C. Appino

One-dimensional/two-dimensional loss measurements up to high inductions

A thermometric-fieldmetric method has been developed by which Fe–Si laminations are characterized under either alternating or rotational excitation up to peak induction Bp=1.85 T in the frequency range 2 Hz≤f≤200 Hz. The measurement is performed on circular samples (diameter=140 mm) under digitally controlled one-dimensional/two-dimensional flux loci. The power losses at high inductions are determined by measurement of the rate of rise of the specimen temperature. Imperfect adiabatic behavior of the material is accounted for by physical modeling of the thermal diffusion process. The low-to-medium induction range, up to Bp∼1.7 T, is covered by conventional fieldmetric measurements, which, in conjunction with the thermometric approach, permit one to achieve a complete characterization of the magnetic sheets versus Bp and f.

Loss separation in soft magnetic composites

We report and discuss significant results on the magnetic losses and their frequency dependence in soft magnetic composites. Two types of bonded Fe-based materials have been characterized at different inductions from dc to 10 kHz and analyzed by extending the concept of loss separation and the related statistical theory to the case of heterogeneous materials. Starting from the experimental evidence of eddy current confinement inside the individual particles, the classical loss component is calculated for given particle size distribution. Taking then into account the contribution of the experimentally determined quasistatic (hysteresis) loss, the excess loss component is obtained and quantitatively assessed. Its behavior shows that the dynamic homogenization of the magnetization process with frequency, a landmark feature of magnetic laminations, is restrained in these materials. This results into a partial offset of the loss advantage offered by the eddy current confinement.

Giant magnetoimpedance and induced anisotropy in joule-heated and conventionally annealed Co-based amorphous materials

Abstract The effect of induced magnetic anisotropy on GMI has been studied in Co 71 Fe 4 B 15 Si 10 melt spun amorphous ribbons, using two different techniques of thermomagnetic annealing. Quasi-static hysteresis loops were measured at controlled induction. From the quasi-static hysteresis loop evolution, the value of the macroscopic anisotropy K u induced by the field annealing treatment was determined. The evolution of the domain structure was observed using a Kerr-effect apparatus.

Computation of Eddy Current Losses in Soft Magnetic Composites

We compute the classical eddy current losses in soft magnetic composite (SMC) materials, taking into account the eddy current paths appearing at the scale of the sample cross-section because of random contacts between the grains. The prediction of this loss contribution is a challenging task, because of the stochastic nature of the associated conduction process. We start our study from an identification of the statistical properties of the contacts between grains, starting from resistivity measurements. We then develop a numerical loss model for random grain-to-grain conduction, by which we demonstrate that the classical loss in SMCs can be decomposed into a contribution deriving from the eddy currents circulating inside the grains and a contribution due to the macroscopic eddy currents flowing from grain to grain via random contacts. An experimental validation of this model is proposed for a representative SMC material, where the magnetic losses are measured in ring samples with a range of cross-sectional areas.

Initial susceptibility vs. applied stress in amorphous alloys with positive and negative magnetostriction

The dependence of initial susceptibility X on applied tensile/compressive stress sigma has been determined for Fe and Co based amorphous alloys characterized by positive and negative magnetostriction, respectively. The contributions to X originating from coherent spin rotation (X/sub r/) and domain wall motion (X/sub w/) are separated by means of suitably devised experiments and found to correlate with the evolution of the domain structure vs. sigma , as directly verified by means of magnetooptical Kerr effect observations. High tensile stresses produce entirely longitudinal (transverse) domain structure in Fe (Co) based ribbons. The longitudinal (transverse) pattern evolves into a transversal (longitudinal) one by progressively changing tension into compression. X/sub r/ and X/sub w/ correspondingly pass through a maximum at intermediate stress values. >

