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Magnetostriction (cf. electrostriction) is a property of magnetic materials that causes them to change their shape or dimensions during the process of
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 ...
. The variation of materials' magnetization due to 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 ...
changes the magnetostrictive strain until reaching its saturation value, λ. The effect was first identified in 1842 by James Joule when observing a sample of
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
. This effect causes energy loss due to frictional heating in susceptible ferromagnetic cores. The effect is also responsible for the low-pitched humming sound that can be heard coming from transformers, where oscillating AC currents produce a changing magnetic field.


Explanation

Internally, ferromagnetic materials have a structure that is divided into '' domains'', each of which is a region of uniform magnetization. When a magnetic field is applied, the boundaries between the domains shift and the domains rotate; both of these effects cause a change in the material's dimensions. The reason that a change in the magnetic domains of a material results in a change in the material's dimensions is a consequence of magnetocrystalline anisotropy; it takes more energy to magnetize a crystalline material in one direction than in another. If a magnetic field is applied to the material at an angle to an easy axis of magnetization, the material will tend to rearrange its structure so that an easy axis is aligned with the field to minimize the free energy of the system. Since different crystal directions are associated with different lengths, this effect induces a strain in the material. The reciprocal effect, the change of the magnetic susceptibility (response to an applied field) of a material when subjected to a mechanical stress, is called the Villari effect. Two other effects are related to magnetostriction: the
Matteucci effect Matteucci effect is one of the magnetomechanical effects, which is thermodynamically inverse to Wiedemann effect. This effect was described by Carlo Matteucci in 1858. It is observable in amorphous wires with helical domain structure, which can be o ...
is the creation of a helical anisotropy of the susceptibility of a magnetostrictive material when subjected to a
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
and the
Wiedemann effect The twisting of a ferromagnetic rod through which an electric current is flowing when the rod is placed in a longitudinal magnetic field. It was discovered by the German physicist Gustav Wiedemann in 1858 . The Wiedemann effect is one of the mani ...
is the twisting of these materials when a helical magnetic field is applied to them. The Villari reversal is the change in sign of the magnetostriction of
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
from positive to negative when exposed to magnetic fields of approximately 40  kA/m. On magnetization, a magnetic material undergoes changes in volume which are small: of the order 10−6.


Magnetostrictive hysteresis loop

Like
flux density Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ...
, the magnetostriction also exhibits
hysteresis Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
versus the strength of the magnetizing field. The shape of this hysteresis loop (called "dragonfly loop") can be reproduced using the Jiles-Atherton model.


