Magnetic And Transport Studies On Nanosystems Of Doped Rare Earth Manganites And VPP PEDOT

Abstract

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.