Magnetostriction: A Property of Magnetic Materials

Magnetostriction is the strain produced in some magnetic materials when their magnetization changes. A specimen may lengthen, shorten or change volume in an applied magnetic field. The size and sign of the response depend on the material, crystallographic direction, initial magnetic state, stress, temperature and the magnitude and direction of the magnetic field.

The direct effect converts a change in magnetic state into mechanical strain. The inverse effect converts applied stress into a change in magnetic state. Engineers use this coupling in actuators, stress and position sensors and acoustic transducers. Magnetostriction also affects transformers, motors and generators because cyclic core strain can create vibration and sound.

What is Magnetostriction?

James Prescott Joule reported field-induced length change in iron in 1842. The longitudinal response is now called the Joule effect. It occurs in many ferromagnetic materials and ferrimagnetic materials, but its magnitude and direction vary.

A magnetic domain is a region in which magnetic moments have a common orientation. Domain patterns minimise the combined exchange, anisotropy, magnetostatic and elastic energies. The lowest-energy magnetization directions depend on the crystal and its stress state.

An applied field can move domain walls and rotate domain magnetization. Magnetoelastic coupling links this magnetic rearrangement to small changes in the spacing of atoms in the crystal lattice. The specimen’s measured magnetostrictive strain is the combined macroscopic response of its domains, grains, texture and mechanical constraints.

The magnetostrictive strain depends on:

• The magnitude, direction and rate of change of the applied magnetic field
• The material’s magnetization curve and magnetic saturation
• Magnetic anisotropy, crystal orientation and polycrystalline texture
• Magnetoelastic coupling, composition, heat treatment and defects
• Temperature, mechanical bias stress and external constraint

Longitudinal magnetostriction is positive when the measured length increases and negative when it decreases relative to the stated reference condition. The sign can differ with material composition and crystal direction. The separate Villari effect is the change in magnetic behaviour caused by applied mechanical stress.

Small length changes can be measured with optical interferometry, strain gauges, piezoelectric transducers, capacitance methods or resonant tests. A common longitudinal magnetostriction coefficient is the fractional length change:

λ = ΔL / L

where ΔL is the change in the measured length and L is the reference length. A reported value should also identify the field direction, magnetic reference state, stress, temperature and whether the result is saturated.

Magnetostrictive Materials

Material choice is not based on strain alone. Engineers also compare hysteresis, required bias field, electrical resistivity, strength, machinability, temperature range, corrosion behaviour and cost. Representative material families include:

• Iron: Pure iron has modest magnetostriction, commonly on the order of a few tens of parts per million. Its measured value varies with purity, crystal direction, stress and magnetic history. In alternating fields, hysteresis loss and eddy current loss depend on the material grade, flux density, frequency and geometry. Laminations restrict induced currents. Iron remains ferromagnetic up to a Curie temperature of about 770 °C, so describing its Curie temperature as low is misleading.
• Nickel: Polycrystalline nickel normally contracts along the magnetizing direction, with saturation magnetostriction often reported between about -30 and -50 ppm. Its hysteresis loss and eddy-current loss depend on specimen processing and operating conditions. Its Curie temperature is about 358 °C.
• Cobalt: Cobalt has a high Curie temperature near 1115 °C and strongly anisotropic magnetic behaviour. Its magnetostriction varies with crystal structure, direction and temperature. High-frequency suitability cannot be inferred from cobalt content alone because resistivity, geometry and hysteresis also control loss.
• Iron-aluminium alloys: Fe-Al compositions can offer useful magnetostriction without rare-earth elements. Composition, texture and heat treatment determine the strain, magnetic loss and mechanical properties, so one coefficient or Curie temperature does not describe the whole family.
• Iron-nickel alloys: Permalloy compositions are selected for high permeability and low coercivity. Some grades are adjusted for near-zero magnetostriction, which reduces stress sensitivity and noise in magnetic shielding, sensing and recording components.
• Cobalt-nickel alloys: Their magnetostriction, anisotropy and Curie temperature change substantially with composition and processing. Some grades combine useful magnetic response with corrosion resistance, but their losses must be measured at the intended frequency and flux density.
• Iron-cobalt alloys: Fe-Co alloys can provide very high saturation flux density and high Curie temperature. Their magnetostriction and mechanical properties depend on composition, ordering, heat treatment and added elements.
• Cobalt-iron-vanadium alloys: Permendur-type alloys use vanadium to improve the workability of high-saturation Fe-Co material. Their Curie temperature is around 980 °C rather than 1400 °C, and magnetostriction and core loss depend on grade and processing.
• Ferrites: Ferrites are electrically resistive ceramic magnetic materials made from iron oxide and other metal oxides. The family includes low- and high-permeability grades and both small- and large-magnetostriction compositions. High resistivity suppresses eddy currents, which supports high-frequency use, while saturation flux density, Curie temperature and corrosion resistance vary by composition.
• Rare-earth compounds: The lanthanides have atomic numbers 57 to 71, and yttrium is also commonly grouped with rare-earth elements. Individual elements do not share one magnetic response. Compounds containing terbium, dysprosium or samarium can produce large magnetostriction, but cost, brittleness, anisotropy and temperature limits often require alloying and careful processing.
• Terfenol-D: Terfenol-D is a terbium-dysprosium-iron intermetallic, commonly written near Tb0.3Dy0.7Fe2. Commercial polycrystalline material can produce roughly 800 to 1200 ppm, while specialised specimens can exceed 2000 ppm under suitable field, bias stress and temperature conditions. It offers large force and strain but is brittle, costly and hysteretic. Devices often need magnetic bias and mechanical prestress.
• Galfenol: Galfenol is a family of iron-gallium alloys, often near 18 to 19 atomic percent gallium. Depending on crystal orientation and processing, reported strain ranges from roughly 100 ppm in common polycrystalline forms to several hundred ppm in textured or single-crystal material. It is more ductile and machinable than Terfenol-D. Hysteresis and frequency response still depend on bias, stress, geometry and lamination.
• Metallic glasses: Metglas is a trade name covering several amorphous alloy compositions, often supplied as thin ribbon. Different grades are chosen for near-zero or non-zero magnetostriction, permeability, saturation flux and loss. The thin, resistive ribbon can limit eddy-current loss in high-frequency cores and magnetoelastic sensors.

