Nanocomposite Magnetic Films Assembled from Nanoparticles

This thesis consists of three main investigations. The first of these is a study of the magnetic properties of Fe nanoparticles embedded in an Al matrix, with different volume fraction. Both Fe nanoparticles, with a diameter of ̴ 2 nm, and the Al matrix were deposited from the gas phase. The atomic Fe moment of the Fe nanoparticles in Al is much less than the bulk Fe value because of considerable alloying at the Fe nanoparticle and Al matrix interface. Two important parameters, the exchange field (Hex) and random anisotropy field (Hᵣ), were investigated using the Random Anisotropy Model (RAM). Fitting the data to this model reveals that with increasing volume filling fraction (VFF) of Fe nanoparticles in Al, both Hex and Hr show an increase, with Hr showing a more significant increase than for Hₑₓ. The second main investigation in this thesis is a study of structure and magnetism in Co nanoparticles embedded in antiferromagnetic Cr. Co K edge and Cr K edge extended x-ray absorption fine structure (EXAFS) experiments were performed in order to investigate atomic structure in the Cr-embedded Co nanoparticles and Cr matrix respectively, whereas magnetism was investigated using a vibrating sample magnetometer (VSM). The atomic structure of the Co nanoparticles is same as the host Cr matrix (bcc), although with a degree of disorder, rather than the bulk Co hcp structure. The net Co moment per atom in the Co/Cr nanocomposite films is significantly lower than bulk Co value, and decreases as the proportion of Co nanoparticles in the film is decreased; for the sample with the most dilute concentration of Co nanoparticles (4.9% by volume), the net Co moment was 0.18 μB/atom. Both the structural and magnetic results show that there is a degree of alloying at the nanoparticle/matrix interface, leading to a core/shell structure in the embedded nanoparticles consisting of an antiferromagnetic CoCr alloy shell surrounding a reduced ferromagnetic Co core. The final part of this work is an investigation into the structure and magnetism of Fe nanoparticles embedded in a Cu1-ₓAlₓ alloy matrix, where the structure of the matrix could be controlled by control over its composition. Cu K edge EXAFS measurements show that there is a slight stretch in the Cu-Cu interatomic distances in the alloy matrix, while the face centred structure in the Cu1-ₓAlₓ matrix is maintained, as the Al-content is increased. Fe K edge EXAFS measurements reveal that for low Al-content in the Cu1-ₓAlₓ matrix, Fe nanoparticles have both fcc and bcc structures, but for higher Al-content the structure of Fe nanoparticles is consistent with bcc. The magnetism measurements, obtained from VSM and SQUID magnetometers, show that the Fe atomic moment increases sharply due to the increasing proportion of bcc Fe nanoparticles. However for Al-content higher than 0.13, the net atomic moment value of Fe decreases slightly, which is consistent with a high degree of alloying between Fe and Al atoms.