ( U ~ 5 meV) in the paramagnetic phase. These results signify the existence of strong magneto-dielectric coupling in LMCO crystals. The field variation of ε′(ω) at fixed temperature and specific frequency highlights the rise in magnetodielectricity (MD) as well as magneto-loss (ML) with increasing magnetic field. It is perceived that this variation is not due to the magneto resistance of LMCO or caused by LMCO - electrode interfaces. The influence of extrinsic parasitic contributions cannot be ruled out entirely, but the presence of positive MD as well as ML at frequencies above the time constant suggests that the relaxation process and the magneto-dielectric coupling are intrinsic to the LaMn0.5Co0.5O3 system. Chapter 6: This chapter describes the successful synthesis of a new perovskite oxide compound, LuMn0.5Ni0.5O3. The structural characterization employs the Rietveld refinement of powder X-ray diffraction pattern. The compound crystallizes in orthorhombic Pbnm crystal structure. dc magnetization reveals ferromagnetic ordering in the sample. However the low temperature glassy phase spotted in the ac susceptibility measurement might classify it as a re-entrant spin-glass compound. But the display of memory effect until the ferromagnetic transition indicates that intrinsic ant ferromagnetic interaction prevails over the dominant ferromagnetic interaction. A critical behaviour study was carried out in the vicinity of the ferromagnetic to paramagnetic phase transition, which provided the critical exponents: α = 0.37, β = 0.241 ± 0.003, γ = 1.142 ± 0.003 and δ = 5.77 ± 0.03. Interestingly, this set of critical exponents does not match with any of the conventional theories of mean field, 3D Heisenberg, and 3D Ising. Rather it fits quite well with data calculated for the stacked triangular 3D version of the (Z2 × S1) model [α = 0.34 ± 0.06, β = 0.25 ± 0.01, γ = 1.13 ± 0.05 and δ = 5.47 ± 0.27]. This study indicates that the magnetic ground state of LuMn0.5Ni0.5O3 is canted ferromagnetic. Chapter 7: Various important results are summarized in this chapter. It also provides a broad outlook in this area of research.

The physics of doped transition metal perovskite has been an area of intense research in the last few decades due to their interesting magnetic and transport properties. Various exciting phenomena such as, colossal magneto resistance, high Tc superconductivity, multiferroicity, ferroelectricity, high temperature ferromagnetism, etc., have made these systems more fascinating in terms of fundamental study as well as technological applications. There are several intrinsic material characteristics in these perovskite oxides that can impact their magnetic properties. Lattice distortion and chemical in homogeneity are two important ones. Changes in valence and ionic radius in rare earth (A- site) and transition metal (B- site) directly result in structural modification through internal pressure. Consequently, atomic distances and bond angles between the transition metals vary. This, intern, influences the nearest neighbour exchange coupling energy and magnetic interaction. A detailed investigation has been carried out on two A-site doped perovskite namely, La0.85Sr0.15CoO3 & La0.5Sr0.5CoO3 and two B-site doped perovskite, LaMn0.5Co0.5O3 & LuMn0.5Ni0.5O3 with a view to study the impact of chemical in homogeneity and lattice distortion on their respective magnetic ground states. The thesis is organized in seven chapters. A brief summary of each is given below: Chapter 1: Provides a brief introduction about the perovskite structure. Origins of lattice distortions and its effect on the magnetic properties are discussed. It includes a discussion on different types of indirect magnetic interactions involved in perovskite oxide structure. The chapter concludes with a description of spin-glass, phase separation/ cluster-glass, memory effect in glassy magnetism, critical behaviour at phase transition and specific heat in magnetic systems. Chapter 2: This chapter outlines basic principles of the experimental techniques employed for the work presented in this thesis. Chapter 3: Details macroscopic as well as microscopic investigations carried out to understand the glassy magnetic anomalies in La0.85Sr0.15CoO3 samples. The origin of phase separation (PS) has been reinvestigated. Since the magnetic behavior of La0.85Sr0.15CoO3 (LSCO15) lies in the border of spin glass (SG) and ferromagnetic (FM) region in the x-T phase diagram, it is subject to controversial debate for the last several years. While some research groups favour PS, others regard SG behaviour as the dominant phenomenon. In-depth investigation carried out to elucidate these views is outlined in this chapter in two sections. The first section deals with the glassy magnetic anomalies in single crystals of LSCO15 grown by optical floating zone method. Since the sample crystallizes from melt, it possesses good compositional homogeneity and the phase purity is confirmed by XRD pattern. Many characteristics of canonical SG systems are discernible in the magnetic study, such as, kink in field-cooling curve below Tf, frequency-dependent peak shift and the time dependent memory effect. The relaxation time in sub-pico second range (~10-13 