Hollow Magnetic Nanoparticles : experimental and numerical studies
This thesis concerns the study of structural and magnetic properties of hollow magnetic nanoparticles (HMNPs), shell and shell/shell. These HMNPs present enhanced surface effects resulting from the presence of both inner and outer surface layers. The experimental investigation combining magne-tic measurements and 57Fe Mossbauer spectrometry of such HMNPs has revealed a complex spin magnetic structure. Small HMNPs with ultrathin thickness show highly disordered magnetic structure and the corresponding in-field hyperfine structure can be described by means of 2 speromagnetic antiferromagnetically coupled, in addition to the significant exchange bias phenomenon. The in-field Mossbauer study of the effect of size and thickness of HMNPs shows that the spin disorder is strongly correlated to the surface to volume ratio. Those experimental magnetic behaviors were confirmed using Monte Carlo simulation. Indeed, after improving the numeric model, it is concluded that surface anisotropy Ks has a dominant role in the magnetic behavior of HMNPs and the value of critical Ks necessary to obtain radial (spike) configuration decreases as the size of HMNPs increases, keeping the same thickness. The numeric study for different sizes and shell thicknesses allows the effect of these parameters on the spin structure of HMNPs to be followed. Then, the experi-mental study extended to shell/shell HMNPs indicates that the spin disorder is enhanced in HMNPs with antiferromagnetic shell (NiO) in addition to larger exchange bias field. From those results, one can try to deduce the effect of spin disorder on the exchange bias phenomena in such system.