STUDY OF A, B-SITE DOPED CHARGE ORDERED Bi1−xCaxMnO3 AND (Nd/La)0.5Sr0.5MnO3 PEROVSKITES

Abstract

Doped RE1−xAExMnO3 manganites (where RE is a rare earth cations and AE is an alkaline earth cation) exhibit various unconventional properties, such as paramagnetic to ferromagnetic (PM–FM) and insulator to metal (IM) transition, colossal magnetoresistance (CMR), antiferromagnetic and charge/orbital ordering (AFM and CO/OO). Past studies show that these unconventional properties arise due to the intricate coupling between spin, lattice, charge and orbital degrees of freedom whose amplitudes strongly depend on the size and ratio of the RE and AE cations. Doping by impurity elements at B-site appear very helpful for a better understanding of the properties of manganites. The Mn ions provide charge carriers, magnetic moments, and local J–T distortions which play a key role in electrical and magnetic properties of the manganites. The manganites are usually annealed in oxygen atmosphere for modification of oxygen stoichiometry. The annealing atmosphere changes the oxygen content which affects the competition between ferromagnetic (FM) and antiferromagnetic (AFM) phases due to the change of Mn4+/Mn3+ ratio, and has a great influence on the magnetic and electrical transport properties of manganites. In contrast to RE1−xAExMnO3, bismuth based manganites (Bi1−xAExMnO3) have not been studied in much detail. One curious property of these manganites is the high value of charge-ordering temperatute (TCO). Bi1−xCaxMnO3 constitutes a very interesting but relatively less-studied system. Despite several investigations relating to the doping in manganites, the physical feature of CO, the interplay between the different magnetic phases and the nature of the resulting metamagnetic phase is still poorly understood. Further, the nature and the impact of the phase separation (P–S) that plays a crucial role in determining the magnetic landscape of manganites is also vaguely understood. The physical properties of manganite thin films are dramatically different from those of bulk manganite samples. The influence of substrate strain is the main factor distinguishing manganite thin films from bulk samples. Since most of the technological iv applications require thin films, it is essential to understand the effects of the substrate-induced strains on the properties of these manganites. The strain in epitaxial thin films can be easily manipulated by varying the film thickness or the substrate material. Thus, in the present thesis we have reported a systematic study of the structural, magnetic and transport properties of A, B-site doped Bi1−xCaxMnO3 (x=0.4, 0.7 and 0.8) and Nd0.5Sr0.5MnO3 (NSMO). The effect of oxygen annealing atmosphere on the magnetic properties of Cu doped Nd0.5Sr0.5MnO3 have also been investigated. Thickness dependent magnetic and transport properties of La0.5Sr0.5MnO3 (LSMO) thin film are also investigated. The present thesis is divided into seven chapters. The first chapter contains an introductory aspects and surveys of the field and describes the basic structure, the known mechanisms to explain the physical properties of perovskite manganites, phase diagram, effects of A-site cation size and doping at B-site on the magnetic properties. In addition to these, synthesis of thin film manganites, strain effect on the structural and physical properties and effect of film-thickness on the magnetic properties have also been discussed in this chapter. Finally at the end of the chapter applications of the manganites and motivation of the present thesis work are described. The second chapter describes the prominent techniques for structural, electrical and magnetic characterization employed in the present investigations. These techniques include X-ray diffraction (XRD) for phase identification, Field emission scanning electron microscopy (FESEM) using secondary electron imaging mode for investigating the surface morphology and energy dispersive X-ray (EDX) spectroscopy for elemental analysis. The techniques that have been used to prepare bulk samples and thin film such as solid state reaction and sputtering have also been described in this chapter. Besides this, superconducting quantum interference device (SQUID) magnetometer used for the magnetic property measurements and the four probe technique for resistivity measurements have also been outlined. In the third chapter, we have presented structural, magnetic and transport properties of hole doped Bi0.6−xRExCa0.4MnO3 (0.0≤x≤0.6), where RE=La, Pr, Nd and Eu. In the present work polycrystalline samples with nominal compositions Bi0.6−xRExCa0.4MnO3 v (0.0≤x≤0.6) have been synthesized via solid state reaction route. The magnetic measurements of the parent Bi0.6Ca0.4MnO3 (BCMO) compound reveal the