Organic-inorganic hybrid perovskites (OIHPs), which are promising materials for electronic and optoelectronic applications (1-10), have made into layered organic-inorganic hybrid perovskites (LOIHPs). These LOIHPs have been applied to thin-film transistors, solar cells and tunable wavelength phosphors (11-18). It is known that devices fabricated with single crystal exhibit the superior performance, which makes the growth of large-sized single crystals critical for future device applications (19-23). However, the difficulty in growing large-sized LOIHPs single crystal with superior electrical properties limits their practical applications. Here, we report a method to grow the centimeter-scaled LOIHP single crystal of [(HOC2H4NH3)2PbI4], demonstrating the potentials in mass production. After that, we reveal anisotropic electrical and optoelectronic properties which proved the carrier propagating along inorganic framework. The carrier mobility of in-inorganic-plane (in-plane) devices shows the average value of 45 cm2 V–1 s–1 which is about 100 times greater than the record of LOIHP devices (15), showing the importance of single crystal in device application. Moreover, the LOIHP single crystals show its ultra-short carrier lifetime of 42.7 ps and photoluminescence quantum efficiency (PLQE) of 25.4 %. We expect this report to be a start of LOIHPs for advanced applications in which the anisotropic properties are needed (24-25), and meets the demand of high-speed applications and fast-response applications.

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

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.