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This paper describes a theoretical approach to calculate the permeability tensor of polycrystalline ferrites in various magnetization states, from the completely demagnetized state to saturation. Demagnetizing dynamic fields related to the magnetic domain and grain shapes are introduced in the initial equations to describe the interactions between domains in a grain and between grains. In this paper, we focus on the calculation of the initial permeability of polycrystalline ferrites. Results obtained from the model are compared in a completely demagnetized state to those given by the well-known Schloemann model and to experiments. In all the models given in the literature, an unusually high value of the damping factor must be introduced into the calculations to correctly describe the low-field loss region. Values of the damping factor comparable to those calculated from the measured resonance linewidth of ferrites permit the prediction of the spreading of the magnetic losses as a function of frequency experimentally observed at low bias fields and especially in the demagnetized state. [ABSTRACT FROM AUTHOR]

We describe a theoretical approach for determining the permeability tensor of polycrystalline ferrites regardless of their magnetization state. To take into account both the demagnetizing dynamic fields related to the magnetic domain and grain shapes and the magnetic interactions between adjoining domains and between adjoining grains, we transform the classical Landau-Lifschitz-Gilbert equation into a coupled two-equation system. We introduce statistical distribution laws for both the domain and grain demagnetizing coefficients into the calculation to take the domain and grain shape diversity into account and derive static vectorial quantities such as the internal magnetic dc field and magnetization in each domain that depends on the applied dc magnetic field from the Stoner-Wohlfarth hysteresis model. We compare results with those for existing models for various magnetized states. Then we apply the model to predict the permeability tensor behavior of barium hexaferrite thin film, especially in the remanent state, which is of great interest for the design of self-biased Y-junction circulators in the millimeter-wave range. [ABSTRACT FROM AUTHOR]

Proposes a model which provides integral expressions for all magnetic permeability tensor components, that can be treated numerically without difficulties, in microwave devices. How the causal aspect that is required when numerical time-domain methods are used is provided by the model; Demonstration of the validity of the model.

The theoretical approach proposed here was aimed at accurately describing the electromagnetic response of self-biased Y-junction circulators made of polycrystalline hexaferrites. The accuracy of circulator response was enhanced by considering the real magnetization state of the material through a calculation of the permeability tensor; this tensor remains valid whatever the strength of the local effective dc field in the ferrite is. Moreover, the hysteresis phenomenon was introduced in the calculation of tensor through the use of the Stoner and Wolhfart [IEEE Trans. Magn. 27, 3475 (1948)] law; as a consequence, the tensor took into account the privileged direction of easy-axis distribution due to the manufacturing process, which allowed us to treat the polycrystalline hexaferrites as being magnetically self-biased. However, the results issued from our approach indicate that this higher accuracy is at the expense of Y-junction circulator performances, which are far from those reported in the literature or obtained with electromagnetic commercial simulators and based on the frequent assumption of saturated hexaferrite, even at the remanent state. [ABSTRACT FROM AUTHOR]

The purpose of this article is to bring out the nonreciprocity of ferrimagnetic powder loaded composites. We first describe their technology of preparation. Then we briefly recall the principle of the broadband measurement method used to determine the permeability tensor components of magnetized materials. Our experimental results performed at X-band frequencies (8-12 GHz) on two different ferrimagnetic loaded composite samples are presented and discussed. We finally show that for each material under test, the off-diagonal component κ of the permeability tensor, which is at the origin of the nonreciprocal effect, is of a magnitude comparable to the magnitude observed in bulk ferrites. This result proves that powders technology can be used to realize composite materials for nonreciprocal microwave applications. [ABSTRACT FROM AUTHOR]

This paper describes a mathematical model enabling the calculation of the effective permeability tensor of heterogeneous magnetic materials set in various magnetization states. The theoretical approach we propose permits to predict the microwave behavior of unsaturated magnetic loaded composites as well as those of unsaturated polycrystalline ferrites. The model gives all the complex components of the permeability tensor in a single calculation phase whatever the magnetization state of the material is. This is useful for the computer-aided design of microwave magnetic devices. The calculation of the effective permeability tensor is carded out from a limited number of parameters: saturation magnetization, anisotropy field, damping factor, external dc field strength, concentration, and shape of the magnetic particles or domains. The model is predictive in the way that the magnetization state of the material is determined from the strength of the external dc magnetic field applied on the material. [ABSTRACT FROM AUTHOR]

Presents a physical model of the properties of polycrystalline ferrites below magnetic saturation. Characterization of properties through permeability tensor as functions of magnetization states, anisotropy field and frequency; Homogeneity of medium through medium approximation; Comparison of results for different states with empirical formulations of permeability tensor components.

In order to develop a new kind of filter that makes a compromise between quasi-optical and more traditional bandpass filtering techniques at millimetre wavelengths, a theoretical modelling of grids in oversize and monomodal waveguides is performed using a variational method. [ABSTRACT FROM AUTHOR]

The topic of this paper is the study of electrically tunable phase-shifters based on the use of liquid crystals. The dielectric characterization of the commercial liquid crystal involved in this study allows us to determine its dielectric permittivities (with and without DC voltage applied) and then its dielectric anisotropy. In this way, two phase-shifter configurations were studied. Every devices propose inverted microstrip line. The two circuits have been realized with coplanar accesses to be use in the millimeter-frequency range. With the aim to improve the phase shift parameter (in °/GHz/cm), our work aims to reduce the transmission line. Instead of a rectilinear transmission line, this one is based on a symmetric stub line. The phase shift obtained is enhanced from 0.87°/GHz/cm to 2.77°/GHz/cm. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 43: 338–341, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20463 [ABSTRACT FROM AUTHOR]

Presents a study that compared the diagonal µ and off-diagonal κ permeability tensor components of magnetized polycrystalline ferrites given by a theoretical model with experimental ones at microwave frequencies. Application of a broad-band and noniterative measurement technique in determining the ferrite magnetization state; Discussion of the experimental validation of the model; Suggestions regarding the association of the model with the technique in the design of nonreciprocal and tunable planar microwave ferrite devices.

Presents a study which outlines the finite-difference time-domain (FDTD) treatment of partially magnetized ferrites which are characterised by a permeability tensor model. Comparison of numerical and analytical results of study; Information on a classical FDTD model; Description of an FDTD algorithm for an unsaturated ferrite.

Presents a study which proposed a measurement method that permits the determination of the complex permeability and permittivity of rectangular samples of various thickness. Electromagnetic analysis of the cell; Experimental results; Conclusion.