Natural remanent magnetization. Natural remanent magnetization is the permanent magnetism of a rock or sediment. This preserves a record of the Earth's magnetic field at the time the mineral was laid down as sediment or crystallized in magma and also the tectonic movement of the rock over millions of years from its original position. Natural remanent magnetization forms the basis of paleomagnetism and magnetostratigraphy.

Igneous rocks
Natural remnant magnetism is important when studying igneous rocks and the majority of the studies are based on. This is because these rocks contain a magnetic field at the time when the rock was formed. By being able measure the angle difference between the current magnetic field and the direction of the rocks the inclination can be discovered as well as seeing at how much the magnetic fields have moved. This is also the most common method used to get the remanence direction and strength. The main difficulty that arises is if the rocks has significant weathering or are overlayed with thick layers of sediments. (Shuang Liu, 2018)

Brunhes in 1906 discovered in the Pliocene lavas in France that showed various directions making the magnetic fields that usually pointed north and down point in south and down. He was able to demonstrate that the baked igneous rocks were magnetized with similar polarity to the other igneous rocks. This created the baked contact test that was able to find relative ages in the areas of igneous rocks. (Neil Opdyke, 1996)

Types 

There are several kinds of natural remnant magnetism that can occur in a sample. Many samples have more than one kind superimposed. Thermoremanent magnetization (TRM) is acquired during cooling through the Curie temperature of the magnetic minerals and is the best source of information on the past Earth's field. Magnetization formed by phase change, chemical action or growth of crystals at low temperature is called chemical remanent magnetization. Sediments acquire a depositional remanent magnetization during their formation or a post-depositional remanent magnetization afterwards.

Some kinds of remanence are undesirable and must be removed before the useful remanence is measured. One is isothermal remanent magnetization, which as a component of natural remnant magnetism induced through exposing a particle to a large magnetic field, causing the field to flip its lower coercivity magnetic moments to a field-favored direction.  A commonly cited mechanism of isothermal remanent magnetization acquisition is through lightning strikes.  Another is viscous remanent magnetization, a remanence acquired when the rock sits in the Earth's field for long periods.

The most important component of remanence is acquired when a rock is formed. This is called its primary component or characteristic remanent magnetization. Any later component is called a secondary component. To separate these components, the natural remnant magnetism is stripped away in a stepwise manner using thermal or alternating field demagnetization techniques to reveal the characteristic magnetic component.

But not "all magnetic changes resulting from mechanical shock can be removed by AF demagnetization". Marine oil-bearing sandstones are physically unstable mineralogies whose low-field susceptibility and isothermal remanent magnetization increase irreversibly, even after weak mechanical shocks and an AF demagnetization in 100 mT peak alternating fields.

Chemical remnant magnetization in magnetite
Magnetite is used for measuring the chemical remnant magnetization. Since it is grown in a magnetic field then after a certain the field is blocked hence acquiring chemical remnant magnetization. However this concept and behavior is still not well understood.(Pick, 1991)
A study was also conducted exploring when magnetite went under low-temperature oxidation to a maghemite. The results showed that this was not a truly effective method die to the separation between the chemical remnant magnetization and viscous remnant magnetization that was formed in the chosen field direction was not as effective.(Gapeev,1991)

Uses
Remnant magnetism specifically measures how much magnetism is left when removed from a magnetic field. This is used to get information on the "consetration, mineralogy, and grain size of the magnetic material". This provided data on the minerals that add to magnetic signal. This provided information on the minerals and where they come from, occurrence in soils, and their magnetic behavior. (Singer, 2013)

See also
Rock magnetism

Notes

References 

 

Ferromagnetism
Stratigraphy