s) is very similar to that of the typical SG systems. Time dependent transport relaxation study exhibits memory effect and the time evolution of resistance scales with magnetization and strictly adheres to the stretched exponential behaviour as commonly expected for a SG-like disordered system. However, a detailed study on transport mechanism and temperature-dependent inverse susceptibility reveals the existence of nanoscopic PS in the sample. In the second section, the origin of PS has been examined through a comprehensive study on two sets of LSCO15 polycrystalline samples prepared from the same initial mixture but subjected to different heat treatment processes. This study depicts the dependence of PS on the preparation conditions. The contrasting magnetic behaviour of PS and SG was resolved by experiments of dc magnetization, linear & non-linear ac susceptibility, neutron depolarization and field-cooled magnetic relaxation. Both samples conform to the general characteristics of a glassy behaviour: a kink in FC magnetization, frequency-dependent peak shift (Vogel–Fulcher law), dc bias-dependent peak shift in accordance with de Almeida–Thouless relation, and characteristic relaxation time in the range of 10-13/10-14 s. This is despite their internal spin structure and interaction being much different at a microscopic level. It is found that the sample processed through a proper homogenization process mimics the SG behaviour, whereas the sample prepared by the conventional method behaves like the PS phase. It is confirmed from neutron depolarization experiments that no ferromagnetic correlation exists in the SG phase of La0.85Sr0.15CoO3, a result in contrast to that of PS phase. Higher harmonic ac susceptibility measurement complements the above observation by the evidence that of 2nd order harmonics are not present in the SG phase of La0.85Sr0.15CoO3. The field-cooled magnetic relaxation study makes a distinct reference to the relaxation process and the strength of interaction between PS and SG like phases. In essence, a concerted effect is made to identify and resolve the spin-glass phase from phase-separated/ cluster-glass. This work shows that chemical in homogeneity is a key factor responsible for phase separation in La0.85Sr0.15CoO3; also intrinsic differences between PS and SG are identified that can serve as guiding tools for research in other similar magnetic oxide systems. It is concluded that the true ground state magnetic property of La0.85Sr0.15CoO3 is spin-glass in nature. Chapter 4: This chapter contains two sections. In the first part, the origin of the re-entrant spin-glass (RSG) behaviour in La0.5Sr0.5CoO3 has been investigated using the conventional magnetometer measurements. Polycrystalline samples prepared by the conventional solid-state synthesis exhibit RSG characteristics with a glassy transition at 190 K. The nature of frequency dependence of χ″(T), a pronounced memory effect and the sluggish response in dc magnetization measurement, all of which clearly indicate the re-entrant behaviour. But, once the sample is taken through a rigorous homogenization procedure of repeated grinding and annealing, its phase turns into pure ferromagnetic one. During the course of this homogenization process, the sample loses oxygen with concurrent degeneration of TC to a lower level. In order to regain the oxygen stoichiometry, it is necessary to anneal the sample in oxygen environment at 900 oC, which triggers deleterious ageing effect by which TC falls progressively with time. In the second part, the effect of oxygen stoichiometry on La0.5Sr0.5CoO3 (LSCO50) thin-films has been investigated. The highest TC reported so far for LSCO50 thin film is 250 K, which is significantly less compared to the bulk TC (262 K) of an oxygen stoichiometric compound. This work focuses on achieving the highest ferromagnetic transition temperature (TC) for LSCO50 films under optimized growth conditions. The analysis of experimental data suggests that the Curie temperature can be enhanced to 262 K, irrespective of whether or not, (a) the film on LAO or STO or (b) any induced strain occurs in the LSCO50 film. Apart from different thin-film growth parameters such as oxygen pressure and substrate temperature during the growth, and post-growth annealing temperature and oxygen pressure, the profile of the laser beam used for ablation of bulk material profile also plays an important role. The elevation of Curie temperature observed in thin-films to that close to the bulk value is believed to be a result of improved stoichiometric composition of oxygen facilitated during thin film growth. However, the strong ageing effect seen is quite close to that is observed in oxygen-annealed polycrystalline sample. Chapter 5: Of the three segments constituting this chapter, the first outlines different magnetic anomalies induced by lattice distortion in LaMn0.5Co0.5O3 (LMCO) single crystals. Single crystals of LMCO compound [(100) orientation] have been successfully grown using the optical floating zone method. Powder as well as single crystal x-ray diffraction analyses provides evidence of large strain dependent structural distortion in as-grown crystals. Spatially resolved 2-D Raman scan reveals that the