Magnetic measurements -Measurement of the magnetic properties of materials

Measuring the magnetic hysteresis cycle and its parameters is at the heart of magnetic materials characterization, and most applications rely on these measurements. Once placed within a methodological framework strictly defined by written standards, it is indispensable for the production, sale and use of materials. We are looking for the experimental relationship M(H) – i.e. J(H) or B(H) – assumed at a macroscopic level, which implies that M is a quantity that comes from a spatial average over the measurement region or over the whole sample.

Measuring the intrinsic M(H) behavior is an ideal, which in practice can only be achieved to a certain degree of approximation, since the long-range effect of demagnetizing fields makes the behavior of the sample sensitive to its geometric characteristics. Practical constraints are imposed by the realization of a suitably sized magnetic circuit, as solutions used on a laboratory scale are not always acceptable from an industrial point of view. This is particularly true for soft materials, whose leakage fields are comparable to (or much larger than) those of the earth, and therefore interfere with the measurement. General recognition goes to methods that ensure good reproducibility, i.e., to those that give equivalent results at different times in different laboratories, and not necessarily to the one that gives the result closest to the intrinsic characteristics of the materials. Measurement standards are supposed to be developed to achieve this objective, at the cost of systematic deviation of the values determined from the intrinsic values of the quantities measured.

In this dossier, we present the basics of measurement methodologies applied to the determination of hysteresis in magnetic materials under DC and AC excitation. Taking into account the problems associated with sample geometry and magnetization system characteristics, and keeping in mind the general framework provided by international standards and associated recommendations, typical measurement procedures are implemented for the characterization of solid samples, sheets, strips or ribbons. The experimental approach evolves with frequency, and specific methods are designed for continuous, or more precisely quasi-static, measurements at radio frequencies. In the low- and medium-frequency range, the focus is on the non-linear, high-induction magnetization regime; in the high-frequency range, the focus is on the linear, low-induction regime.

The methods for producing and measuring magnetic fields have already been introduced in the previous dossier