SINGULAR POINT DETECTION IN MULTIDOMAIN SAMPLES

In the present work the problem of singular point detection (SPD) is examined in the case of multidomain crystallites. As is known, a uniaxial polycrystalline ferromagnet displays a sharp peak in the second derivative of magnetization M with respect to magnetic field H versus H, located at the anisotropy field H = H(A). This singularity was explained on the basis of the classical SPD theory which considers only single-domain crystallites. Here this matter is analyzed in the frame of the Neel phase theory and leads to an interpretation of the SPD peaks which seems to be more realistic. A theoretical study of the magnetization curve of a rotational ellipsoid single crystal clearly shows existence of a critical field H(c) separating the multiphase from the single phase mode. The analysis of the magnetization curve of a polycrystalline material is based on the assumption that the main features of the SPD peaks are only determined by the discontinuity in the susceptibility at H(c). Contrary to the classical SPD theory, the shape of the grains is of fundamental importance. If the grains all have the same shape, the peak turns out to be a divergence; on the other hand, aggregates of crystallites having different demagnetizing factors show finite peaks closely resembling the experimental ones. The effects on the peak of crystallites' shapes and angular distribution have been investigated by means of numerical calculations. The validity of the present approach has also been tested by computer simulations on the basis of an extended Stoner-Wohlfarth model including the Neel phase theory.