Percolative behaviour and demagnetizing effects in disordered heterostructures

This study provides a behavioural analysis of the particular demagnetizing effects displayed by disordered heterostructures close to a particular value (C-p) of their magnetic load C. The emphasis is put on the convergence of evidences, got from several independent ways (experimental, analytical and numerical), of a percolative phenomenon in random distributions of magnetic particles that rules the sharp variations of the inner and outer demagnetizing effects at concentrations close to C-p. When C < C-p, magnetic gaps, which originate from the magnetic dilution, may cause a cut-off of the magnetic flux paths in the composite medium, leading to inner demagnetizing effects only. When C > C-p, the delocalization of the magnetic poles leads the outer demagnetizing factor of the sample to switch from zero to its maximum value. The analytical approach of this phenomenon is based on an effective medium theory. It leads to a description of the interactions between magnetic inclusions that show a deep similarity with the main notions the reciprocity principle calls for. (C) 2002 Elsevier Science B.V. All rights reserved.