Design, Synthesis And Characterization Of Magnetic Ferrite Nanostructures : Toward Novel Permanent Magnets

Abstract: Magnetic oxide nanoparticles (NPs) may interact with each-other for example via dipolar or superexchange interactions, depending whether they are in direct contact. These interparticle interactions yield both ferro- and/or antiferromagnetic coupling and modify the energy barrier of the magnetic particle, depending upon the strength of the coupling and orientation of the particles. The corresponding perturbation of the magnetic order can readily be investigated by measuring the dc-magnetization or/and the ac-susceptibility of the samples as a function of the temperature or magnetic field. Furthermore, remanence plots, First Order Reversal Curves (FORCs) analysis and magnetic relaxation measurements are ideal methods to investigate reversal mechanisms and magnetic interactions. As we show in this thesis for a set of reference nanoparticle systems comprising magnetically hard/soft ferrites, the strong interaction regime leads to interesting phenomena, including collective dynamics, exchange bias-like hysteresis loop shifts, and interface-mediated exchange-coupling of hard and soft phases. The strength of the interparticle interactions has been investigated for a set of dense assemblies of equally sized magnetically soft maghemite NPs coated with different fractions of oleic acid layer, and compared to the dilute case of silica-coated NPs. When hard exchange-biased Co-doped maghemite NPs are mixed with unbiased soft particles with equal size, we observe that dipolar interactions yield a horizontal magnetic hysteresis loop shift. We observe that the measured hysteresis bias of this system is larger than that of the exchange bias of the unmixed Co-doped particles, and assign the extra contribution to have dipolar origin ("dipolar bias"). A more complex scenario is reported for hard/soft nanostructured powder systems of Sr and Co ferrite whose morphology (epitaxial texture or lacking coherence) strongly alter the existing interparticle magnetic interactions, and in turn the reversal process of magnetization: the magnetic coherence length scales have been estimated and thus limit for rigid coupling uncovered. Doping strategies by chemical substitution with diamagnetic cations have been also investigated, to tailor the hard/soft properties: eventually, the plasma sintered compacted ferrite composites exhibit a larger energy product compared to the single phased components, establishing a strategy to produce permanent magnets with large coercivity. We believe that our studies provide new and useful knowledge into the role of magnetic interactions at the nanoscale.