Nanostructured Magnetic Films Produced by Magnetic Nanoparticles

摘要

Gas-phase Fe nanoparticles with a diameter ~ 2nm, have been used in all the nanostructured material in this thesis. In pure Fe nanoparticle systems with different thicknesses, two important parameters the exchange interaction (Hex) and random anisotropy (Hr) were investigated using the Random Anisotropy Model (RAM). This reveals that for the same particle size Hex remains almost constant for varying Fe thicknesses; whereas Hr increases with the increase of Fe film thickness. This is ascribed to increasing strain imposed at the nanoparticle level. The observed high values of Hr are related to an oxide on the cluster surface in these films, whose effect is also observed in low temperature magnetometry data. This shows the appearance of exchange bias in the films. The RAM approach when applied to Fe clusters in Co matrices, reveals much lower values of Hr than found in pure Fe nanoparticles and both Hr and Hex show an increase with the Volume Fraction (VF) of Fe in Co. The increase in Hex is ascribed to the increasing spin moment with Fe volume fraction. The nature of Fe clusters in very thick layers produce a high frequency Ferromagnetic Resonance response in the radio frequency range, which is an important finding for many applications. \udThe EXAFS study of Fe nanoparticles in Cr matrices show no structural modification relative to the bulk bcc structure of both elements. The magnetometry results suggest that in dilute Fe concentration films, the observed decrease in the overall magnetization is due to the development of a nonmagnetic shell at the interface between Fe and Cr at each cluster boundary. This is reinforced by the lack of any evidence of EB. With increasing VF at about 10% of Fe there is strong evidence of the formation of a super-spin-glass (SSG) that shows the characteristic memory effect. Increasing the Fe nanoparticles VF to 20% Fe in Cr, the magnetization exceeds that expected for Fe indicating that the interaction induces some of the Cr to order ferromagnetically. \udCore-shell nanoparticle systems have been synthesised by a method that allows a complete control over the morphology of these assemblies. Atomic investigations in Fe@Cu CS nanoparticles reveal that Fe nanoparticles adopt the fcc structure with a 20 monolayer Cu shell thickness and stay in the bcc structure for 1-2 monolayer thick Cu shells. No alteration in the Fe atomic structure has been reported for different Au shell thicknesses in Fe@Au. The magnetic data show a reduced magnetization of the FM-AFM Fe@Cr CS nanoparticles as compared to the bulk value which is also ascribed to the formation of a non-magnetic Fe shell at the interface.