Remanence
Encyclopedia
Remanence or remanent magnetization is the magnetization
left behind in a ferromagnetic material (such as iron
) after an external magnetic field
is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence. The remanence of magnetic materials provides the magnetic memory in magnetic storage
devices, and is used as a source of information on the past Earth's magnetic field in paleomagnetism
.
The equivalent term residual magnetization is generally used in engineering applications. In transformer
s, electric motors and generators a large residual magnetization is desirable (see also electrical steel
). In many other applications it is an unwanted contamination, for example a magnetization remaining in an electromagnet
after the current in the coil is turned off. Where it is unwanted, it can be removed by degaussing
.
Sometimes the term retentivity is used for remanence measured in units of magnetic flux density.
). A magnetic hysteresis loop is measured using instruments such as a vibrating sample magnetometer
and the zero-field intercept is a measure of the remanence. In physics
this measure is converted to an average magnetization
(the total magnetic moment
divided by the volume of the sample) and denoted in equations as
. If it must be distinguished from other kinds of remanence it is called the saturation remanence or saturation isothermal remanence (SIRM) and denoted by 
.
In engineering applications the residual magnetization is often measured using a B-H Analyzer, which measures the response to an AC magnetic field (as in Fig. 1). This is represented by a flux density
. This value of remanence is one of the most important parameters characterizing permanent magnets; it measures the strongest magnetic field they can produce. Neodymium magnets, for example, have a remanence approximately equal to 1.3 teslas
.
), and these particles are not identical. Magnetic minerals in rocks may have a wide range of magnetic properties (see rock magnetism
). One way to look inside these materials is to add or subtract small increments of remanence. One way of doing this is first demagnetizing the magnet in an AC field, and then applying a field
and removing it. This remanence, denoted by 
, depends on the field. It is called the initial remanence or the isothermal remanent magnetization (IRM).
Another kind of IRM can be obtained by first giving the magnet a saturation remanence in one direction and then applying and removing a magnetic field in the opposite direction. This is called demagnetization remanence or dc demagnetization remanence and is denoted by symbols like
, where 
is the magnitude of the field. Yet another kind of remanence can be obtained by demagnetizing the saturation remanence in an ac field. This is called ac demagnetization remanence or alternating field demagnetization remanence and is denoted by symbols like 
.
If the particles are noninteracting single-domain particles with uniaxial anisotropy, there are simple linear relations between the remanences.
in rocks.
Magnetization
In classical electromagnetism, magnetization or magnetic polarization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material...
left behind in a ferromagnetic material (such as iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
) after an external magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence. The remanence of magnetic materials provides the magnetic memory in magnetic storage
Magnetic storage
Magnetic storage and magnetic recording are terms from engineering referring to the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetization in a magnetizable material to store data and is a form of non-volatile memory. The information is accessed using...
devices, and is used as a source of information on the past Earth's magnetic field in paleomagnetism
Paleomagnetism
Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of...
.
The equivalent term residual magnetization is generally used in engineering applications. In transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s, electric motors and generators a large residual magnetization is desirable (see also electrical steel
Electrical steel
Electrical steel, also called lamination steel, silicon electrical steel, silicon steel, relay steel or transformer steel, is specialty steel tailored to produce certain magnetic properties, such as a small hysteresis area and high permeability.The material is usually manufactured in the form of...
). In many other applications it is an unwanted contamination, for example a magnetization remaining in an electromagnet
Electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...
after the current in the coil is turned off. Where it is unwanted, it can be removed by degaussing
Degaussing
Degaussing is the process of decreasing or eliminating an unwanted magnetic field. It is named after Carl Friedrich Gauss, an early researcher in the field of magnetism...
.
Sometimes the term retentivity is used for remanence measured in units of magnetic flux density.
Saturation remanence
The default definition for remanence is the magnetization remaining in zero field after a large magnetic field is applied (enough to achieve saturationSaturation (magnetic)
Seen in some magnetic materials, saturation is the state reached when an increase in applied external magnetizing field H cannot increase the magnetization of the material further, so the total magnetic field B levels off...
