1,721,085 research outputs found
Misure su nuovi tipi di fotorivelatori per calorimetria ad alta energia in campi magnetici
No full textHysteresis and nucleation field in two-dimensional magnetization process
No full textWe have investigated the vector nature of the magnetization process in disk-shaped Co-based amorphous samples,
providing a joint description of spin rotation and domain nucleation processes. Thermo-magnetic treatments have been
exploited to induce a macroscopic uniaxial anisotropy in the specimen plane. An alternating field Ha has been applied
to the specimens along a direction forming with the easy axis an angle yHa between 0! and 90!: The two orthogonal
components of the magnetization Mk and M? (along the easy axis and perpendicularly to it), and of the effective field
Hk and H?; have been measured by means of a compensated winding and a many-turn H-coil placed on the sample
surface. We have studied the evolution of the Mk vs. M? and Hk vs. H? loops as a function of yHa : In particular we
have pointed out the condition leading to the nucleation of domains in a two-dimensional magnetization process, and
obtained the nucleation field values HN for any yHa : These results have been interpreted proposing a model accounting
for nucleation in the framework of the Stoner–Wohlfarth theory
The Memory in Magnetic Systems
No full textMost of the features of ferromagnetic materials are embedded in their hysteresis loop [1]: a matter compelling one to focus the attention on the memory properties of magnetic systems. Starting from the landmark work of Preisach, where the memory effects of scalar hysteresis are suitably accounted for by means of the so called Preisach plane, a vector generalization to ensembles of Stoner-Wohlfarth particles has been recently proposed [2]. In this approach the stability properties of particles are described by adopting the conventional astroid representation, a strategy also exploited to discuss incoherent particle rotations [3]. On the other hand, in order to include the possibility of reversal by domain wall motion, a modification of the coherent rotation mechanism, graphically represented by a “truncated astroid”, has been proposed as well [4].
Within this framework, we present a general mathematical tool able to track the history of any magnetic system, with reversal of magnetization driven by reversible and irreversible rotations and domain wall displacements. Coupled to a vector hysteresis model, the procedure outlined allows one to decrease to a large extent the computational time needed to reproduce the hysteresis loop. Eventually, this instrument supplies a set of thermodynamic variables providing an estimate of the magnetic energy loss entailed by irreversible processes, and the consequent entropy production.
[1] “The Science of Hysteresis”, G Bertotti and I. Mayergoyz Editors, Academic Press (Elsevier), 2006
[2] Carlo Appino, “Representation of Memory in Particle Assembly Hysteresis Models”, IEEE Trans. on Mag., VOL. 45, NO. 11, Nov. 2009
[3] A. Stancu, C.Papusoi, L. Spinu, “Mixed –type model of hysteresis”, J. Magn. Magn. Mater. 150 (1995) 124-130.
[4] C. Appino, M. Valsania, V. Basso, “A vector hysteresis model including domain wall motion and coherent rotation”, Physica B 275 (2000) 103-10
Determinazione del gap energetico dell'ossido superconduttore YBCO per mezzo di misure di tunnel
No full textVector Model for Losses in Non-Oriented Steel sheets
No full textThe presence of two-dimensional (2D) magnetization processes in devices and electrical machines calls for the development of vector model for loss prediction. Starting from the Del Vecchio-Charap work [2], we developed a static model able to reproduce the 2D evolution of magnetization in soft non-oriented steel sheets: the most interesting case for applications.
The material is assimilated to an ensemble of biaxial grains, each associated with a hysteron having two orthogonal easy axes. In each of them, the field driven irreversible switch of magnetization between the easy directions (Barkhausen jump: BJ) is governed by the value of local coercive field (about ten times smaller than the anisotropy fields); the ensemble of all these BJs accounts for the domain wall displacement. After the BJ, the local magnetization is brought to its energy minimum by the antagonism between anisotropy and Zeeman energy, so determining the reversible magnetization component. Domain wall reversible processes (bending) are not considered, whereas the role of macroscopic and internal demagnetizing fields is accounted for.
The system average magnetization is obtained after integrating the outputs of single hysterons, each weighted by probability density functions suitably characterizing the material properties (i.e.: grain orientations, coercive and anisotropy fields). We have been able to reproduce the loss vs. polarization W(Jp) evolution in several non-oriented materials, subjected to alternating and rotating fields. In particular, under circular induction, the W drop at high Jp was always found, without introducing “ad hoc” fitting functions. It is remarkable that the outlined approach can be extended to systems magnetized in dynamic conditions.
[1] C. Appino, C. Ragusa, and F. Fiorillo, “Can rotational magnetization be theoretically assessd?”, IJAEM 44 (20144), 355-370
[2] R.M. Del Vecchio, S. H. Charap, “Two dimensional hysteresis model”, IEEE Trans. Mag. 20 (1984), 1437-143
Magnetic characterization of Fe-Si non-oriented laminations under alternating, rotational, and 2D flux loci
No full textPerdite di potenza nei materiali ferromagnetici amorfi in funzione dell'induzione e dello sforzo applicato
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