presence of charge ordered antiferrromagnetic (COAFM) phase with charge ordering temperature (TCO) ~289 K and AFM Neel temperature (TN) ~136 K. In case of La doped samples, the COAFM phase disappears. Furthermore, the samples with x=0.2 to x=0.6 exhibit paramagnetic to ferromagnetic (PM–FM) transition and PM–FM transition temperature (TC) decreases progressively from 270 K to 203 K as x increases from 0.2 to 0.5. However, the samples with x=0.0 and 0.1 do not show PM–FM transition. The temperature dependent resistance (R–T) measurements of the samples with x=0.0 to 0.4 exhibit insulating behaviour, while samples with x=0.5 and 0.6 exhibit metal-insulator (M−I) transition at ~ 163 K and 198 K, respectively. In the second section of this chaper, effect of Pr doping at A-site of BCMO have been discussed. Systematic substitution of Pr at Bi site induces an interesting interplay between the charge ordering and antiferromagnetism. The charge ordering temperature (TCO) decreases with increasing x. The antiferromagnetic (AFM) ordering temperature (TN) increases sharply at both the extremes but remains nearly constant from x=0.2 – 0.4. At temperatures lower than TN a transition to the glassy state is observed in all doped samples. The Pr doping also lead to enhancement in the magnetic moment. In case of Nd doped samples, similar variations in TCO and TN are observed as in case of Pr doped samples. Furthermore, Nd-doping promotes an antiferromagnetic to a ferromagnetic type fluctuation in the materials at room temperature evidenced by the change in the value of P . The value of resistivity increases with the increasing Pr content from x=0.0 to x=0.2 and afterwards it decreases with increasing values of x up to x=0.5. Resistivity of x=0.6 is higher than the other samples. In case of Nd doped samples, a concomitant decrease in resistivity up to x=0.3 and then an increase in resistivity up to x=0.5 have been found. In case of Eu doped sample quite peculiar magnetic properties are obtained. TCO decreases with increasing x up to x=0.4 and then slightly increases with further increasing x up to x=0.6. Further, TN decreases with increasing x. Eu doping also leads to enhancement in the magnetic moment and a concomitant decrease in resistivity up to x=0.2 and then an increase in resistivity up to x=0.5. In the last section of this chapter, all the results are correlated and discussed in terms of A-site cation radii (<rA>) mismatch. The Bi3+ lone pair is vi responsible for the observed complexity in the magnetic and electrical properties of the samples. The experimental results are discussed on the basis of the character of the Bi3+ lone pair electron and the average A-site cation radius (<rA>). The fourth chapter embodies the study of structural, magnetic and transport properties of electron doped Bi0.2−xPrxCa0.8MnO3 (x=0.00, 0.02, 0.04, 0.08, 0.12, 0.16 and 0.20), Bi0.2Ca0.8Mn0.9X0.1O3 (where X=Ti, Cr, Fe, Co, Ni, Cu) and Bi0.3Ca0.7Mn1−xTixO3, (where x=0.0, 0.015, 0.03, 0.05, 0.08, 0.12 and 0.16). In the first section of this chapter, it has been found that the substitution of Pr at Bi site has stronger effects on charge ordering properties than hole doped manganites. TCO decreases with increasing x up to x = 0.12 and then after it increases with x up to x=0.20. A spin glass (SG) state has also been observed at ~105 K in all the samples. Pr doping also leads to enhancement in the magnetic moment up to x=0.12 and then after a decrease in magnetic moment up to x=0.20. A concomitant increase in resistivity up to x=0.04 and then decrease in resistivity up to x=0.20 is also observed. In the second section of this chapter, the structural, magnetic and transport properties of Bi0.2Ca0.8Mn0.9X0.1O3 (where X=Ti, Cr, Fe, Co, Ni, Cu) have been investigated. The parent sample Bi0.2Ca0.8MnO3 (BCMO) exhibits robust charge-ordered antiferrromagnetic (COAFM) phase with TCO ~ 155 K and TN ~105 K. TCO decreases by ~ 10 K and ~ 33 K, respectively, in Ni2+ and Cu2+ doped samples, while it increases by 42 K in Ti4+ doped sample. In case of Fe3+, Co3+ and Cr3+ doped samples charge-ordering (CO) completely melts. The paramagnetic (PM) to ferromagnetic (FM) transition temperatures (TC) of doped samples have lower values as compared to undoped one. In addition, a spin glass (SG) state is observed in all the samples and the magnetic state at T < TC is akin to a cluster glass (CG) for undoped and Ni, Cu, Ti doped samples formed due to the presence of FM clusters in COAFM matrix. Furthermore, the enhancement in resistivity in all the doped samples with respect to undoped one has been observed. In the last third section of this chapter, the structural, magnetic and transport properties of Bi0.3Ca0.7Mn1−xTixO3, (where x=0.0, 0.015, 0.03, 0.05, 0.08, 0.12 and 0.16) have been