Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a
ferromagnetic
Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...
material to withstand an external
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
without becoming
demagnetized. Coercivity is usually measured in
oersted or
ampere
The ampere (, ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to elect ...
/meter units and is denoted .
An analogous property in
electrical engineering
Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...
and
materials science, electric coercivity, is the ability of a
ferroelectric material to withstand an external
electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field ...
without becoming
depolarized.
Ferromagnetic materials with high coercivity are called magnetically ''hard'', and are used to make
permanent magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel ...
s. Materials with low coercivity are said to be magnetically ''soft''. The latter are used in
transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
and
inductor
An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...
cores,
recording heads,
microwave
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...
devices, and
magnetic shielding.
Definitions
Coercivity in a
ferromagnetic material is the intensity of the applied
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
(''H'' field) required to demagnetize that material, after the magnetization of the sample has been driven to
saturation by a strong field. This demagnetizing field is applied opposite to the original saturating field. There are however different definitions of coercivity, depending on what counts as 'demagnetized', thus the bare term "coercivity" may be ambiguous:
* The ''normal coercivity'', , is the ''H'' field required to reduce the
magnetic flux
In physics, specifically electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B over that surface. It is usually denoted or . The SI unit of magnetic flux is the weber ...
(average ''B'' field inside the material) to zero.
* The ''intrinsic coercivity'', , is the ''H'' field required to reduce the
magnetization
In classical electromagnetism, magnetization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material. Movement within this field is described by direction and is either Axial or D ...
(average ''M'' field inside the material) to zero.
* The ''remanence coercivity'', , is the ''H'' field required to reduce the
remanence to zero, meaning that when the ''H'' field is finally returned to zero, then both ''B'' and ''M'' also fall to zero (the material reaches the origin in the hysteresis curve).
The distinction between the normal and intrinsic coercivity is negligible in soft magnetic materials, however it can be significant in hard magnetic materials.
The strongest
rare-earth magnets lose almost none of the magnetization at ''H''
Cn.
Experimental determination
Typically the coercivity of a magnetic material is determined by measurement of the
magnetic hysteresis loop, also called the ''magnetization curve'', as illustrated in the figure above. The apparatus used to acquire the data is typically a
vibrating-sample or alternating-gradient
magnetometer
A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. A compass is one such device, ...
. The applied field where the data line crosses zero is the coercivity. If an
antiferromagnet
In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighboring spins (on different sublattices) pointing in opposite directions. ...
is present in the sample, the coercivities measured in increasing and decreasing fields may be unequal as a result of the
exchange bias
Exchange bias or exchange anisotropy occurs in bilayers (or multilayers) of magnetic materials where the hard magnetization behavior of an antiferromagnetic thin film causes a shift in the soft magnetization curve of a ferromagnetic film. The exc ...
effect.
The coercivity of a material depends on the time scale over which a magnetization curve is measured. The magnetization of a material measured at an applied reversed field which is nominally smaller than the coercivity may, over a long time scale, slowly
relax to zero. Relaxation occurs when reversal of magnetization by domain wall motion is
thermally activated and is dominated by
magnetic viscosity
Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particles ...
. The increasing value of coercivity at high frequencies is a serious obstacle to the increase of
data rates in high-
bandwidth magnetic recording, compounded by the fact that increased storage density typically requires a higher coercivity in the media.
Theory
At the coercive field, the
vector component of the magnetization of a ferromagnet measured along the applied field direction is zero. There are two primary modes of
magnetization reversal
In classical electromagnetism, magnetization is the vector field that expresses the density of permanent or induced magnetic dipole moments in a magnetic material. Movement within this field is described by direction and is either Axial or Dia ...
:
single-domain rotation and
domain wall
A domain wall is a type of topological soliton that occurs whenever a discrete symmetry is spontaneously broken. Domain walls are also sometimes called kinks in analogy with closely related kink solution of the sine-Gordon model or models with pol ...
motion. When the magnetization of a material reverses by rotation, the magnetization component along the applied field is zero because the vector points in a direction orthogonal to the applied field. When the magnetization reverses by domain wall motion, the net magnetization is small in every vector direction because the moments of all the individual domains sum to zero. Magnetization curves dominated by rotation and
magnetocrystalline anisotropy are found in relatively perfect magnetic materials used in fundamental research. Domain wall motion is a more important reversal mechanism in real engineering materials since defects like
grain boundaries
In materials science, a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are two-dimensional defects in the crystal structure, and tend to decrease the electrical and therma ...
and
impurities
In chemistry and materials science, impurities are chemical substances inside a confined amount of liquid, gas, or solid, which differ from the chemical composition of the material or compound. Firstly, a pure chemical should appear thermodynami ...
serve as
nucleation
In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that deter ...
sites for reversed-magnetization domains. The role of domain walls in determining coercivity is complicated since defects may ''pin'' domain walls in addition to nucleating them. The dynamics of domain walls in ferromagnets is similar to that of grain boundaries and
plasticity in
metallurgy
Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys.
Metallurgy encompasses both the sc ...
since both domain walls and grain boundaries are planar defects.
Significance
As with any
hysteretic process, the area inside the magnetization curve during one cycle represents the
work that is performed on the material by the external field in reversing the magnetization, and is dissipated as heat. Common dissipative processes in magnetic materials include
magnetostriction and domain wall motion. The coercivity is a measure of the degree of magnetic hysteresis and therefore characterizes the lossiness of soft magnetic materials for their common applications.
The saturation remanence and coercivity are figures of merit for hard magnets, although
maximum energy product is also commonly quoted. The 1980s saw the development of
rare-earth magnets with high energy products but undesirably low
Curie temperatures. Since the 1990s new
exchange spring hard magnets with high coercivities have been developed.
See also
*
Magnetic susceptibility
In electromagnetism, the magnetic susceptibility (Latin: , "receptive"; denoted ) is a measure of how much a material will become magnetized in an applied magnetic field. It is the ratio of magnetization (magnetic moment per unit volume) to the ap ...
*
Remanence
References
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External links
Magnetization reversal applet (coherent rotation)*For a table of coercivities of various magnetic recording media, see
Degaussing Data Storage Tape Magnetic Media (
PDF
Portable Document Format (PDF), standardized as ISO 32000, is a file format developed by Adobe in 1992 to present documents, including text formatting and images, in a manner independent of application software, hardware, and operating systems. ...
), at fujifilmusa.com.
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Physical quantities
Magnetic hysteresis