| dc.description.abstract | 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. | en_US |