presented. TCO decreases gradually with increasing Ti doping content, and finally disappears completely for x= 0.16. The Neel temperature (TN) also decreases with increasing Ti doping content. At T ≤ TN a transition to cluster glass like state is also seen. vii The zero field cooled/field cooled (ZFC/FC) magnetization at high temperature (T > 200 K) decreases with increasing Ti content, whereas opposite trend is observed at low temperature (T < 200 K). The small exchange bias effect is also observed for x=0.08 at 10 K. The resistivity increases with the increasing Ti doping content. Based on the present study it has been proposed that the disorder induced by doping on the Mn site and magnetic exchange interactions between Mn and doped ions play a key role in explaining magnetic and electrical properties. In the first section of the fifth chapter, the polycrystalline samples with nominal compositions Nd0.5Sr0.5Mn1−xCuxO3 (x = 0.0, 0.01, 0.03, 0.05 and 0.10) have been synthesized with the aim to study the change in the structural, magnetic and electrical properties due to oxygen-annealing and substitution of Cu for Mn. The temperature dependent magnetization measurement shows TCO at 240 K in Nd0.5Sr0.5Mn03 sample. TN gradually increases with increasing the Cu doping content. The value of TN is found more for the oxygen annealed samples than the air sintered samples. The enhancement of the magnetization and sharpening of peak are observed in the M−T plots by oxygen annealing. It is also found that TC increases and TN decreases with an increase in the applied magnetic-field. The value of resistivity decreases with the increasing Cu content from x=0.0 to x=0.03 and afterwards it increases with increasing values of x up to 0.10 for both air and oxygen sintered samples. It is also found that oxygen-annealed samples exhibit higher resistivity than the air sintered samples except for x=0.0. The results are discussed according to the change of magnetic exchange interaction caused by Cu-doping. It is also found that the amount of Mn4+ appears to be the main variable which influences the physical properties. In the second section of this chapter, the structural, magnetic and electrical properties of Ln0.5Sr0.5Mn0.9Cu0.1O3 (Ln= La, Pr, Nd or Ho) perovskite manganites have been investigated to explore the influence of A-site cation radius (<rA>) and the A-site cation size-disorder (σ2) on the various interdependent phenomena such as ferromagnetism (FM), phase separation (PS) and charge ordering (CO). The temperature dependent magnetization (M−T) curve of La-based sample shows four distinct points at ~ 269 K, 255 K, 200 K and 148 K corresponding to strong FM, cluster glass (CG), weak FM, and antiferromagnetic (AFM) transitions, respectively. Our viii investigation shows that TN increases, whereas TC and irreversibility temperatures (Tirr) decrease with decreasing <rA>, i.e. with increasing σ2. Furthermore, the value of the magnetization decreases and resistivity increases with decreasing <rA>. All samples exhibit insulating behavior in the temperature range 77 K to 300 K and above 110 K the electronic conduction mechanism has been described within the framework of the variable range hopping (VRH) model. The sixth chapter describes the thickness dependent structural, magnetic and transport properties of La0.5Sr0.5MnO3 (LSMO) thin films. A series of LSMO films with thicknesses of 30, 60, 125 and 300 nm have been epitaxially deposited on (001)-oriented LaAlO3 (LAO) substrate using dc magnetron sputtering technique. All the LSMO films have TC lower than that of bulk LSMO. The paramagnetic (PM) to ferromagnetic (FM) transition at TC is followed by antiferromagnetic (AFM) ordering at TN in all the films. Very small variation in TC with increasing film thickness is observed. However, AFM temperature (TN) increases by 15 K with increasing film thickness from 30 to 300 nm. But increase in TN for 60 nm film is about 21 K. The resistivity of the thin films decreases with increasing film thickness up to 125 nm. The temperature dependent resistivity (ρ–T) plot for 60 nm thick film shows two insulator to metal (I–M) transitions at ~ 215 K and 156 K and a metal to insulator (M–I) transition at ~ 85 K. The resistivity above 240 K of the films with various thicknesses is consistent with small polaronic hopping conductivity. The polaronic formation energy EA increases with the increase of film thickness except for 60 nm thick film where a small decrease in EA is observed. Many results are explained in terms of phase-separation and Jahn–Teller (J–T) distortion. The seventh chapter contains the brief summary and conclusions on the work presented in the thesis through chapters three to six. The overall comments and recommendations have also been added in this chapter