Magnetostriction Applications

Applications use either field-induced strain or the inverse stress-induced magnetic response:

• Actuators: A drive coil changes the magnetic field and produces strain in a magnetostrictive element. These actuators can provide high force and fast, small motion, but the design may need a bias field, prestress, cooling and feedback to manage nonlinearity and hysteresis. Whether they outperform a piezoelectric or another actuator depends on stroke, force, bandwidth, size and power requirements.
• Sensors: Stress, force, torque or position can change a material’s magnetization, permeability or resonant response. A coil or magnetic-field sensor detects that change. Magnetostrictive sensors can support sealed or contactless measurements, while accuracy and stability depend on calibration, temperature compensation, magnetic bias and hysteresis. They are not universally more accurate than strain gauges or capacitive sensors.
• Transducers: Alternating magnetic fields can generate sound or ultrasound, and mechanical waves can induce a magnetic signal in the reverse direction. Sonar, ultrasonic processing and vibration control are established uses. Performance relative to other types of transducers depends on power, bandwidth, distortion, biasing and mechanical impedance matching.
• Transformers: Alternating core flux produces cyclic magnetostrictive strain, which contributes to core vibration and audible hum. Designers select low-magnetostriction core materials and control joints, clamping and flux density to reduce noise. Raising the applied voltage above the design value can increase core flux, saturation, vibration and loss; magnetostriction is not a standard method for improving transformer efficiency.
• Motors and generators: Specialised devices can use magnetostrictive rods, rings or laminations for incremental motion, torque production or vibration-energy harvesting. Their compact mechanisms can suit niche applications, but stroke, hysteresis, drive coils and magnetic bias prevent a universal advantage over conventional rotating machines.

Magnetostriction Effects

Several named magnetoelastic effects describe different field, stress and torsion conditions:

• Villari effect: Mechanical stress changes magnetization, permeability or magnetic susceptibility. This inverse magnetostrictive response supports mechanical-load sensing when the magnetic state is measured and calibrated.
• Matteucci effect: Torsional stress in a magnetised ferromagnetic wire changes its circular magnetization and can produce an axial magnetic signal. The response depends on the wire’s helical anisotropy and magnetic susceptibility, which allows non-contact torque sensing in suitable materials.
• Wiedemann effect: A ferromagnetic wire twists when longitudinal and circumferential magnetic fields combine to rotate its magnetization helically. The torsional response can drive or detect rotational motion, but its size depends on material, bias, geometry and current.

Conclusion

Magnetostriction couples a material’s magnetic state to elastic strain. Longitudinal strain is commonly reported as λ = ΔL / L, together with the field, direction, stress, temperature and reference state used for the measurement.

The direct effect supports actuators and acoustic transmitters. The inverse response supports stress, torque, position and vibration sensors. In conventional magnetic cores, the same coupling can be unwanted because it produces vibration and sound.

The Villari, Matteucci and Wiedemann effects describe related but distinct responses to axial stress, torsion and combined magnetic fields. Device designs must use the definition that matches their field and loading geometry.

Iron, nickel, cobalt, Fe-Al, Fe-Ni, Fe-Co, ferrites, rare-earth compounds, Terfenol-D, Galfenol and metallic glasses cover a wide range of strain, bias-field, loss and mechanical properties. Published coefficients are not interchangeable because composition, crystal texture, heat treatment, field, frequency, stress and temperature affect the result.

A useful design therefore starts with the required force, displacement, bandwidth, sensing range and environment, then selects and tests a material under those operating conditions.