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

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