Limitations On High-frequency Permeability Of Magnetic Materials

Engineering of magnetic composites with high microwave permeability is important for EMC/EMI and other applications. One of the challenging problems is to determine constraints on the achievable high-frequency permeability of magnetic composites. The objective of the present paper is to analyze the data available from the literature on the microwave permeability constraints and derive a generalized constraint condition. It is well known that in many practical occasions the actual magnetic frequency dispersion curves differ from the Lorentzian behavior. For these cases, an integral form of the constraint may be useful. Possible applications of the integral constraint are discussed for the cases of large damping, pronounced effect of eddy currents, and inhomogeneous materials. It is shown that the derived constraint can be successfully used in these cases for both estimating microwave performance of devices containing magnetic materials and obtaining additional data on the magnetic structure of materials. © 2013 IEEE

Application Of Generalized Snoek\u27s Law Over A Finite Frequency Range: A Case Study

Generalized Snoek\u27s law proposed in an integral form by Acher and coauthors is a useful tool for investigation of high-frequency properties of magnetic materials. This integral law referred to as Acher\u27s law allows for evaluating the ultimate performance of RF and microwave devices which employ magnetic materials. It may also be helpful in obtaining useful information on the structure and morphology of the materials. The key factor in practical application of Acher\u27s law is an opportunity to employ either measured or calculated data available over a finite frequency range. The paper uses simple calculations to check the applicability of Acher\u27s law in cases when the frequency range is limited, and the magnetic loss peak is comparatively wide and has a distorted shape. The cases of large magnetic damping pronounced skin effect, and inhomogeneity of the material are considered. It is shown that in most cases calculation of the integral through fitting of actual magnetic frequency dispersion by the Lorentzian dispersion law results in accurate estimations of the ultimate high-frequency performance of magnetic materials

Corporate governance and insider trading

Thesis (Ph. D.)--Massachusetts Institute of Technology, Sloan School of Management, 2008.Includes bibliographical references (p. 58-64).I investigate the relation between corporate governance and insider trading by corporate executives. Despite the general view that trade on non-public information adversely affects capital market participants, the impact of corporate governance on such trading remains relatively unexplored in prior research. I propose an empirical measure that relies on a predicted pattern in stock returns to identify transactions that are more likely to be based on private information and provide evidence to validate the construct. Using this measure, I find that good corporate governance, identified through board and ownership characteristics that have been linked to more effective monitoring of management in prior research, is negatively related to opportunistic insider trading. In supplementary analysis, I provide evidence on the robustness of this relation to an alternative hypothesis and to potential endogeneity. Overall, I conclude that good corporate governance helps to attenuate opportunistic insider trading.by Konstantin A. Rozanov.Ph.D

Influence Of Higher-order Modes In Coaxial Waveguide On Measurements Of Material Parameters

The use of a coaxial air-filled line as a test fixture for measuring complex permittivity and permeability often shows odd resonance-like behavior of material parameters as functions of frequency. This effect is typically either ascribed to the half-wavelength resonance at the sample length, or erroneously misinterpreted as intrinsic resonance behavior of the material. However, as is shown in this paper, such behavior can be attributed to excitation of the higher-order modes on the surface of the sample resulting in resonance absorption of electromagnetic energy in the test fixture. Herein, analytical, numerical, and experimental results show that there can actually be a significant impact of higher-order modes in a coaxial line on the extracted constitutive material parameters of samples

Microwave Permeability of CO₂ Z Composites

The microwave permittivity and permeability of Co2Z barium ferrite composite samples are measured as functions of frequency and volume fraction of the ferrite. Magnetostatic properties of the bulk ferrite are determined. This allows Snoek\u27s law [J. L. Snoek, Physica 14, 204 (1948)] to be verified by comparing the microwave and magnetostatic Snoek\u27s constants. The modification of Snoek\u27s law for hexagonal ferrites suggested recently by Acher et al. [Phys. Rev. B 62, 11324 (2000)] is also verified. Acher\u27s constant is found from microwave measurements to agree with the value calculated from the magnetostatic properties of bulk ferrite, but microwave and magnetostatic Snoek\u27s constant do not agree. This may be attributed to the effect due to demagnetizing factors of ferrite inclusions that are not considered in the derivation of Snoek\u27s and Acher\u27s laws. The measured frequency-dependent permeability of composites satisfies the Lorentzian dispersion law and is consistent with the Maxwell Garnett approximation [J. C. Maxwell Garnett, Philos. Trans. R. Soc. London 203, 385 (1904)]. According to the theoretical analysis based on the Lorentzian dispersion law and the Maxwell Garnet mixing rule, both Snoek\u27s and Acher\u27s constants must be linear functions of the volume fraction, independent of whether microwave values of the constants are in agreement with the magnetostatic values. In contrast, the experimental measurements reveal a steady decrease of both constants with the volume fraction. The disagreement is discussed in terms of the influence of effective medium in composite on the inherent permeability of ferrite particles

Restoration of the Lorentzian and Debye Curves of Dielectrics and Magnetics for FDTD Modeling

The algorithms of extracting the Lorentzianand Debye-curve parameters of dielectric and magnetic materials from the results of measurements at several frequency points are presented. These algorithms are based on an analytical solution of systems of non-linear equations with physical constraints that follow from the fundamental principle of causality. The extracted parameters are useful for FDTD modeling of electromagnetic structures containing such dispersive media. Some examples are presented

Representation of Permittivity for Multiphase Dielectric Mixtures in FDTD Modeling

A simple method of approximating frequency characteristics of composites in a form convenient for time-domain numerical modeling is proposed. The frequency characteristics can be obtained from experiment or calculations based on the Maxwell Garnett mixing formalism. The resultant frequency characteristic might be of a complex shape corresponding to a combination of a number of absorption peaks. The approximation is made by a series of Debye-like terms using a genetic algorithm (GA). This leads to the necessity of taking a number of terms in the approximating series. Every term corresponds to its pole, i.e., the frequency where the maximum loss occurs. Some numerical examples are represented

Wide-Band Lorentzian Media in the FDTD Algorithm

This paper considers the case of a wide-band Lorentzian (WBL) algorithm in the finite-difference time-domain (FDTD) modeling of dispersive media. It is shown herein that the WBL model is a physically meaningful and practically useful case of the frequency behavior of materials along with the Debye and narrow-band Lorentzian (NBL). The recursive convolution algorithms for the finite-difference time-domain technique for NBL and WBL models differ. The Debye model, which is suitable for comparatively low-frequency dispersive materials, may not have sufficient number of parameters for describing the wide-band material, especially if this material exhibits pronounced absorption at higher frequencies. It is shown that the Debye model can be used, if the Q-factor of the linear circuit analog corresponding to the Lorentzian model of the material is less than approximately 0.8. If the quality factor is in the limits of about 0.8 \u3c Q ≤ 1, then the WBL model is appropriate. For Q \u3e 1, the NBL model must be applied. The NBL model is suitable for dielectrics exhibiting resonance effects in the microwave frequency range. The WBL model is typical for composites filled with conducting fibers