The scientific activity of the Materials Department 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 the year 2005 are summarized here.
Metallic alloys with 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 characterized. Thin film preparation was chiefly focused on permalloy, Fe/Co alloys and Fe/Tb giant magnetostriction alloys.
Domain structure of a nanogranular FeCoAlO thin film near an irregular edge by high-resolution Kerr microscopy. The magnetic field is applied in the horizontal direction, with intensity increasing from top to bottom.
Iron thin film meander, RF-sputtered and optically lithographed in the magnetic thin film laboratory. Domain structure visualized by High-resolution Kerr microscopy, developed in the Magnetooptical laboratory. The applied magnetic field lies on the horizontal direction, with increasing intensity (from left to right, from top to bottom).
Very high coercivity Nd-Fe-B and Sm-Co permanent magnets have been characterized in a complete fashion exploiting Pulsed Field Magnetometry (PFM). Comparison with conventional methods making use of superconducting solenoids evidences the role of the thermal fluctuation aftereffect. It has been shown that the behavior of permeability and losses in Mn-Zn ferrites from DC to the MHz range can be accurately predicted by exploiting the concept of loss separation. To this end, the dissipation mechanisms arising from spin damping associated with domain wall relaxation and precessional resonance are invoked and quantitatively described in the framework of the Landau-Lifshitz-Gilbert equation for spin dynamics. A novel setup for two-dimensional (2D) measurements in soft magnetic laminations has also been developed. A three-phase supply is employed, with flux loci control based on the principle of contraction mapping.
Hysteresis loop by PFM and Extraction Magnetometer (EM). Magnetization rates: ~ 450 T·s-1 with PFM and ~ 5 ·10-4 T·s-1 with EM.
Induction B and field H loci in a grain-oriented Fe-Si lamination tested under controlled rotational flux at 50 Hz.
A reference system for magnetic flux density is maintained at INRIM in the range 1 T - 10-5 T making use of a stable wide pole face electromagnet, a couple of Helmholtz coils with current-controlled supply system, flowing-water and still-water NMR magnetometers. The calibration of the reference coils partly relayed on the application of a standard Garrett solenoid, previously subjected to an intercomparison exercise. The relative expanded uncertainty of the system is estimated to range between 2·10-3 (B ~ 10-5 T) and 7·10-6 (B = 1 T).
Relative expanded uncertainty versus magnetic flux density strength in the INRIM reference system.
Solid state cooling using the magnetocaloric effect (MCE) is an attractive novel technology which promises efficient refrigeration with a lower environmental impact with respect to current gas based techniques. Particularly large MCE has been observed in ferromagnetic materials presenting a first-order magnetostructural phase transition. In adiabatic conditions the transitions are accompanied by large temperature changes up to 8-10 K under a field change H ~ 5 T. The results of Ni2MnGa are of particular interest due to the specific characteristics of the alloy which presents, in the oriented single crystal form, a rich magnetic, mechanical and thermal phenomenology. A new experimental setup was built and operated to obtain the presented data.
Adiabatic temperature changes ΔTad(T)H induced by a variable magnetic field change in Ni2MnGa single crystal samples presenting a magneto-structural phase transition.
A current of spin-polarized electrons can apply appreciable torques to a nanoscale ferromagnet, which may result in current-induced switching or microwave oscillations of magnetization. By applying methods of nonlinear dynamic system theory and bifurcation theory we have analytically constructed the complete stability diagram characterizing the response of the nanomagnet when spin-polarized currents and external magnetic fields are simultaneously present. Detailed quantitative predictions have been derived for the critical currents and fields inducing magnetization switching, for the amplitude and frequency of magnetization self-oscillations, and for the conditions leading to hysteretic transitions between self-oscillation and stationary regimes.
Top. Stability diagram for current-induced magnetization dynamics. Vertical axis: current density; horizontal axis: external field, applied along the in-plane easy-axis of the nanomagnet. Symbols P, A, and S2 indicate stationary magnetization regimes, O and O2 self-oscillatory regimes. Bottom. Schematic representation of typical magnetization states for different dynamic regimes. Circles: stable (solid) and unstable (open) stationary states. Bold lines: stable (continuous) and unstable (dashed) self-oscillatory motions.
Mainstream research concerning hysteresis properties, both from the theoretical and experimental points of view, is mainly focused on scalar properties, with the loss of some fundamental aspects of the magnetization process. In fact the reversal of magnetization has an intrinsic vector nature, whose grasp needs a unified description of spin rotations and wall assisted magnetization reversals: Barkhausen jumps and nucleation. In order to investigate these phenomena a suitable experiment has been devised, which makes it possible to highlight hysteresis features at the same time. A Co-based circular amorphous sample, with a macroscopic uniaxial anisotropy, is submitted to an alternating magnetic field Ha, applied along a direction forming with the easy axis e∥ an angle θHa between 0° and 30°. The two orthogonal components of the magnetization M∥ and M⊥(along the easy axis and perpendicularly to it), and of the effective field H∥ and H⊥, have been measured. The evolution of the M∥ vs. M⊥ and H∥ vs. H⊥ loops allows one to work out the conditions leading to the nucleation of domains. The case θHa=30° is displayed in the figure, where the role played by reversible and irreversible magnetization processes along the first magnetization curve and the major loop is also highlighted.
Locus followed by the tip of the vector effective field H=Ha+Hd (Hd = demagnetizing field) when the alternating field Ha is applied. Starting from the demagnetized state (1) one can observe, in addition to the reversible spin rotation, the following magnetization regimes: (1-2) Bloch wall reversible displacement; (2-3) irreversible motion of Bloch walls; (3-4) and (4-5) absence of Bloch walls; (5-6) and (6-7) irreversible motion of Bloch walls. HN can be assimilated to the nucleation field.