Magnetostrictive materials

Magnetostrictive materials can convert magnetic energy into
kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acce ...
, or the reverse, and are used to build
actuator An actuator is a component of a machine that is responsible for moving and controlling a mechanism or system, for example by opening a valve. In simple terms, it is a "mover". An actuator requires a control device (controlled by control signal) a ...
s and
sensor A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
s. The property can be quantified by the magnetostrictive coefficient, λ, which may be positive or negative and is defined as the fractional change in length as the magnetization of the material increases from zero to the saturation value. The effect is responsible for the familiar " electric hum" () which can be heard near
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' ...
s and high power electrical devices. Cobalt exhibits the largest room-temperature magnetostriction of a pure element at 60 microstrains. Among alloys, the highest known magnetostriction is exhibited by Terfenol-D, (Ter for terbium, Fe for
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
, NOL for Naval Ordnance Laboratory, and D for
dysprosium Dysprosium is the chemical element with the symbol Dy and atomic number 66. It is a rare-earth element in the lanthanide series with a metallic silver luster. Dysprosium is never found in nature as a free element, though, like other lanthanide ...
). Terfenol-D, , exhibits about 2,000 microstrains in a field of 160 kA/m (2 kOe) at room temperature and is the most commonly used engineering magnetostrictive material.
Galfenol In materials science, galfenol is the general term for an alloy of iron and gallium. The name was first given to iron-gallium alloys by United States Navy researchers in 1998 when they discovered that adding gallium to iron could amplify iron's ma ...
, , and
Alfer Alperm (also alfenol or alfer) is a class of alloys comprising 83-90% of iron and 10-17% of aluminium. The most widely used composition is with 16% Al. An alloy with 13% Al is also sometimes referred to as alfer. It exhibits large magnetostriction ...
, , are newer alloys that exhibit 200-400 microstrains at lower applied fields (~200 Oe) and have enhanced mechanical properties from the brittle Terfenol-D. Both of these alloys have <100> easy axes for magnetostriction and demonstrate sufficient ductility for sensor and actuator applications. Another very common magnetostrictive composite is the amorphous alloy with its trade name
Metglas Metglas is a thin amorphous metal alloy ribbon produced by using rapid solidification process of approximately . This rapid solidification creates unique ferromagnetic properties that allows the ribbon to be magnetized and de-magnetized quickly an ...
2605SC. Favourable properties of this material are its high saturation-magnetostriction constant, λ, of about 20 microstrains and more, coupled with a low magnetic-anisotropy field strength, HA, of less than 1 kA/m (to reach magnetic saturation).
Metglas Metglas is a thin amorphous metal alloy ribbon produced by using rapid solidification process of approximately . This rapid solidification creates unique ferromagnetic properties that allows the ribbon to be magnetized and de-magnetized quickly an ...
2605SC also exhibits a very strong ΔE-effect with reductions in the effective
Young's modulus Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied ...
up to about 80% in bulk. This helps build energy-efficient magnetic
MEMS Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...
. Cobalt ferrite, (CoO·Fe2O3), is also mainly used for its magnetostrictive applications like sensors and actuators, thanks to its high saturation magnetostriction (~200 parts per million). In the absence of rare-earth elements, it is a good substitute for Terfenol-D. Moreover, its magnetostrictive properties can be tuned by inducing a magnetic uniaxial anisotropy. This can be done by magnetic annealing, magnetic field assisted compaction, or reaction under uniaxial pressure. This last solution has the advantage of being ultrafast (20 min), thanks to the use of
spark plasma sintering Spark plasma sintering (SPS), also known as field assisted sintering technique (FAST) or pulsed electric current sintering (PECS), or plasma pressure compaction (P2C) is a sintering technique. The main characteristic of SPS is that the pulsed or u ...
. In early
sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances (ranging), communicate with or detect objects on o ...
transducers during World War II,
nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow t ...
was used as a magnetostrictive material. To alleviate the shortage of nickel, the Japanese navy used an
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
-
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...
alloy from the Alperm family.


Mechanical behaviors of magnetostrictive alloys


Effect of microstructure on elastic strain

Single-crystal alloys exhibit superior microstrain, but are vulnerable to yielding due to the anisotropic mechanical properties of most metals. It has been observed that for polycrystalline alloys with a high area coverage of preferential grains for microstrain, the mechanical properties (
ductility Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile str ...
) of magnetostrictive alloys can be significantly improved. Targeted metallurgical processing steps promote abnormal grain growth of grains in
galfenol In materials science, galfenol is the general term for an alloy of iron and gallium. The name was first given to iron-gallium alloys by United States Navy researchers in 1998 when they discovered that adding gallium to iron could amplify iron's ma ...
and alfenol thin sheets, which contain two easy axes for magnetic domain alignment during magnetostriction. This can be accomplished by adding particles such as boride species and
niobium Niobium is a chemical element with chemical symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it has s ...
carbide () during initial chill casting of the ingot. For a polycrystalline alloy, an established formula for the magnetostriction, λ, from known directional microstrain measurements is: λs = 1/5(2λ100+3λ111) During subsequent
hot rolling In metalworking, rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness, to make the thickness uniform, and/or to impart a desired mechanical property. The concept is simi ...
and recrystallization steps, particle strengthening occurs in which the particles introduce a “pinning” force at
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 ...
that hinders normal (
stochastic Stochastic (, ) refers to the property of being well described by a random probability distribution. Although stochasticity and randomness are distinct in that the former refers to a modeling approach and the latter refers to phenomena themselv ...
) grain growth in an annealing step assisted by a atmosphere. Thus, single-crystal-like texture (~90% grain coverage) is attainable, reducing the interference with magnetic domain alignment and increasing microstrain attainable for polycrystalline alloys as measured by semiconducting strain gauges. These surface textures can be visualized using
electron backscatter diffraction Electron backscatter diffraction (EBSD) is a scanning electron microscope–based microstructural-crystallographic characterization technique commonly used in the study of crystalline or polycrystalline materials. The technique can provide in ...
(EBSD) or related diffraction techniques.


