When an
igneous
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
rock cools, it acquires a thermoremanent magnetization (TRM) from the Earth's field. TRM can be much larger than it would be if exposed to the same field at room temperature (see
isothermal remanence). This remanence can also be very stable, lasting without significant change for millions of years. TRM is the main reason that
paleomagnetists are able to deduce the direction and magnitude of the ancient Earth's field.
History
As early as the eleventh century, the Chinese were aware that a piece of
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
could be magnetized by heating it until it was red hot, then
quenching
In materials science, quenching is the rapid cooling of a workpiece in water, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as pha ...
in water. While quenching it was oriented in the Earth's field to get the desired polarity. In 1600,
William Gilbert published ''
De Magnete
''De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure'' (''On the Magnet and Magnetic Bodies, and on That Great Magnet the Earth'') is a scientific work published in 1600 by the English physician and scientist William Gilbert. A h ...
'' (1600), a report of a series of meticulous experiments in magnetism. In it, he described the quenching of a steel rod in the direction of the Earth's field, and he may have been aware of the Chinese work.
In the early 20th century, a few investigators found that
igneous rocks
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
had a
remanence
Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed. Colloquially, when a magnet is "magnetized", it has remanence. The ...
that was much more intense than remanence acquired in the Earth's field without heating; that heating rocks in the Earth's magnetic field could magnetize them in the direction of the field; and that the Earth's field had reversed its direction in the past.
In paleomagnetism
Demagnetization
It has long been known that a TRM can be removed if it is heated above the
Curie temperature
In physics and materials science, the Curie temperature (''T''C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Cur ...
of the minerals carrying it. A TRM can also be partially demagnetized by heating up to some lower temperature
and cooling back to room temperature. A common procedure in
paleomagnetism
Paleomagnetism (or palaeomagnetismsee ), is the study of magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.''
Certain magnetic minerals in rock ...
is ''stepwise demagnetization'', in which the sample is heated to a series of temperatures
, cooling to room temperature and measuring the remaining remanence in between each heating step. The series of remanences can be plotted in a variety of ways, depending on the application.
Partial TRM
If a rock is later re-heated (as a result of burial, for example), part or all of the TRM can be replaced by a new remanence. If it is only part of the remanence, it is known as ''partial thermoremanent magnetization (pTRM)''. Because numerous experiments have been done modeling different ways of acquiring remanence, pTRM can have other meanings. For example, it can also be acquired in the laboratory by cooling in zero field to a temperature
(below the
Curie temperature
In physics and materials science, the Curie temperature (''T''C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Cur ...
), applying a magnetic field and cooling to a temperature
, then cooling the rest of the way to room temperature in zero field.
Ideal TRM behavior
The Thellier laws
The ideal TRM is one that can record the magnetic field in such a way that both its direction and intensity can be measured by some process in the lab. Thellier showed that this could be done if pTRM's satisfied four laws. Suppose that A and B are two non-overlapping temperature intervals. Suppose that
is a pTRM that is acquired by cooling the sample to room temperature, only switching the field
on while the temperature is in interval A;
has a similar definition. The ''Thellier laws'' are
*''Linearity'':
and
are proportional to
when
is not much larger than the present Earth's field.
*''Reciprocity'':
can be removed by heating through temperature interval
, and
through
.
*''Independence'':
and
are independent.
*''Additivity'': If
is acquired by turning the field on in both temperature intervals,
.
If these laws hold for any non-overlapping temperature intervals
and
, the sample satisfies the Thellier laws.
A simple model for the Thellier laws
Suppose that a sample has a lot of magnetic minerals, each of which has the following property: It is
superparamagnetic
Superparamagnetism is a form of magnetism which appears in small ferromagnetic or ferrimagnetic nanoparticles. In sufficiently small nanoparticles, magnetization can randomly flip direction under the influence of temperature. The typical time be ...
until the temperature reaches a ''blocking temperature''
that is independent of magnetic field for small fields. No irreversible changes occur at temperatures below
. If the resulting TRM is heated in zero field, it becomes superparamagnetic again at an ''unblocking temperature''
that is equal to
. Then it is easy to verify that reciprocity, independence and additivity hold. It only remains for linearity to be satisfied for all the Thellier laws to be obeyed.
The Néel model for single-domain TRM
Louis Néel
Louis Eugène Félix Néel (22 November 1904 – 17 November 2000) was a French physicist born in Lyon who received the Nobel Prize for Physics in 1970 for his studies of the magnetic properties of solids.
Biography
Néel studied at the Lycé ...
developed a physical model that showed how real magnetic minerals could have the above properties. It applies to particles that are
single-domain, having a uniform magnetization that can only rotate as a unit.
See also
*
Rock magnetism
Rock magnetism is the study of the magnetic properties of rocks, sediments and soils. The field arose out of the need in paleomagnetism to understand how rocks record the Earth's magnetic field. This remanence is carried by minerals, particularly ...
References
{{Reflist, 3
Rock magnetism
Geomagnetism
Ferromagnetism