M. Pasquale

Measurement and prediction of dynamic loop shapes and power losses in soft magnetic materials

Measurements of dynamic hysteresis loops on Fe/sub 78/B/sub 13/Si/sub 9/ amorphous ribbons, in the range 0.5 Hz >

Structural, magnetic and anisotropic properties of Ni2MnGa melt-spun ribbons

Abstract The structural and magnetic characterisation of the melt-spun Ni 2 MnGa alloy is presented. The rapid solidification technique produces nanocrystalline ribbons with composition very close to the master alloy, which are single phase, chemically homogeneous and not brittle. The samples are highly textured with the easy magnetisation axis of the martensitic phase in the ribbon plane.

Power losses in magnetic laminations with hysteresis: Finite element modeling and experimental validation

Dynamic hysteresis loop shapes and magnetic power losses are studied in nonoriented Fe-Si laminations exhibiting significant excess losses. Measurements are carried out under controlled sinusoidal induction in the frequency range from 1 Hz to 1.6 kHz, at various peak inductions from 0.25 to 1.5 T. Excess losses are found to obey a f3/2 law up to frequencies of 200–400 Hz, depending on peak induction. Beyond this limit, definite deviations are observed, due to eddy current shielding. Detailed information on the flux and field distribution in this high frequency regime is obtained by finite element solutions of Maxwell equations employing the dynamic Preisach model to describe quasi-static hysteresis and dynamic wall processes. The agreement between theoretical predictions and measurements is discussed.

Loss measurements on amorphous alloys under sinusoidal and distorted induction waveform using a digital feedback technique

A newly developed digital feedback wattmeter, allowing power loss measurements in soft laminations under generic induction waveform is presented. The system is intrinsically free of auto‐oscillations, typical drawback of analog feedback circuits, and can therefore be operated in a wide frequency range (0.5 Hz–100 kHz). Loss measurements performed by this setup under sinusoidal and distorted induction waveforms on Co based amorphous ribbons are reported. It is shown that the classical approximation to the prediction of power losses under distorted induction largely fails to account for the experimental results. A novel theoretical approach, based on the statistical theory of losses, is discussed and successfully applied to the experiments. In particular, it is shown that knowledge of the loss components under sinusoidal induction at a given magnetizing frequency permits one to make an accurate prediction of the effect of distortion at that frequency as well as other ones. Illustrative applications at 1 kHz...

Temperature dependence of magnetically induced strain in single crystal samples of Ni-Mn-Ga

We investigate the effect of temperature and applied magnetic field on the strain behavior of oriented single crystal samples of Ni2MnGa. It is shown that the martensite twins can be partially oriented by the application of a magnetic field during cooling from the austenite state, observing a strain of the order of 1%. Depending on the final cooling temperature, a different value of magnetically induced strain is observed in the martensite phase, an effect due to the irreversible alignment of a fraction of the martensite twins to the magnetic field direction. The results are discussed with reference to different experimental procedures and the shape memory characteristics of the alloy.

Reversible and irreversible magnetization in soft iron‐based polycrystalline materials

The magnetization process has been investigated in nonoriented SiFe alloys over a wide induction range, from about 10−4 T to technical saturation. Special attention has been devoted to the role of grain size 〈s〉. The low‐field behavior of the initial magnetization curve is characterized by a number of remarkable features: (1) the permeability μr tends, in the limit of vanishing applied field H, to a constant value, independent of 〈s〉; (2) the increase of μr with H exhibits a nonlinear behavior, in contrast with the Rayleigh law; and (3) the larger the grain size, the steeper the rise of μr with H. This effect is attributed to an increased ease of propagation of locally induced magnetization reversals at larger 〈s〉 values. Experiments put in evidence a correlation between μr and the length of propagation λ, which is found to vary as μ1/2r. However, at high inductions (above about 1 T), larger grains imply a reduction of permeability, due to the growth of unfavorable crystallographic textures. Grain size al...

Field-driven structural phase transition and sign-switching magnetocaloric effect in Ni–Mn–Sn

Depending on the starting equilibrium temperature, the application of a magnetic field on a sample of Ni–Mn–Sn produces sample heating or cooling during adiabatic experiments. The competition between endothermal and exothermal effects is observed close to the martensite-to-austenite magnetostructural phase transition. A model assuming the coexistence of two phases and a field dependence of their volume allows to compute the evolution of entropy and heat capacity during the phase transition. The correct fitting of the results suggests that the field-induced reduction of the martensite-to-austenite transition temperature is responsible for the observed sign switching of the magnetocaloric effect.

Analysis of mechanical and magnetic instabilities in Ni-Mn-Ga single crystals

In this paper experimental stress-strain and magnetization versus field curves measured for Ni2MnGa alloys are compared with modeling results obtained using a statistical model. The model is based on the assumption of equivalence between the magnetic field and mechanical stress through magnetoelastic interactions. The approach is extended to describe the complex irreversible behavior of strain and magnetization using a statistical distribution of the energy barriers that separate the metastable states of the martensite twins. A theoretical value of the energy conversion coefficient cme=1.57×10−4 MPa (kA/m)−2 is found, which is in good agreement with the experimentally determined value of 1.25×10−4 MPa (kA/m)−2.

Stress sensing with Co based ferrite composites

We measure the magnetic and magneto-mechanical properties of polymer-bonded Co ferrites to analyze their behavior as stress sensors in a wide temperature and compressive stress range. The composites, possessing a magnetostriction of 30 10 6 and a Young’s modulus of 50 GPa, are good candidates for a range of sensing applications, with good chemical stability and good high frequency characteristics. r 2002 Elsevier Science B.V. All rights reserved.

Microwave Behavior of Polymer Bonded Iron Oxide Nanoparticles

Samples composed of a polymer matrix were loaded with different fractions from 0% to 30% of Fe oxide magnetic nanoparticles with an average size ranging from 5 to 25 nm. The permittivity and permeability of the composites were determined upon a very wide frequency range, spanning from DC to 30 GHz using different methods with overlapping frequency intervals. The nanoparticle content was observed to significantly modify the permeability and permittivity behavior of the polymer. The frequency and intensity of both dielectric and ferromagnetic resonances of the composites are clearly affected by the presence of the magnetic nanoparticles, and the composites can be optimized for specific applications requiring absorption of electromagnetic radiation from the MHz to the GHz range.