When an igneous 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).
TRM is the main reason that paleomagnetists are able to deduce the direction and magnitude of the ancient Earth's field.
[1] As early as the eleventh century, the Chinese were aware that a piece of iron could be magnetized by heating it until it was red hot, then quenching in water.
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.
A common procedure in paleomagnetism 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.
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.
Because numerous experiments have been done modeling different ways of acquiring remanence, pTRM can have other meanings.
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.
is a pTRM that is acquired by cooling the sample to room temperature, only switching the field
If the resulting TRM is heated in zero field, it becomes superparamagnetic again at an unblocking temperature
Louis Néel developed a physical model that showed how real magnetic minerals could have the above properties.