Synthesis, structure and magnetic properties of low dimensional spin systems in the 3d transition metal oxides and superconductivity in magnesium borate

The major part of this thesis deals with the synthesis and magnetic characterization of low dimensional spin systems in the 3d transition metal oxides. Such systems are of interest due to the simplicity of their structures, allowing theoretical modeling of their electronic and magnetic behavior. Exotic properties are also often encountered. Studies involving layered magnetic materials based on triangle lattices, in particular, have resulted in many observations of unusual low temperature spin dynamics, and have presented new challenges for the theoretical understanding of magnetic systems. The magnetic properties of some compounds exhibiting these triangle-based lattices are described here in detail.; BaNi₂V₂O₈ is a spin-1 antiferromagnet on a honeycomb net. Susceptibility χ(T), specific heat C(T), and neutron diffraction measurements on this compound reveal the onset of antiferromagnetic (AFM) long-range ordering (LRO) close to 50 K. Diffuse diffraction peaks that are characteristic of two-dimensional (2D) short-range order are also observed up to 100 K. χ(T) of Ba(Ni_1−xMg_x)₂V ₂O₈ shows the gradual disappearance of LRO with doping.; Ni₃V₂O₈, Co₃V₂O ₈, and β-Cu₃V₂O₈ have spin-1, spin-3/2, and spin-1/2 magnetic lattices that are a new anisotropic variant of the Kagomé net, wherein edge-sharing MO₆ octahedra form the rises and rungs of a “Kagomé staircase”. The anisotropy largely relieves the geometric frustration, but results in rich magnetic behavior. Characterization of the magnetization of polycrystalline samples of Ni ₃V₂O₈ and Co₃V₂O₈ reveals that the compounds are ferrimagnetic in character. C(T) show four distinct magnetic phase transitions below 9 K for Ni₃V₂O ₈ and two below 11 K for Co₃V₂O₈. In the case of β-Cu₃V₂O₈, χ(T) and C(T) show the onset of short-range ordering at approximately 75 K, and a magnetic phase transition with the characteristics of antiferromagnetism at around 29 K.; The second part of this thesis describes the bulk synthesis of M9132 at temperatures as low as 500 to 550°C with good superconducting properties. Most reported bulk synthesis of MgB₂ has been done at high temperatures (e.g. 900°C), which is not favorable for thin film synthesis. The results of this work confirm the potential of developing an “in-situ” thin film technology based on MgB₂.