Classical eddy current losses in Soft Magnetic Composites

This paper deals with the problem of loss evaluation in Soft Magnetic Composites (SMCs), focusing on the classical loss component. It is known that eddy currents can flow in these granular materials at two different scales, that of the single particle (microscopic eddy currents) and that of the specimen cross-section (macroscopic eddy currents), the latter ensuing from imperfect insulation between particles. It is often argued that this macroscopic loss component can be calculated considering an equivalent homogeneous material of same bulk resistivity. This assumption has not found so far clear and general experimental validation. In this paper, we discuss energy loss experiments in two different SMC materials, obtained using different binder types, and we verify that a classical macroscopic loss component, the sole size-dependent term, can be separately identified. It is also put in evidence that, depending on the material, the measured sample resistivity and the equivalent resistivity entering the calculation of the macroscopic eddy currents may not be the same. A corrective coefficient is, therefore, introduced and experimentally identified. This coefficient appears to depend on the material type only. An efficient way to calculate the macroscopic classical loss in these materials is thus provided.

Can rotational magnetization be theoretically assessed

We review state-of-the-art modeling of two-dimensional (2D) magnetization process in soft magnets. Emphasis is placed on the phenomenological assessment of the energy losses in steel sheets under rotational field. It is stressed that a general physically based theoretical framework, exploitable for applications in practical materials and machine cores, is not available at present. However, a rational approach to the 2D magnetic losses and their frequency dependence can be pursued. This is based on the separation between quasi-static (hysteresis) and dynamic magnetization processes and the connection with their unidirectional (scalar) counterpart. A number of vector models of the 2D quasi-static hysteresis behavior, typically residing on assessed scalar models, are discussed. They have prevalent formal-mathematical character, and, while being often helpful in numerical calculations, they leave unanswered the quest for solid physical interpretation. The dynamic magnetization process can, on the other hand, rely on a 2D phenomenological extension of the rmly established physical approach to the unidirectional scalar case provided by the statistical theory of losses. This is especially true for isotropic (i.e. nonoriented) magnetic laminations, where both rotational and elliptical flux loci can be conveniently handled. It permits one to achieve general appraisal of the experiments and flexible formulation, useful for numerical applications in machine cores

Effect of stress anisotropy on hysteresis and Barkhausen noise in amorphous materials

Hysteresis, power losses, and the Barkhausen effect are investigated in an Fe-based highly magnetostrictive amorphous material, as a function of applied stress. By means of the static and dynamic Preisach model, and of existing theories of the Barkhausen effect, the results are shown to be compatible with the existence of a characteristic structural length δc, playing a role similar to that of grain size in crystalline materials. At low applied stresses, where the magnetization process is dominated by quenched-in stresses σi, δc is identified with the typical wavelength of σi fluctuations. The theoretical analysis leads to the estimate δc∼70–100 μm and 〈σi〉∼3.5 MPa.

Broadband Magnetic Losses in Fe-Si and Fe-Co Laminations

We discuss comprehensive broadband (dc-10 kHz) investigations on magnetic losses in Fe-(3 wt%)Si and Fe-Co laminations. In this range of frequencies, the prediction of loss is not easy, because skin effect can be quite important. The theoretical approach generally relies on a dynamic hysteresis model in association with a diffusion equation, but it imposes heavy computational burden. We present here a computationally efficient dynamic hysteresis model based on the dynamic Preisach model (DPM), by which one can achieve fast and precise solution of the diffusion equation considering the hysteretic constitutive equation of the material. This is achieved at a greatly reduced computational cost with respect to the standard DPM. The loss results provided by this simplified model are at all frequencies in a very good agreement with the prediction by the full DPM and with the experiments.

Alternating and Rotational Losses Up to Magnetic Saturation in Non-Oriented Steel Sheets

A three-phase magnetizer has been developed, by which non-oriented Fe-Si steel sheets can be characterized under alternating and rotational flux up to a polarization value Jp ≈ 0.98 Js, where Js is the saturation magnetization. The loss measurements, performed in the frequency range 2 Hz-1 kHz, require the combination of field-metric and thermometric methods, besides fine control of the induction wave-shape/loci under the required demanding exciting conditions. By exploiting the loss separation concept, it is observed that under rotational flux, both the hysteresis and excess loss components monotonically decrease with Jp, to disappear at saturation. The measured losses then become equal to the calculated classical losses. This could actually be predicted, because of the expected disappearance of the domain walls under saturating rotational field, but it has never been previously verified by the experiments.