Dielectric and magnetic studies of wide-bandgap Mott insulators and their composites
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High-permittivity magneto-dielectric materials and their composites have drawn a tremendous attraction in recent past due to their versatile applications such as in piezoelectric transducers, low-power CMOS devices, sensors and spintronics [1-11]. Apart from the Perovskite systems (Pb[Zr,Ti]O3, CaCu3Ti4O3, KNaNbO3, etc.), lead-free 3d transition metal oxide based magnetic ceramics with static dielectric constant (εr) more than 1000 are very limited in the literature [12-17]. Compounds such as La1-xCax,MnO3, BaTiO3, YBa2Cu3O6, and FeTiTaO6 are considered as important systems because of their negligible temperature variation of εr and relaxor-like ferroelectric behavior [18-25]. There has not been enough attention paid to the high-frequency dielectric behaviour of alkali metal (Na or Li) doped nonperovskite wide-bandgap oxides such as ZnO and NiO, and their composites. Among various lead-free 3d transition metal oxide systems Li and Ti doped wide-bandgap Mott insulators are the best known examples because of their colossal dielectric permittivity and temperature independent behavior of & [12, 26]. Internal grain-boundary layer mechanism and thermally excited relaxation process are the key sources for such enhanced εr, and non-zero dipole moment . Dielectric relaxation in Fe, Mg, Al, Co, Zr and V doped wide-bandgap oxides are also widely investigated aiming to understand the ac-conductivity, space-charge polarization (Maxwell-Wagner type) at low frequencies and defect-dipole induced polarization mechanism at high frequency regions [27-30]. The present research work has primarily motived to develop the two-phase composite systems of Zn1-xNixO/NiO and Ni1-xNaxO whose physical properties are comparable to those of Li, Ti doped NiO. When we initiated this problem no report was available focusing on the structural, dielectric and magnetic properties of above composites either in the form of both bulk or nanostructures. Hence, an attempt was made to understand the growth mechanism and other physical properties of the above mentioned composites.
Supervisor: Subhash Thota