Thermoremanent magnetism is a term given to the remanent magnetism in the magnetic materials found in clay and rocks which have been heated above Curie temperature.
The study of this phenomena in archaeology is called archaeomagnetism.
Why it occurs:
The most obvious example of this effect is in the study of the material in ancient kilns. What happens is that the clay that lines the walls of the kiln act the same way that iron does, because clay contains the iron oxides magnetite and hematite. Magnetite grains consist of small, multiple domains, whilst hematite consists of a large, single domain. When it’s heated above Curie temperature, (that is, when the kiln is used) the domains in both iron oxides change - that is, the domains in magnetite shift so that the ones that are closest to the Earth’s magnetic field grow; the domain in hematite rotates so that it is more closely aligned with the Earth’s magnetic field.
The resultant of these changes is that the clay now has a magnetic field that is aligned with that of the Earth’s – and when the clay is cooled, that magnetic field become fixed the way it was.
Archaeologists have used this to study changes in the Earth’s magnetism. For example, an archaeologist would have measured precisely what orientation a section of the clay had in the kiln, and then removed it, later determining the magnetic field in relation to the dimensions of the section. What he/she gets out of this is the orientation of the magnetic field at the time the kiln was last used. Some archaeological samples can thus be dated with this method.