A.C. Barone

Ultrashort laser ablation of solid matter in vacuum: a comparison between the picosecond and femtosecond regimes

We have investigated the main features of ultrashort laser ablation of solid matter by using laser pulses of different durations, ranging from fs to ps timescales, and wavelengths, in the visible-near infrared spectral range. The analysis has been carried out on Si and Ni in terms of the two main characteristics of the ablation process: vacuum expansion of the ablated material and generation of nanoparticles of the target material. Fast photography and optical emission spectroscopy were used to characterize the plume expansion dynamics, while atomic force microscopy analysis of less than one layer deposits was employed to analyse the size distribution of the produced nanoparticles. Our analysis indicates that the properties characterizing the plume expansion in vacuum as well as the size distribution of the nanoparticles produced with laser pulses in the range of 100 fs–1 ps are almost independent of the specific material properties and laser pulse characteristics, thus representing general features of the process in these conditions.

Magnetic and morphological characteristics of nickel nanoparticles films produced by femtosecond laser ablation

We have used the technique of femtosecond (fs) laser ablation in a vacuum to produce films of nickel nanoparticles. A peculiarity of this fs laser deposition is the significant shape and orientation anisotropy of the nanoparticles, which are oblate ellipsoids with the major axis parallel to the deposition substrate. The deposited films present unique magnetic properties, and, in specific conditions, very high remanence ratios (up to 0.7) accompanied with relatively low values of saturation and coercive fields can be obtained. We have interpreted these results in terms of the mentioned anisotropies, and of the occurrence of a thermally induced in-plane tensile stress, which is a function of the nanoparticles size.

Morphology, structure and magnetic properties of (Tb0.3Dy0.7Fe2)100−xFex nanogranular films produced by ultrashort pulsed laser deposition

Magnetic films were produced by ultrashort pulsed laser deposition (uPLD) using a rotating multitarget of terfenol-D (nominal composition: Tb0.3Dy0.7Fe2) and iron. The composite films obtained have a nanoparticle morphology typical of material produced by the uPLD technique, where each particle retains the stoichiometry of the parent target material. The co-deposition allows the production of (Tb0.3Dy0.7Fe2)100−x Fex films, where x can range from 0% to 100%. Unlike films obtained by standard nanosecond PLD, pure terfenol-D layers (x = 0) are amorphous, while the addition of iron induces the formation of Fe crystalline nanoparticles inside an amorphous nanogranular matrix of terfenol-D. The magnetic properties depend on the nanoparticle morphology and more strictly on the fraction of iron particles. In particular, it was demonstrated that the exchange interaction between hard magnetic terfenol-D nanoparticles and iron nanoparticles is active in the uPLD films, giving a cumulative magnetic response resulting from an averaging of the properties of the two component phases.

Inhomogeneity effects on magnetic and magnetotransport properties of CoxCu100−x films produced by ultrashort pulsed laser deposition

Films produced by means of ultrashort pulsed laser deposition (uPLD) and constituted of Co nanoparticles (NPs) mixed with Cu ones exhibit peculiar morphological and topological properties. In particular, the NPs obtained by uPLD retain their individuality with a moderate coalescence which permits interparticle discontinuities. Due to this condition, the NP interface contributes significantly to the electron transport mechanisms depending on the magnetizing field. As a consequence, magnetoresistance effects are evidenced which do not saturate up to external magnetizing fields much higher than the value required for the saturation of the macroscopic magnetization. Moreover, when the cobalt volume fraction (x) is lower than 35%, the set of magnetic, resistive and magnetoresistive data shows that the magnetic percolation among the Co particles is not completely obtained and a transport mechanism of the giant magnetoresistance (GMR) kind is active in the uPLD films, notwithstanding the fact that the average particle size is higher in comparison with that expected for GMR optimization. On the other hand, if the contact among Co particles is obtained (x ≥ 50%) the anisotropic magnetoresistance (AMR) becomes the prominent effect. In this circumstance, the presence of interparticle discontinuities is still deducible by the experimental evidence of a negative derivative of the magnetoresistance ratio versus the applied magnetic field. In the same x range (x ≥ 50%), the AMR increases with the Co content, while zero field resistivity does not change.

Stepwise behaviour of magnetization temperature dependence in iron nanoparticle assembled films

An unusual stepwise behaviour is reported in the temperature dependence of the zero field cooled magnetization in iron nanoparticle dense films produced by ultra-short pulsed laser deposition assisted by irradiation of nanoparticles with a nanosecond UV laser pulse, appropriately delayed, during their flight from the target to the substrate. This behaviour, induced by the particle systems morphology, characterized by clusters of tightly coupled nanoparticles as well as by some voids between them, is ascribed to the competition between Zeeman energy density, intracluster anisotropy energy density and intercluster exchange energy density. A phenomenological model and Monte Carlo simulations are reported, which support the proposed interpretation.