strain generates a distribution of octahedral distortion in the lattice. While some are compressive in nature, others in the nearby territory relate to tensile distortion. The ac susceptibility measurement elucidates distinct changes in the ferromagnetic transition temperature (TC) in the as grown (strained) crystal. It is possible to release strain by rigorous annealing process. Which also results in a uniform TM-O octahedral deformation. Room temperature 2-D Raman spectra bears testimony to this. Upon annealing, the single crystalline order is diminuend by the atomic rearrangement. This causes tilting of the oxygen octahedra, by decreasing intra-octahedral angle θTM-O-TM, and lowering of exchange energy Jex between the magnetic ions. The transition temperature falls and the magnetic phase merges with that in the strain-free polycrystalline material. A detailed critical analysis performed in the vicinity of paramagnetic to ferromagnetic phase transition in both the samples establishes that the ground state magnetic behaviour, assigned to the strain-free LMCO crystal is of 3D Heisenberg type. But the local octahedral distortion present in the as-grown crystal causes mean field like magnetic interaction at few local sites. This serves as a key drive for the critical exponents to distance from the 3D Heisenberg model towards the mean-field type. The second part of this chapter concerns the anomalous re-entrant glassy magnetic behaviour observed in LMCO single crystals. The ac susceptibility study illustrates the low temperature anomalous glassy magnetic ordering in these crystals. The material behaves like a normal magnetic glass, (frequency-dependent peak-shift in ac susceptibility) in conformance with the phenomenological Vogel-Fulcher law, of spin flips time: ~10-4 s. However, the crystal does not respond to the external dc bias and just as well remains free from memory effect. Anomalous behaviour of this kind is rare in magnetic oxides. The magneto-dielectric effect in LMCO is discussed in the third section of this chapter. The real part of dielectric permittivity (ε′) has a colossal value of 1800 at 220 K and 10 kHz. However as the sample is cooled further, ε′ decreases slowly; followed by dielectric relaxation in the region, 120 - 150 K. Detailed analysis of the temperature dependence of the imaginary part of the dielectric permittivity (ε″) show that there is no relaxor-like phenomena in this compound. The frequency dependence of ε″ reveals that the low frequency region is dominated by Maxwell-Wagner relaxation, whereas, at high frequency, a Debye type relaxation persists. The temperature dependent full-width at half-maximum for this Debye relaxation, peaks at the corresponding TC. The temperature variation of the relaxation time has two domains of different slopes. At zero external field, ε″(ω) has a low activation energy (U = 46.4 meV) in the ferromagnetic region, compared to that in the paramagnetic (60.1 meV) phase. The boundary lies near the corresponding TC. In the presence of external applied field 5 T, U remains unchanged in the ferromagnetic region, but decreases ( U ~ 5 meV) in the paramagnetic phase. These results signify the existence of strong magneto-dielectric coupling in LMCO crystals. The field variation of ε′(ω) at fixed temperature and specific frequency highlights the rise in magnetodielectricity (MD) as well as magneto-loss (ML) with increasing magnetic field. It is perceived that this variation is not due to the magneto resistance of LMCO or caused by LMCO - electrode interfaces. The influence of extrinsic parasitic contributions cannot be ruled out entirely, but the presence of positive MD as well as ML at frequencies above the time constant suggests that the relaxation process and the magneto-dielectric coupling are intrinsic to the LaMn0.5Co0.5O3 system. Chapter 6: This chapter describes the successful synthesis of a new perovskite oxide compound, LuMn0.5Ni0.5O3. The structural characterization employs the Rietveld refinement of powder X-ray diffraction pattern. The compound crystallizes in orthorhombic Pbnm crystal structure. dc magnetization reveals ferromagnetic ordering in the sample. However the low temperature glassy phase spotted in the ac susceptibility measurement might classify it as a re-entrant spin-glass compound. But the display of memory effect until the ferromagnetic transition indicates that intrinsic ant ferromagnetic interaction prevails over the dominant ferromagnetic interaction. A critical behaviour study was carried out in the vicinity of the ferromagnetic to paramagnetic phase transition, which provided the critical exponents: α = 0.37, β = 0.241 ± 0.003, γ = 1.142 ± 0.003 and δ = 5.77 ± 0.03. Interestingly, this set of critical exponents does not match with any of the conventional theories of mean field, 3D Heisenberg, and 3D Ising. Rather it fits quite well with data calculated for the stacked triangular 3D version of the (Z2 × S1) model [α = 0.34 ± 0.06, β = 0.25 ± 0.01, γ = 1.13 ± 0.05 and δ = 5.47 ± 0.27]. This study indicates that the magnetic ground state of LuMn0.5Ni0.5O3 is canted ferromagnetic. Chapter 7: Various important results are summarized in this chapter. It also provides a broad outlook in this area of research.