1,721,010 research outputs found
13th International Workshop on 1&2 Dimensional Magnetic Measurement and Testing, 3-6 September 2014, Turin, Italy
No full textMagnetic loss versus temperature and role of doping in Mn-Zn ferrites
Full text linkWe investigate the effect of different doping schemes on the broadband magnetic losses and their temperature dependence in Mn-Zn ferrites. CaO, Nb2O5, ZrO2, and SiO2 are added with increasing proportions to TiO2-doped prefired powders and, after sintering at either 1275 °C or 1300 °C, the obtained ring samples are tested versus frequency f (DC-1 GHz) and peak polarization Jp (2 mT – 200 mT) up to T = 160 °C. Appropriately enhanced impurity contents are shown to induce further decrease of the energy loss in materials already prepared for best performance at high temperatures (140 – 160 °C). This behavior can be hardly ascribed to the impurity-related increase of the electrical resistivity brough about by extra-doping, being it rather connected to a corresponding monotonical decrease of the effective magnetic anisotropy < Keff > with T. The decreasing anisotropy makes the balance between the contributions of domain wall (dw) displacements and reversible rotations to the magnetization process evolving in favor of the latter. The energy loss correspondingly develops with frequency and peak polarization in a complex fashion, according to the specific dissipative mechanisms sustained by the spins precessing either inside the moving walls or in the bulk. A dividing line in the (Jp − f) plane is identified, which separates dominant dw- and rotation-generated losses. It moves downward (i.e. lower f) with increasing temperature, the higher T the lower the frequency at which the rotations, theoretically assessed via the Landau-Lifshitz equation, supersede the domain wall contribution. Once accomplished, however, the transition to rotations can lead, according to the theoretical model, to higher losses when moving to higher temperatures. Following the experimental trend of the complex resistivity versus frequency at different T values, the calculations and the experiments show that eddy currents start to contribute to the energy loss, in the 5 mm thick ring samples, around a few MHz, accounting for about 50 % of measured loss beyond some 50 MHz. The chief dissipative process at applicative frequencies and induction values is therefore identified with spin damping, to which the generalized loss decomposition method can be applied
Magnetic Loss Decomposition in Co-Doped Mn-Zn Ferrites
Full text linkThe magnetic properties of sintered Mn-Zn ferrites, Co2+ enriched by addition of CoO up to 6000 ppm, were measured in ring samples for a broad range of peak polarization values (2 mT – 200 mT) and frequencies (dc – 1 GHz). The results were analyzed by separating the contributions to the magnetization process of domain-wall motion and magnetization rotation, and applying the concept of loss decomposition. By determining the value and behavior of the rotational permeability μrot as a function of the CoO content, we obtain the average effective magnetic anisotropy and its effect on the loss. We thus identify the hysteresis (quasi-static) Wh, rotational Wrot, and excess Wexc loss components and their dependence on CoO. The quasi-static loss Wh, the domain wall permeability μdw, and have minima, and μrot has a maximum, for CoO in the range 3000 – 4000 ppm. The rotational loss by spin damping Wrot,sd is calculated by use of the Landau-Lifshitz equation by assuming distributed anisotropy field amplitudes. Wrot,sd covers the experimental loss behavior beyond about 1 MHz. Wexc and Wh, both directly generated by the moving domain walls, share the dissipative response of the material at lower frequencies and show similar trends versus CoO content. It is concluded that the modulation of the magnetic anisotropy of Mn-Zn ferrites through Co2+ enrichment, leading to maximum magnetic softening for CoO = 3000 – 4000 ppm, can be assessed in terms of separate effects of domain wall motion and moment rotations and their related dissipative properties
Sensing dynamic forces by Fe-Ga in compression
Full text linkThis paper concerns the sensing of dynamic force/stress by means of a measuring system based on the use of an Fe-Ga sample (Galfenol) coupled to a magnetic circuit. The study is focused on the measurement of the effective magnetic field, detected at the rod specimen surface, and its variation DeltaHeff under time-dependent applied sinusoidal stress DeltaSigma oscillating at frequencies between 5 Hz and 20 Hz at different values of applied bias field Sigmabias (2.5 kA/m < Sigmabias < 27 kA/m). For the considered frequency and Hbias range, the measured DeltaHeff tends to linearly depend on DeltaSigma in contrast with the corresponding induction variation behavior (DeltaB - DeltaSigma), where hysteresis effects appear. With DeltaSigma in the range ± 15 MPa we obtain 0.06 MPa resolution in the determination of the alternating stress DeltaSigma, a result pointing to the effectiveness and sensitivity of this Galfenol-based method for detection and measurement of time-dependent stresse
The temperature dependence of magnetic losses in CoO-doped Mn-Zn ferrites
Full text linkCoO-doping is known to stabilize the temperature dependence of initial permeability and magnetic losses in Mn-Zn ferrites, besides providing, with appropriate dopant contents, good soft magnetic response at and around room temperature. These effects, thought to derive from the mechanism of anisotropy compensation, are, however, poorly assessed from a quantitative viewpoint. In this work, we overcome such limitations by providing, besides extensive experimental investigation vs frequency (DC–1GHz), CoO content (0 ≤ CoO ≤ 6000 ppm), and temperature (−20 °C ≤ T ≤ 130 °C) of permeability and losses of sintered Mn-Zn ferrites, a comprehensive theoretical framework. This relies on the separate identification of domain wall motion and moment rotations and on a generalized approach to magnetic loss decomposition. The average effective anisotropy constant ⟨Keff⟩ is obtained and found to monotonically decrease with temperature, depending on the CoO content. The quasistatic energy loss Wh is then predicted to pass through a deep minimum for CoO = 3000–4000 ppm at and below the room temperature, while becoming weakly dependent on CoO under increas- ing T. The rotational loss Wrot(f) is calculated via the complex permeability, as obtained from the Landau-Lifshitz equation for distributed values of the local effective anisotropy field Hk,eff (i.e., ferromagnetic resonance frequency). Finally, the excess loss Wexc(f) is derived and found to comply with suitable analytical formulation. It is concluded that, by achieving, via the rotational permeability, value and behavior of the magnetic anisotropy constant, we can predict the ensuing properties of hysteresis, excess, and rotational losses
Alternating and Rotational Losses Up to Magnetic Saturation in Non-Oriented Steel Sheets
Full text linkA 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 experiment
Influence of mechanical and water-jet cutting on the dynamic magnetic properties of NO Fe-Si steels
Full text linkMagnetization curve and energy losses have been analyzed in non-oriented (NO) Fe-Si alloys with variable thickness (0.20 mm–0.35 mm), cut at widths ranging between 5 mm and 60 mm, in order to assess the impact of cutting, either done by punching or water-jet techniques. Measurements were performed by means of a digitally controlled single strip tester, from dc up to 1.5 kHz, at peak polarization values Jp = 1.0 T and 1.5 T. The evolution of the magnetization curve and the structure-dependent hysteresis Wh and excess Wexc loss components have been assessed as a function of the strip width using a simple phenomenological model, by which the extension of the damaged area at the edges of the cut sheets is estimated. Such a model assumes a hyperbolic dependence of the measured polarization on the cut strip width
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