[3] The earliest studies in this direction used the simplest models of a ferromagnetic rock, each describing a single type of magnetization. However, researchers arrived at the understanding of the fact that interrelations between various types of magnetization should be examined in terms of a general model because the demagnetization technique aimed at the removal of secondary magnetization inevitably affected the primary magnetization. In particular, such was the Preisach-Neel model supplemented with concepts of interrelated hysteretic and thermal-activation processes [Belokon et al., 1973; Sholpo, 1977]. In our opinion, this model fits best a system of single-domain (SD) or pseudosingle-domain (PSD) particles such that each particle can be brought into correspondence with an elementary rectangular hysteresis cycle with critical fields of normal ( a ) and reversed ( b ) magnetizations. In this case an initial isolated SD or PSD particle has a symmetric hysteresis loop with a critical field H0 corresponding to the magnetic anisotropy: a = H0 + H and b = H0 - H, where H is the random field of interaction with other particles. Evidently, the calculation of the fields H0 and H, as well as the spontaneous magnetization Is, is critical for the construction of the Preisach-Neel model, because the response of a ferromagnetic grain to external effects is determined by the dependence of Is, H0 and H on their intensity. In this work, based on the simplest models, we examine the response of small ferromagnetic grains ("elements" of the Preisach-Neel model) to external effects, taking into account lattice imperfections and chemical heterogeneity of ferromagnetic material.
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