Magnetic properties of single-domain particles and the rock remanence

1. Introduction

[2] The remanent magnetization of rocks is a metastable magnetic state of a system of ferromagnetic (antiferromagnetic) particles in a nonmagnetic matrix; this state preserves, to an extent, the information on rock transformations due to various factors (the geomagnetic field, temperature, pressure, chemistry, and time). The reconstruction of effects applied to a rock (the solution of an inversion problem) can be effected only if the response of the system of ferromagnetic particles to each of these factors is clearly understood. In the presence of the external field, this response involves the formation of a remanent magnetization, often carrying pertinent information. Therefore, rock magnetism studies have been often applied to the construction of theories describing the properties of various types of remanent magnetization under conditions of a weak magnetic field (of the same order as the Earth's field) and significant effects of other factors [Khramov et al., 1982; Nagata, 1961; Neel, 1955; Petrova, 1961].

[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 I_s, 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 I_s, 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.