An analysis of entropy production and its evolution in the frequency domain upon application of electromagnetic fields to ferrimagnetic material has been investigated. The spectral entropy of NiZn ferrites doped with different amounts of Yttrium has been studied by numerical simulation. Under the application of an oscillating electromagnetic field, the system is out of equilibrium as there is entropy production due to the dielectric and magnetic polarization. A system subjected to electromagnetic fields E.!/ and H.!/ at constant temperature T has been considered. For the numerical simulation of the behavior of the entropy production due to magnetic relaxation, experimental data have been correlated to a recently published causality model. For the entropy production due to dielectric relaxation, experimental data have been used. Entropy production related to magnetic response could be associated with two mechanisms: domain wall motion and spin rotation. Entropy production related to dielectric processes could be associated with the delay of the different components of the polarizability as a function of the frequency of the applied field and the correlation with the transport charge mechanism in the material. At the frequency range under study, the entropy production due to dielectric relaxation is negligible. Thus entropy spectra could be a suitable tool for the analysis of the ferrimagnetic materials. 176
Abstract-This paper presents the study of an artificial material, made up of a periodic structure, defined by a unit cell, consisting of a finite number N of periodic layers of thin conducting cylinders placed between two dielectric planes. These artificial materials known as metamaterials can be regarded as a homogeneous material with effective constitutive parameters impossible to achieve with naturally occurring materials, such as negative values for both magnetic permeability and electric permittivity. An analytical model has been developed to study the effective electric permittivity of the whole system in terms of the unit cell dimensions and the frequency of the incident electromagnetic wave. Simulations of the effective electric permittivity of the metamaterial were performed by varying the geometry of the metamaterial. This analysis enables the design and construction of structures with properties that make them an attractive candidate for shielding applications in the range of microwave frequencies. The metamaterial has been constructed with four rows of 5 bronze conducting rods each. We have made experimental measurements of the shielding effectiveness of these materials when subjected to a electromagnetic plane wave with electric field polarized along the direction of the conducting rods, and conversely, with electric field polarized perpendicular to the rods. Non-zero values for shielding effectiveness were observed in the first polarization, and zero values in the second case.
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