Compressive stress to induce domain alignment

For actuator applications, maximum rotation of magnetic moments leads to the highest possible magnetostriction output. This can be achieved by processing techniques such as stress annealing and field annealing. However, mechanical pre-stresses can also be applied to thin sheets to induce alignment perpendicular to actuation as long as the stress is below the buckling limit. For example, it has been demonstrated that applied compressive pre-stress of up to ~50 MPa can result in an increase of magnetostriction by ~90%. This is hypothesized to be due to a "jump" in initial alignment of domains perpendicular to applied stress and improved final alignment parallel to applied stress.


Constitutive behavior of magnetostrictive materials

These materials generally show non-linear behavior with a change in applied magnetic field or stress. For small magnetic fields, linear piezomagnetic constitutive behavior is enough. Non-linear magnetic behavior is captured using a classical macroscopic model such as the Preisach model and Jiles-Atherton model. For capturing magneto-mechanical behavior, Armstrong proposed an "energy average" approach. More recently, Wahi ''et al.'' have proposed a computationally efficient constitutive model wherein constitutive behavior is captured using a "locally linearizing" scheme.


Applications

* Electronic article surveillance – using magnetostriction to prevent
shoplifting Shoplifting is the theft of goods from an open retail establishment, typically by concealing a store item on one's person, in pockets, under clothes or in a bag, and leaving the store without paying. With clothing, shoplifters may put on item ...
* Magnetostrictive delay lines - an earlier form of computer memory * Magnetostrictive loudspeakers and
headphones Headphones are a pair of small loudspeaker drivers worn on or around the head over a user's ears. They are electroacoustic transducers, which convert an electrical signal to a corresponding sound. Headphones let a single user listen to an a ...


See also

* Electromagnetically induced acoustic noise and vibration * Inverse magnetostrictive effect *
Wiedemann effect The twisting of a ferromagnetic rod through which an electric current is flowing when the rod is placed in a longitudinal magnetic field. It was discovered by the German physicist Gustav Wiedemann in 1858 . The Wiedemann effect is one of the mani ...
– a torsional force caused by magnetostriction *
Magnetomechanical effects {{Unreferenced, date=May 2019, bot=noref (GreenC bot) Magnetomechanical effects connect magnetic, mechanical and electric phenomena in solid materials. * Magnetostriction * Inverse magnetostrictive effect * Wiedemann effect * Matteucci effect * G ...
for a collection of similar effects *
Magnetocaloric effect Magnetic refrigeration is a cooling technology based on the magnetocaloric effect. This technique can be used to attain extremely low temperatures, as well as the ranges used in common refrigerators. A magnetocaloric material warms up when a m ...
* Electrostriction *
Piezoelectricity Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied mechanical stress. The word ' ...
* Piezomagnetism *
SoundBug Soundbug is a small speaker that can turn a resonant flat surface into a flat panel speaker. The Soundbug is attached to a smooth resonant surface, this surface then acts as speaker. The tone differs depending on the surface (wood, metal, glass etc ...
*
FeONIC Feonic is a commercial company specialising in the design and development of magnetostrictive audio products and is a spin-off from Hull University. Products The products use a smart material that changes shape in a magnetic field. This mat ...
– developer of audio products using magnetostriction * Terfenol-D *
Galfenol In materials science, galfenol is the general term for an alloy of iron and gallium. The name was first given to iron-gallium alloys by United States Navy researchers in 1998 when they discovered that adding gallium to iron could amplify iron's ma ...


References


External links


Magnetostriction
*
Invisible Speakers from Feonic that use Magnetostriction

Magnetostrictive alloy maker: REMA-CN
{{Authority control Magnetic ordering