Growth, Transport, Magnetic And Thermal Studies On Single Crystals Of Pr1-xPbxMnO3

Abstract

Mixed valence manganites with the perovskite structure R1-xAxMnO3 (where R = La, Nd, Pr and A = Ba, Ca, Sr, Pb) have been a popular subject of contemporary research because of their interesting physical properties such as competing magnetic orders, metal-insulator transitions and colossal magnetoresistance. A complex interplay between structure, electronic and magnetic properties results in rich phase diagrams involving various metallic, insulating and magnetic phases. A review of the literature related to rare-earth managnites clearly reveals that the systems with Pb as a divalent dopant are relatively less explored. This may be due to the volatile nature of lead based compounds which are used as precursors for preparing these systems. This has motivated us to take up research on Pb doped rare earth manganites. This thesis is divided into eight chapters. The first introductory chapter gives a brief review of the work on manganites which have already been reported in the literature following which the motivation for carying out the present investigation is given. The second chapter deals with technical details of various instruments used in the present reasearch work. The third chapter is related to growth of single crystals, their preliminary characterization, magnetization and resistivity studies. Single crystals of Pr1-xPbxMnO3 are grown by flux technique for different compositions. Crystals are characterized by energy dispersive x-ray analysis (EDAX) and inductively coupled plasma atomic emission spectroscopy (ICPAES) for compositional analysis. Magnetization and resistivity studies are carried out on Pr1-xPbxMnO3 for three compositions viz. x = 0.2, 0.23 and 0.3. The magnetization vs. temperature plots show that all the three compositions undergo a transition from paramagnetic to ferromagnetic state. The magnetization in the low temperature ferromagnetic region obeys Bloch`s law. The susceptibility in the paramagnetic region is fitted to Curie Weiss law. Deviation of susceptibilty from Curie Weiss law, a feature observed in all the three crystals has been attributed to formation of ferromagnetic clusters at ~ 250 K. The cluster formation has its implications on all other properties in the temperature range from TC to 250 K where TC is the magnetic transition temperature. Resistivity measurements are carried out on the same three compositions. The x = 0.2 and 0.23 compositions undergo a transition from paramagnetic insulating to ferromagnetic insulating phases. The x = 0.3 composition shows a metal – insulator transition at nearly 35 K above TC. Chapter 4 describes the critical behaviour of Pr1-xPbxMnO3 for two compositions, viz. x = 0.23 and 0.3. For critical studies, magnetization vs. field measurements are carried out in the temperature range TC ± 10 K. Using modified Arrott plots and Kouvel-Fisher method the critical exponents and precise value of TC are obtained. The x = 0.23 composition shows results which indicate a conventional second order phase transition shown by a 3D Heisenberg ferromagnet. It also obeys the universal scaling behaviour. However, the x = 0.3 composition shows deviation from this behaviour. Hence to probe further into the nature of magnetic transition of this compound the effective critical exponents are calculated as a function of reduced temperature ε (=(T-TC)/TC). Based on the behaviour of effective exponents the nature of the transition in the x = 0.3 composition is described in detail. The unconventional ordering is attributed to presence of possible magnetic frustration in the system. In chapter 5 the resistivity and magnetoresistance behaviour of the x = 0.23 and 0.3 crystals are discussed. Initially the nature of plots of temperature and field variation of resistivity are described for both the cases. Detailed measurements are carried out at the magnetic transition region. The analysis is carried out in terms of critical scattering behaviour at the transition region. The zero field resistivity is analyzed in terms of theory of Fisher and Langer, while the magnetoresistance is fitted to scaling theory at the critical region developed by Balberg and Helman. It is seen that the x = 0.23 crystal shows a critical behaviour in resistivity for zero field as well as in magnetoresistance close to TC. However, the behaviour of the x = 0.3 composition is more complex. A simpler critical scattering theory alone cannot explain its large negative magnetoresistance. Chapter 6 contains the EPR studies on the x = 0.23 and 0.3 compositions. Analysis is carried out in the paramagnetic region. The EPR signals are fitted to a modified Dysonian equation. The intensity, linewidth, and asymmetry parameter are obtained as a function of temperature from fitting. The parameters are obtained till 210 K for both compositions. The intensity is fitted to a Curie Weiss law. The linewidth shows a “bottleneck” mechanism and is proportional to conductivity. Hence it is fitted to activated behaviour. In addition, a secondary signal develops at low fields from 240 K and is present till 200 K in both the compositions. This is explained by means of phase separation. In chapter 7 the specific heat of the x = 0.23 and 0.3 compositions are discussed. The measurements are carried out from 2 to 300 K in zero field and also in the presence of 3 Tesla magnetic field. The analysis is carried out in two separate sections. The first section deals with the low temperature analysis from 2 to 80 K where apart from the usual lattice, electron and magnetic terms, presence of Schottky anomaly is also discussed. The Schottky peak occurs at a relatively higher temperature of around 40 K. Due to presence of higher order lattice terms the Schottky effect is not easily discernible. It is extracted only from fitting. In the second section, the specific heat associated with ferromagnetic – paramagnetic transition is extracted. The lattice term in the entire temperature range from 10 to 300 K except at the transition region is fitted to Einstein function. The magnetic specific heat is obtained by subtracting the Einstein specific heat from the total specific heat. The change in entropy due to magnetic transition is also calculated for both compositions. In chapter 8 the general conclusions derived from the work presented in this thesis are summarized along with the scope for future work in this system.