The scientific activity covers a wide range of topics in the field of magnetism: basic research and physical modelling, preparation of materials, structural and magnetic characterisation, applications and metrological activities. Significant results achieved in 2006 are summarised here.

Metallic alloys with an amorphous, nanocrystalline and microcrystalline structure were prepared by rapid solidification from the melt under controlled atmosphere. Nanogranular alloys displaying high magnetoresistance effect (Cu-Fe-Ni, Cu-Fe, Au-Co, Au-Fe) were obtained and investigated. Magnetic shape memory alloys (FePd, NiMnGa thin ribbon) were also characterised. Magnetic thin films have been produced by DC/RF sputtering. Thin film preparation was chiefly focused on Fe/Co, Fe/Tb giant magnetostriction alloys, amorphous CoFeBSi and FeBSi alloys.

Decay of the longitudinal magnetisation component after saturation at H =-15kA/m in a FeNbCuSiB thin film. Insets: representative cases of domain structure.

Permanent magnets are comprehensively characterised in our laboratory by closed-circuit and open-sample methods. The Pulsed Field Magnetometer (PFM) method has been developed to test extra-hard rare-earth based magnets. The dynamic phenomena engendered by the application of the fast exciting field pulse, which descend from the thermal fluctuation after-effect and eddy currents, are brought to light and separately evaluated. A new model has been developed by which the frequency dependence of the magnetic energy losses in sintered soft ferrites can be predicted from DC to the MHz range. It is based on the appraisal of the contributions by domain wall (d.w.) displacements and magnetisation rotations and the related relaxation/resonant type dissipation mechanisms. The frequency dependence of the d.w. generated losses is calculated from the quasi-static losses, whereas the contribution by rotations is obtained via the Landau-Lifshitz-Gilbert equation.

A Nd-Fe-B cylindrical sample is tested first using a closed-circuit hysteresisgraph (T = 100 s). The hysteresis loop is then obtained by PFM (T = 11 ms) and corrected for the eddy current swelling (dashed line). Solid line: further correction for the magnetic viscosity field ΔH_c.

Experimental behaviour of the real and imaginary relative permeability components in a spinel ferrite μ^'(f) and μ^''(f)(symbols) described (solid lines) by adding the predicted contributions of domain wall displacements (dashed lines) and rotational processes (dash-dotted lines).

Evolution of the FMR frequency in nanogranular Fe-Co-B-Ni films having different thickness with the applied field H_a. Open squares, dots and triangles measurements; solid, dashed and dotted lines data fit using the Kittel formula.

The picture shows the model prediction of a magnetic refrigeration cycle in the s-T diagram computed for a cross section of an Active Magnetic Regenerative refrigeration device with Gd₅Si₂Ge₂ alloy as working material. The model is a superposition of bistable units with distributed energy barriers and energy levels that make it possible to compute the efficiency and the power of the refrigeration machine. The parameters have been identified from experimental isothermal magnetisation curves and the s-T transformations are computed without additional parameters. Continuous lines: irreversible transformations accounting for hysteresis effects. Dashed lines: reversible transformations by anhysteretic curves.

Model prediction of a magnetic refrigeration cycle for a Gd₅Si₂Ge₂ alloy.

The theory of ferromagnetic resonance and spin-wave instabilities is based on the assumption of small magnetisation motions around the saturated state. In nanomagnets, larger motions can be realised and consequently the conditions under which these motions are stable have to be determined. An analytical theory of spin-wave instabilities in systems with uniaxial symmetry has been developed, together with a micromagnetic code for the numerical simulation of magnetization dynamics in ultra-thin disks, subject to a spatially uniform dc field and radio-frequency excitation. The simulations give evidence of the onset of a quasi-periodic regime, in good agreement with analytical theory.

Numerical simulation of foldover and instability processes in ultra-thin magnetic disk. Solid squares: numerical prediction for transverse magnetisation as a function of dc field. Jumps in the curve represent foldover instabilities. The shaded region is the numerical prediction for a quasi-periodic response (see inset for time dependence).

The archaeo-magnetism of ancient ceramics and natural rocks is usually employed for dating inferences. Hysteresis loop measurements have been combined with colour surveys to characterise the properties of burnt clays and archaeological tiles, to evaluate the conditions applied during production and to distinguish groups of objects of different provenance. The properties of ancient artefacts have been observed to depend on the number and volume of iron oxide particles and can be classified in terms of high coercivity (ferrimagnetic) or low coercivity (paramagnetic and super-paramagnetic) magnetic behaviour.

Obsidian samples caught from ores situated on Etna (Sicily, Italy) and Mylos island (Greece).