). A magnetic hysteresis loop is measured using instruments such as a vibrating sample magnetometer
Vibrating sample magnetometer
A vibrating sample magnetometer or VSM is a scientific instrument that measures magnetic properties, invented in 1955 by Simon Foner at Lincoln Laboratory MIT. The paper about his work was published shortly afterward in 1959 A sample is placed inside a uniform magnetic field to magnetize the sample...
and the zero-field intercept is a measure of the remanence. In physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
this measure is converted to an average magnetization
Magnetization
In classical electromagnetism, magnetization or magnetic polarization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material...
(the total magnetic moment
Magnetic moment
The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...
divided by the volume of the sample) and denoted in equations as
. If it must be distinguished from other kinds of remanence it is called the saturation remanence or saturation isothermal remanence (SIRM) and denoted by .In engineering applications the residual magnetization is often measured using a B-H Analyzer, which measures the response to an AC magnetic field (as in Fig. 1). This is represented by a flux density
Flux density
-Formal Statement:The flux density is simply defined as the amount of flux passing through a unit-area. -Mathematical Statement:The flux density would essentially be the number of field lines passing through a defined unit-area...
. This value of remanence is one of the most important parameters characterizing permanent magnets; it measures the strongest magnetic field they can produce. Neodymium magnets, for example, have a remanence approximately equal to 1.3 teslasTesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
.
Isothermal remanence
Often a single measure of remanence does not provide adequate information on a magnet. For example, magnetic tapes contain a large number of small magnetic particles (see magnetic storageMagnetic storage
Magnetic storage and magnetic recording are terms from engineering referring to the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetization in a magnetizable material to store data and is a form of non-volatile memory. The information is accessed using...
), and these particles are not identical. Magnetic minerals in rocks may have a wide range of magnetic properties (see rock magnetism
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 certain strongly magnetic minerals like magnetite...
). One way to look inside these materials is to add or subtract small increments of remanence. One way of doing this is first demagnetizing the magnet in an AC field, and then applying a field
and removing it. This remanence, denoted by , depends on the field. It is called the initial remanence or the isothermal remanent magnetization (IRM).Another kind of IRM can be obtained by first giving the magnet a saturation remanence in one direction and then applying and removing a magnetic field in the opposite direction. This is called demagnetization remanence or dc demagnetization remanence and is denoted by symbols like
, where is the magnitude of the field. Yet another kind of remanence can be obtained by demagnetizing the saturation remanence in an ac field. This is called ac demagnetization remanence or alternating field demagnetization remanence and is denoted by symbols like .If the particles are noninteracting single-domain particles with uniaxial anisotropy, there are simple linear relations between the remanences.
Anhysteretic remanence
Another kind of laboratory remanence is anhysteretic remanence or anhysteretic remanent magnetization (ARM). This is induced by exposing a magnet to a large alternating field plus a small dc bias field. The amplitude of the alternating field is gradually reduced to zero to get an anhysteretic magnetization, and then the bias field is removed to get the remanence. The anhysteretic magnetization curve is often close to an average of the two branches of the hysteresis loop, and is assumed in some models to represent the lowest-energy state for a given field. ARM has also been studied because of its similarity to the write process in some magnetic recording technology and to the acquisition of natural remanent magnetizationNatural Remanent Magnetization
Natural remanent magnetization is the permanent magnetism of a rock or sediment. In some forms, it can preserve a record of the Earth's field and the tectonic movement of the rock over millions of years...
in rocks.
External links
See also
- CoercivityCoercivityIn materials science, the coercivity, also called the coercive field or coercive force, of a ferromagnetic material is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation...
- HysteresisHysteresisHysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
- Rock magnetismRock magnetismRock 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 certain strongly magnetic minerals like magnetite...
- Thermoremanent magnetizationThermoremanent magnetizationWhen an igneous rock cools, it acquires a thermoremanent magnetization from the Earth's field. TRM can be much larger than it would be if exposed to the same field at room temperature . This remanence can also be very stable, lasting without significant change for millions of years...
- Viscous remanent magnetizationViscous Remanent MagnetizationViscous remanent magnetization , also known as viscous magnetization, is remanence that is acquired by ferromagnetic materials by sitting in a magnetic field for some time. The natural remanent magnetization of an igneous rock can be altered by this process...