Advanced Magnetic and Dielectric Nanomaterials
Abstract: Multiferroics are a novel class of next generation multifunctional materials that exhibit simultaneous magnetic spin, electric dipole, and ferroelastic ordering. It gives an additional degree of freedom to design new devices. Magnetoelectric effect in these materials result in the manipulation of magnetic spins via applied electric field and vice versa, making them suitable for next generation applications. Single phase multiferroics show low magnetoelectric coefficient, hence there is a need to look at composite multiferroic structures with respective magnetic and ferroelectric phase. The magnetoelectric coefficient in the composite structures depends on the magnitude of strain induced by one phase to the other. This requires the need to study suitable magnetic and ferroelectric materials that can be combined to create magnetoelectric multiferroic composite structures. Also, higher surface to volume ratio at the nanoscale should enhance the interaction between the two phases. Here, in we synthesised and studied magnetic and ferroelectric structures that have potential to be used as the respective phases of multiferroic magnetoelectric composites. Magnetic materials with high magnetostriction and low coercivity are suitable candidates for the formation of multiferroic composite. The size dependent and tuneable magnetic properties of cobalt ferrite and nickel-iron composites, respectively fulfil the above-mentioned criteria. Herein, the properties of the above magnetic materials were explored at nanoscale where efficient techniques such as thermal decomposition and electrospinning were applied. Cobalt ferrite nanoparticles with varying sizes were synthesised at the nanoscale and magnetic studies were performed to study their size dependent suitability to be used as a potential magnetic material in multiferroic composite formation. The nanoparticle synthesis by thermal decomposition of metal oleate precursors displayed reaction time dependent growth. The nanoparticles sized below superparamagnetic limit showed a negligible coercivity fulfilling an essential requirement to display magnetoelectric effect. Alongside, a successful synthesis of novel cobalt iron oxide (Co0.33Fe0.67O) nanoparticles was also performed. This displayed a synthesis dependent ferrimagnetic to antiferromagnetic phase transition in Co Fe-O structure at nanoscale. A controlled oxidation of Co0.33Fe0.67O could lead to the formation of antiferromagnetic-ferrimagnetic core-shell nanostructure that can overcome the superparamagnetic limit in nanoparticles system. They are potential materials in ME-RAMs. 1-D magnetic nanostructure show a sharp shape anisotropy and hence can be used as magnetic components of composite multiferroic structures. Nickel-iron composites in FCC phase were studied at the nanoscale in the form of fibres. Electrospinning of suitable metal precursors with PVP polymer followed by the reduction of nanofibres in H2 led to the formation of Ni0.47Fe0.53 fibre mats. They were ferromagnetic and displayed high saturation magnetisation along with low coercivity fulfilling the requirement to be used in magnetoelectric applications. 1-D flexible ferroelectric composite structures were studied alongside to be used as the ferroelectric component of multiferroic composites. Polyvinylidene fluoride was doped with DIPAB at varying ratios to study the improvement in the ferroelectric properties of the composite structure in comparison to just PVDF with low dielectric constant. Electrospinning of composite polymer solution led to the formation of DIPAB doped PVDF nanofibres. They displayed improved relative dielectric constant and low loss tangent and find use in composite magnetoelectric materials formation. The ease of processability of DIPAB doped PVDF nanofibres aids in incorporating the above studied magnetic materials. The studies proved the worth of as-synthesised magnetic and ferroelectric materials at the nanoscale for the formation of magnetoelectric multiferroic composite nanomaterials. The cobalt ferrite nanoparticles doped in DIPAB-PVDF nanofibres can result in core-sheath ME composite structure. A coating of DIPAB-PVDF composite on the formed Ni0.47Fe0.53 fibres will result to the formation of 1-D magnetoelectric structures.