G. A. Badini-Confalonieri

Magnetic anisotropy in ordered textured Co nanowires

The magnetization reversal in ordered arrays of Co nanowires with tailored hcp-phase texture, controlled by pH synthesis and nanowires length, has been investigated. The angular dependence of coercivity has been experimentally determined for different crystal textures, and the corresponding magnetization reversal mode is interpreted by analytical modelling. The results show that reversal takes place by propagation of a transverse-like domain wall mode. The fitting of experimental and calculated data allows us the quantitative evaluation of the magnetocrystalline anisotropy constant strength whose magnetization easy direction evolves from parallel to the wires toward in-plane orientation with the change of hcp-phase texture.

The effect of transverse field on fast domain wall dynamics in magnetic microwires

We have studied the domain wall longitudinal propagation and its dynamics under the influence of transverse magnetic field in thin magnetic wires. A different behavior was observed for strong and weak transverse fields. In weak transverse field Ht, the domain wall dynamics depends on the direction of Ht. Transverse field applied in one direction increases the Walker limit and shifts the existence of transverse domain wall to higher axial field. Transverse magnetic field applied in opposite direction decreases the Walker limit and favors vortex domain wall even at low fields. Different behavior was obtained in strong transverse field which speeds up the domain wall velocity to its saturation value of 9 km/s independently on the orientation of transverse field.

Magnetotunable left-handed FeSiB ferromagnetic microwires.

The magnetotunable left-handed characteristics of Fe(77.5)Si(12.5)B(10) glass-coated ferromagnetic microwires are analyzed in array and single microwire configuration, employing a rectangular waveguide working in X band. While the negative permeability is ascribed to the natural ferromagnetic resonance (NFMR) of the highly and positive magnetostrictive microwire, the negative permittivity features of the medium are attributed to the interaction of the microwires with the metallic rectangular waveguide. The dependence of the NFMR frequency on the applied external magnetic field enables the design of magnetotunable left-handed systems with wide-frequency band.

Fabrication and magnetic properties of hard/soft magnetostatically coupled FePt∕FeNi multilayer microwires

A family of multilayer microwires with hard/soft biphase magnetic behavior is here introduced. The microwires consist of a Fe63Pt27Si10 hard magnetic nucleus and a Fe20Ni80 soft outer shell separated by an intermediate insulating Pyrex glass microtube. The precursor FePtSi glass-coated microwire is fabricated by quenching and drawing technique, and its L10 hard magnetic phase is grown by postannealing treatment technique. The polycrystalline FeNi soft magnetic outer shell has been deposited by electroplating. The analysis of the low-field hysteresis loops of the FeNi soft phase after premagnetizing until near magnetic saturation provides information about the magnetostatic coupling between phases. The FeNi magnetization curve is shifted toward positive field when the FePt remanent magnetization is positive and vice versa. A systematic analysis of the magnetostatic coupling and the corresponding bias field arising from uncompensated poles of the premagnetized FePt hard phase has been performed. The strengt...

Study of the Switching Field in Amorphous and Nanocrystalline FeCoMoB Microwire

We have studied the frequency dependence of switching field in a wide range of frequencies in amorphous and nanocrystalline microwires with nominal composition Fe40Co38Mo4B18. Samples were heat treated for 1 h at different temperatures in a wide temperature range 20-600°C. Three regions in the frequency dependence of the switching field were identified. Drop of switching field at low frequencies up to 50 Hz is explained in term of structural relaxation. Above 50 Hz the magnetoelastic contribution of the switching field is dominant. The magnetoelastic contribution of the switching field can be fitted by the power law (H sw ¿ ~ f1/n), giving exponent n equal 2 for frequency below 1000 Hz for all studied samples. Above 1000 Hz, the switching field reflects the structure of microwire being highly frequency dependent in as-cast sample and sample annealed at 450°C (where the microwire is quite inhomogeneous) while its frequency dependence is very weak for other annealing temperatures. Moreover, power exponent n gives non-physical values (~ 100) in this range.

Bistable FeCoMoB microwires with nanocrystalline microstructure and increased Curie temperature

Novel nanocrystalline glass-coated Fe40Co38Mo4B18 microwires are introduced. They combine the advantages of nanocrystalline alloys exhibiting simultaneously increased Curie temperature and magnetic bistability, which is required for modern sensoric and spintronic devices. Positive magnetostriction of the crystalline FeCo grains results in a magnetic bistability, whereas good soft magnetic properties remain stabilized. As a result of the mechanical stress induced by the glass coating, the optimum temperature range for thermal treatment is enhanced up to 600 °C.

Nonlinear magnetoimpedance and parametric excitation of standing spin waves in a glass-covered microwire

The giant magnetoimpedance of an 8.5 μm glass-covered amorphous microwire was investigated in the frequency range of 10 MHz–3.5 GHz. It was found that when the exciting microwave current exceeds some threshold value, a periodic fine structure appears in the frequency dependence of the complex impedance. The appearance of this nonlinear phenomenon is interpreted to be a consequence of the parametric excitation of standing spin waves.

Locally induced domain wall damping in a thin magnetic wire

The damping mechanisms affecting the motion of a single domain wall were studied in a thin bistable magnetic wire. It was found that the overall damping is frequency and temperature dependent through the locally induced anisotropy via structural relaxation. This phenomenon can increase the overall damping by one order of magnitude and enables an effective tailoring of the domain wall dynamics according to required application.

Multipeak ferromagnetic resonance behaviour tailored by magnetoelastic coupling in FeSiB/CoNi layered microwires

The aim of this work is to report on the influence of the presence of a magnetically hard layer with large thickness on the ferromagnetic resonance (FMR) behaviour of soft/hard biphase microwires. Such microwires consist of a FeSiB soft nucleus, an intermediate glass layer and a CoNi hard outer shell, and are prepared by the combined quenching and drawing technique with electrochemical deposition. The unusual FMR response observed for this biphase system is characterized by the presence of a multipeak absorption spectrum that depends on the static field and on the thickness of the hard phase by means of magnetoelastic coupling. For a thick enough CoNi layer, the internal stresses induced in the nucleus during the electroplating process are very strong, giving rise to three electromagnetic absorptions: one at a low frequency related to the CoNi shell and two at higher frequencies corresponding to the FeSiB nucleus which are ascribed to two well-defined magnetization regions with different magnetic anisotropies.

Ni–Mn–Ga ferromagnetic shape memory wires

Ni–Mn–Ga ferromagnetic shape memory wires (Ni2.10Mn0.98Ga0.92, mean diameter 170 μm) are obtained by the rotating water bath melt spinning technique. The compositional heterogeneity linked to its dendritelike structure gives rise to a complex and broad martensitic transformation (MT). The reduced value of magnetization in the as-spun sample is ascribed to Mn–Mn antiferromagnetic interactions at structural defects as atomic disorder, vacancies, and antiphase boundaries structures. Moreover, the observed low temperature magnetic relaxation process is characterized by a splitting in the zero-field-cooled/field-cooled magnetization curves and the frequency shift in the ac magnetic susceptibility. The results are interpreted in terms of the coexistence of a reduced magnetization state and nearly noninteracting magnetic clusters. A high temperature treatment optimizes both the MT and the magnetic characteristics (i.e., decrease in the hysteresis of the MT and magnetization recovery, respectively).