The interplay between magnetism, defects and disorder in polycrystalline oxides

This thesis investigates the crucial role of defects and disorder in determining the structural, electrical and magnetic properties of a few transition metal oxides. In solid state physics, crystallinity or the periodicity of lattice plays a very significant role. All the basic theories are formulated considering perfectly ordered crystalline materials because this simplifies the structural description. However in reality, the arrangement of atoms or molecules in most crystalline materials is not perfect and often interrupted by crystallographic defects disrupting the perfect crystalline order in solids. Still, many technologically significant materials are based on weakly or strongly disordered materials. A group of such materials is dilute magnetic semiconductors, where random site occupancy of doped transition metal ions introduces disorder and an associated ferromagnetism critically dependent on defect concentration is often found. Following a general introduction in chapter 1, sample growth and characterization techniques are described in chapter 2. In chapters 3 and 4, rigorous macro- and microscopic studies have been carried out on two dilute magnetic oxides like float-zone grown single crystalline BaTi0.95Fe0.05O3-d with different oxygen stoichiometry as well as Ca and Fe co-doped BaTiO3 with different Ca and Fe concentrations. Here, the room temperature ferromagnetism and crystal structures are shown to be modified strongly with increasing doping and oxygen vacancies. In chapter 5, presence of larger amount of bound oxygen vacancy defects at the cores is shown to induce room temperature ferromagnetism in solvothermally synthesized Co-doped BaTiO3 nanocrystals. In chapter 6, a strong covalency driven structural instability is reported to modulate the transport, dielectric and magnetic properties of flux grown Ba5Co5ClO13 and Ba6Co6ClO16 single crystalline as well as polycrystalline samples. In chapter 7, Sr3FeMoO7 (n=2 Ruddlesden-Popper compound) polycrystalline powder synthesis and detailed studies revealing the possible effects of disorder (due to the presence of antisite defects and antiphase boundaries) on the magnetic and magnetotransport properties of the system is described. In the end, a brief discussion and summary of all the works in the thesis are presentedThe research was conducted under the supervision of Prof. Sugata Roy of the Materials Science division under SMS [ School of Materials Science]The research was carried out under DST grant & IACS fellowshi