M. Angiolini

Size and shape effect on the magnetic properties of α-Fe2O3 nanoparticles

The magnetic properties of α-Fe 2 O 3 nanoparticles, prepared as powders by chemical route and as granular films by sputtering, have been investigated for different size (3 - 700 nm) and shape (rhombohedral, acicular and spherical). The particle size and shape effect on the Morin transition (T M = 263 K in the bulk system) has been investigated by analyzing the temperature dependence of the zero-field-cooled (ZFC) and field-cooled (FC) magnetization. For rhombohedral (30 - 350 nm) and spherical (10 - 50 nm) particles, the Morin temperature was found to decrease with decreasing particle size and increasing magnetic field. On the other hand, acicular particles (350 - 700 nm) do not show the Morin transition, unless annealed. In samples characterized by a large distribution of particle size, the ZFC and FC measurements show the superimposed effects of the superparamagnetic behaviour of very small particles and the Morin transition for the larger ones. Only the superparamagnetic component is observed in granular films consisting of very small spherical particles (3 nm), whose average blocking temperature is about 140 K.

Size and shape effect on the canted antiferromagnetism in α-Fe2O3 particles

Abstract The effect of thermal treatments on both the Morin transition and the spin flop field (Hsf) of α-Fe2O3 acicular particles (major axis:(350 ± 50) nm; minor axis: (85 ± 15) nm.) has been investigated by means of magnetization vs temperature-magnetic field measurements. Both the Morin transition and the spin flop process have been observed only after an annealing treatment at 500 °C. Due to the finite size effect, the values of the Morin temperature and spin-flop field are significantly below the bulk value.

Structural and magnetic properties of granular Co-Ag thin films deposited by pulsed laser deposition

Abstract An investigation of microstructural properties of CoxAg1 − x films grown by Pulsed Laser Deposition, as a function of deposition and post-deposition annealing temperature, is reported. Surface morphology and microstructure were investigated by XPS, X-ray diffraction, SEM and TEM measurements. Information on panicle size distributions has been extracted from the analysis of the temperature dependence of the irreversible magnetization. Depending on cobalt content, deposition and post-deposition annealing temperature, the maximum of the Co grain size distribution was estimated to be in the range 2–6 nm.

Structural Refinement of Ag-Fe Blends during High Energy Ball Milling

The effect of high energy ball milling on the structural refinement of the binary system Ag-Fe has been studied by Transmission and Scanning Electron Microscopy, Small Angle Neutron Scattering and Mossbauer spectroscopy. Samples containng 10 wt% of Fe were submitted to high energy ball milling for times up to 50 hours. The process produces a fine dispersion of Fe into the Ag matrix; the size distribution function is quite broad with an average radius of the order of a few nm. The minimum average particle size is obtained after about 10 hours of milling, longer term processing leads to a detectable particle coarsening. TEM analyses give evidence of a well defined orientation relationships between the Fe particles and the Ag matrix. Mossbauer spectroscopy reveals a modification of the hyperfine field experienced by the Fe atoms only for short term milling. On the basis of the experimental results one can assume that the fine dispersion of Fe into the Ag matrix occurs by mutual solubilisation probably induced by plastic shear, followed by fast decomposition by precipitation. After this step, further milling can induce only coarsening of the microstructure.