| dc.description.abstract | a) Thin films of LBCO and LBCNO grown by PLD on LAO (100) and STO (100) substrates showed (001) growth.
b) Study of their transport properties showed semiconducting behavior in the a–b plane and metallic along the c-direction.
c) The anisotropic transport property associated with this material is attributed to the short and long Cu–O bonds connected by nearly 180° and 90° in the a–b plane and uniformly connected Cu–O–Cu chains by 180° along the c-direction. YBCO grown on LBCO has shown a Tc of 88 K.
a) The non-equilibrium nature of the pulsed laser ablation process is exploited to make thin films of RNiO? without actually taking the single-phase compound as the target.
b) PrNiO? shows metallic behavior on LAO (100) substrate.
c) The first-order metal-to-insulator transition has been suppressed in these films.
d) The deviation from bulk properties in thin films has been attributed to the effect of substrate strain on the films. The substrate-induced strain on LAO (100) acts in a similar way as external pressure.
a) Thin films of PCMO, NCMO, and NSMO have been fabricated by PLD.
b) The films grow in (101) direction on LAO (100).
c) All the films show d?/dT < 0. Resistivity of PCMO and NCMO films is fitted to the polaronic hopping mechanism.
a) We have fabricated LPMO/LBCO multilayers and studied their magnetic and electrical properties.
b) With increasing cuprate layer thickness, a decrease of Tc is observed without any evidence for oscillation in the sign of the exchange coupling, in contrast to the situation in manganite/nickelate-based multilayers.
c) The magnetization and magnetoresistance decrease as the thickness of the cuprate layer increases.
d) The insulator-metal transition is suppressed in the multilayers, and changes in the dimensionality of electrical conduction with magnetic field and layer thickness have been observed.
a) We have fabricated LCMO/PCMO multilayers and showed epitaxial growth and superlattice. LCMO/NCMO and LCMO/LMO multilayers are also fabricated.
b) The insulator-to-metal transition is observed in all the multilayers.
c) LCMO/PCMO multilayers showed large MR of 98%. At a field of 0.5 T, MR of 41% is observed. LCMO/NCMO multilayers have also shown such a large MR.
In this thesis, electrical transport, magnetic, and magnetoresistance properties have been investigated on thin films of perovskite oxide. Thin films were fabricated by pulsed laser deposition. The deviation from bulk transport properties because of substrate strain has been studied. Manganite superlattices have been fabricated, and their magnetic coupling and metallic coupling are presented in detail. A way to enhance the magnetoresistance in the manganite multilayers is shown.
We have made thin films of LBCO and LBCNO and examined the anisotropy associated with their conductivity. As single-crystal studies on these compounds do not exist, this study gives a new direction to explore anisotropy by thin-film techniques. This film gives good lattice and chemical compatibility with YBCO, and YBCO shows a Tc of 88 K grown on the LBCO layer.
We have proven the advantage of using the pulsed laser deposition technique to make thin films of RMO? without actually taking the single-phase compound as a target. We have investigated the influence of substrate strain on the transport properties of RNiO? and charge-ordered material films. It is worth noting that both compounds crystallize in the orthorhombic structure in their bulk solids. From Chapters 3 and 4, we conclude that the growth of an orthorhombic phase on a cubic lattice substrate undergoes strain, and its bulk transport behavior gets modified.
Magnetic multilayers are interesting from both basic and application points of view. We have investigated the magnetic, electrical, and magnetoresistance properties in ferromagnetic/non-ferromagnetic metallic oxide multilayers. In this study, keeping the LPMO layer thickness constant and increasing the thickness of the LBCO from 1 unit cell to 8 unit cells, we have observed a decrease in magnetic coupling and magnetoresistance. The salient finding in the metallic oxide multilayers is the dimensionality crossover in electrical conductivity in the presence of a magnetic field. The formation of disorder at the boundary is believed to be the cause of such properties.
We have made multilayers with LCMO as the FMM and PCMO as the spacer layer, proposing a large MR in such systems. Indeed, we have shown a large MR of 98% in these multilayers. A magnetoresistance of 40% has been observed in response to a magnetic field of 0.5 T at 215 K in the LCMO/PCMO multilayer system. We have shown the effect of the spacer layer (cuprate) on the magnetic properties in the manganite multilayers. The idea of using an insulating manganite layer (such as charge-order materials) as the spacer layer to enhance MR in manganite multilayers is demonstrated. This study gives a new way to enhance magnetoresistance in manganite systems.
Though the results gave sufficient proof for the drawn inference, a few more experiments in each case would have provided crucial information in support of the presented argument. The deviation in the transport properties of RMO? and charge-ordered films has been explained on the basis of strain caused by the substrate on the film. To quantify the effect of strain, one needs to do experiments such as grazing incidence XRD to evaluate the in-plane lattice parameters. Then parameters need to be substantiated from electron diffraction data. In the present investigations, the superlattice peaks obtained for the multilayers are not satisfactory. Therefore, in addition to the in-plane scan, ? and ? scans of the films from a four-circle diffractometer are essential to elucidate the structure of the multilayers.
In the case of multilayers, we have said that the influence of disorder at the boundary between the bilayers modifies the magnetic properties. To show the clean interface between the manganite multilayers, it is essential to examine these films by high-resolution TEM (transmission electron microscopy). Also, to have a complete understanding of multilayer properties, there is a need to have high-quality magnetization data such as that from a SQUID magnetometer. One could have supported the arguments presented in the thesis provided such measurements were done on these films.
One can study the SNS junction properties of the YBCO/LBCO/YBCO trilayer. Since LBCO offers a large resistance that can increase the I?Rn product, LBCO can have an edge over other normal metal barriers. Fabrication of SNS junction needs the junction area of the order of a few microns.
A detailed study of the thin-film properties of charge-ordered materials is necessary to go further with the fabrication of their multilayers. A better possibility to investigate strain on the CO material films is to grow them on different substrates and study their lattice parameters by grazing incidence XRD and TEM for the twin boundaries. The thin-film study of CO materials is important as they show huge MR.
In the case of multilayers of LPMO/LBCO, it would be interesting to study the properties in the current-perpendicular-to-the-plane geometry. Since the spin diffusion length is a better parameter, one can expect large MR in such configurations. A similar study on LCMO/PCMO multilayers would also be interesting.The multilayers can be of interest in superlattices such as La?.?Sr?.?MnO?/Y?.?Ca?.?MnO?, where a large MR at room temperature is expected. CMR/HTSC superlattices are another area of research from both understanding and application points of view. In these structures, one can study the flux dynamics of the superconductor and magnetic exchange coupling between the ferromagnetic layers above the superconducting transition temperature. CMR/HTSC heterostructures can also be of interest in microwave filters.
Actually, the fabrication and study of spin-valve structures using manganites can be of interest for spin-valve transistors. The advantage with manganites is that both ferromagnetic and antiferromagnetic layers can be deposited from similar materials by slightly changing the composition. There are only a few reports in the literature on manganite multilayers. A lot more understanding of multilayer growth, interface, and heteroepitaxial strain is needed to see if the MR properties can be used for device applications.
In spite of limited facilities, attempts were made to show new results in each one of the chapters. Wherever feasible, more systems have been studied in support of the new observations. We hope that the work presented here is a worthwhile contribution to the area of magnetic and metallic oxide thin films and their heterostructures. | |
