Principles of the radiofrequency and microwave characterization of materials



according to the proprietes of your materials


Free-space characterization test bench: paraxial beam



Listing of the team publication
Permeability tensor of magnetized ferrites



Description
This technique allows a broadband measurement of the permeability tensor components together with that of the complex permittivity of ferrimagnetics and/or of partly magnetized or saturated composite materials.

 

Principle
The method has been developed within the LEST [1, 2]; it is based on the measurement of the distribution parameters, Sij, of a rectangular waveguide whose section is partly filled with the material under test (Top figure). The Sij-parameters are measured with a vector network analyzer. The sample is rectangular and consists in a plate or a thick layer.


Analysis of measured data

 The analysis of measured data, that is the determination of e, m and k of the  material from the waveguide Sij requires to associate an optimization program (inverse problem) to the dynamic electromagnetic analysis of the cell (direct problem) as follows:

  • The direct problem implies to calculate the propagation modes in the various areas of the cell, and then to match the modes in the plane of empty-cell/under-load-cell discontinuities. Such an analysis allows a rigorous description of the dynamic behavior of the cell.
  • The inverse problem is solved through a series of iterations of unknowns at a given frequency and from an initial state: this gives the convergence of the theoretical Sij-parameters computed by the simulation software of the cell to the measured Sij-parameters.

 Complex susceptibility of a ferromagnetic versus frequency


Results

  • Frequency range used: 7  - 13  GHz (network analyzers HP8510B).
  • Type of material tested: ferrimagnetic and ferrocomposite materials with mean to high losses (tan d >10-3).
  • Additional equipment needed: electromagnet with its power supply to magnetize the sample.
  • Network analyzer calibration: TRL.
  • Dimensions of the tested samples:
    • length: 25 mm at the most (maximum),
    • width: 10.16 mm,
    • thickness: from several hundreds micrometers to few millimeters.

Comments
Till nowadays, the permeability tensor components were measured by resonant methods [3]. But, these methods, despite their very high accuracy, have the drawback of being single-frequency. In addition, they give unexploitable data in the zone of gyromagnetic resonance (high-loss zone) due to a strong reduction of cavity quality factor. If the broadband measurement methods for isotropic materials are well known, on the other hand those suitable for anisotropic media are less common. Thus, the characterization method we have developed using a rectangular waveguide is the first one that allows the broadband measurement of the permeability tensor components of magnetized ferrites. It led to two different publications [1, 2].


1. P. Quéffélec, M. Le Floc'h and Ph. Gelin, "Nonreciprocal cell for the broad band measurement of tensorial permeability of magnetized ferrites : direct problem", IEEE Transactions on Microwave Theory and Techniques, Vol. 47, No. 4, pp. 390-397, April 1999.

2. P. Quéffélec, M. Le Floc'h and Ph. Gelin, "New method for determining the permeability tensor of magnetized materials in a wide frequency range", IEEE Transactions on Microwave Theory and Techniques, Vol. 48, No. 8, pp. 1344-1351, August 2000.

3. J. Krupka, "Measurements of all complex permeability tensor components and the effective line widths of microwave ferrites using dielectric ring resonators", IEEE Trans. Microwave Theory Tech., Vol. 39, N°7, pp. 1148-1157, July 1991.