### 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
*H*_{0} corresponding to the magnetic anisotropy:
*a* = *H*_{0} + *H* and
*b* = *H*_{0} - *H*, where
*H* is the random field of interaction with other particles.
Evidently, the calculation of the fields
*H*_{0} 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}, *H*_{0} 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.

**Powered by TeXWeb (Win32, v.2.0).**