A very different picture of magnetic domains has emerged over
the past decade, which should eventually have profound consequences
for models of various remanent magnetizations as well as for a
host of magnetic properties such as susceptibility and coercivity.
Most work during the past few years has been directed at developing
a better picture of domain structures and their variations with
size. It appears that the size above which a grain becomes very
nonuniformly magnetized is very close to the critical SD-MD
transition size, d
, calculated from conventional methods.
However, below d
the ``SD'' grain is not uniformly magnetized
(although it is nearly so), and more importantly, the structures
immediately above d
do not resemble conventional MD ones. The
greater the size above d
the closer the domains resemble the
conventional domain picture, except that with increase in size the
number of LEM states also increases. Domain imaging and theory are
in qualitative agreement but not in quantitative agreement. For
example, the observational case for the existence of LEM states is
convincing but the theoretically predicted number of LEM states is
sometimes significantly higher than observed. Residual stress and
systematic errors in domain imaging appear to be likely candidates
for explaining this discrepancy. Remanence associated with the
change in the number of domains, i.e. transdomain remanence, seems
likely in some forms of CRM, possible in TRM, and unlikely in VRM
at ambient temperature. The distribution of domains (particularly
the range of the distribution) may vary significantly depending on
the origins of the remanence. Consequently, domain observations
could become important tools to distinguish between some forms of
primary and secondary remanences.
The study of the type, distribution, and stability of domain structures is a very specialized subject within rock magnetism. However, if a goal is to understand why paleomagnetism works so well, then realization of this goal requires that the properties of domains be understood. The major rock magnetic result of the past decade has been the recognition that there are LEM states and that their distribution in a given grain can depend on that grain's history. During the past few years, and for the first time, it has become possible to observe and calculate three-dimensional structures in some small grains that contribute significantly to the stable magnetization in rocks. In the future this information should be extended to develop better models for the magnitude and stability of magnetization found in rocks.
One of the important developments during the past few years has been the establishment of a national rock magnetic facility, the Institute for Rock Magnetism, at the University of Minnesota under the leadership of Subir Banerjee. This institute has had a very positive impact on rock magnetism, and the broad range of equipment at the Institute has been used by numerous visitors, including scientists from countries other than the United States.
We have been very selective in the rock magnetic topics we chose to cover in this report. In accordance with the new format of these reports, no attempt has been made to reference all the papers published during the past few years on magnetic domains and related subjects. (Indeed, we have not even referenced all the recent publications of the authors.) We direct those readers who preferred the previous report format to the IRM Quarterly published by the Institute of Rock Magnetism: the IRM Quarterly contains references to the majority of papers published in rock magnetism and related subjects.
We thank Andrew Newell for valuable discussions and a review of an early draft of this manuscript. David Dunlop and Ken Verosub also provided us with very useful comments. Finally, we thank the National Science Foundation for support of this work.