Description
This technique allows a broad-band measurement of the complex magnetic susceptibility of isotropic, ferro or ferrimagnetic materials. Its application is limited to the characterization of only ring-shaped samples because of the coaxial structure of the testing device.

Principle
The ring-shaped sample constitutes the magnetic nucleus; two coils are wound around it.  When an alternating current is applied to the primary coil composed of the outer conductor together with the central core of the coaxial line, it causes a magnetic field while si the secondary circuit consisting in a measuring loop records the changes in magnetic flux. Faraday's law rules the voltage at the terminals of the secondary coil. The complex magnetic permeability is determined from the differential measurement of Sij parameters in transmission (empty and under load).  

Analysis of measured data
The ring-shaped structure of magnetic fields matches perfectly the sample shape. In consequence, this method allows one to get rid of shape-demagnetizing effects. Thus, by using a very simple analytical relation, the analysis of S₂₁ parameters in both conditions, i.e. empty and under load, gives the complex susceptibility of the materials under test. Only the geometrical dimensions of the primary and secondary coils of the transformer as well as those of the sample are needed.

Résults

Frequency range used : 1 KHz - 6 GHz (network analyzers HP4195A and HP8753ES).
Type of material tested: ferrimagnetic and ferromagnetic materials.
Accuracy:relative errors
• < 1% for 100 <c ' ;
• < 3% for 10 < c' < 100;
• 10% for c'< 10.
Network analyzer calibration: SOLT.
Dimensions of the tested samples:
• inner diameter: 5 mm at least,
• outer diameter: 50 mm at the most,
• thickness: 12 mm at the most.

Comments
This type of characterization cells has been developed as part of the research activities carried out in our laboratory before being the subject of an industrial transfer with SAGEM company at Montluçon (France). Another cell based on the same principle has been developed  to characterize magnetic liquids, e.g.  ferrofluid,  up to 3 GHz [1].

1. A.M. KONN, P. TALBOT, J.L. MATTEI and M. LE FLOC'h, "High sensitivity and broadband technique applied to magnetic measurements in ferrofluids", Journal of Magnetism and Magnetic Materials, Vol. 122, pp 164-165, 1993.