Magnetostrictive materials are broadly defined as materials that undergo a change in shape due to change in the magnetization state of the material.  Nearly all ferromagnetic materials exhibit a change in shape resulting from magnetization change.  In most common materials, nickel, iron, and cobalt, the change in length is on the order of 10 parts per million (see figure at right).  In addition, the change in volume is very small. This type of magnetostriction has been termed Joule magnetostriction after James P. Joule’s discovery in the 1850’s.   The relatively small change in shape of these materials limited their use in engineering.  Initial sonar designs contemplated exploiting the magnetostrictive effect, but were left unexplored due to advances in piezoelectric materials such as quartz and Rochelle salt, and later lead zirconium titanate (PZT). 

The engineering era of magnetostrictive materials began with the discovery of giant (1000’s of ppm) magnetostriction in rare earth alloys during the 1960’s by A.E. Clark and others.  The culmination of research into a engineering alloy incorporating rare earth materials was Terfenol-D, a specially formulated allow of Terbium, Dysprosium, and Iron that exhibits large magnetostriction at room temperature and relatively small applied fields.  Earlier alloys exhibited large magnetostriction, but either at very large magnetic fields, or at cryogenic temperatures, or both.  Terfenol-D overcame the temperature difficulty by incorporating a RFe₂ microstructure which raised the curie temperature above room temperature.  The necessary magnetic field was reduced by balancing the ratio of Terbium and Dysprosium, two elements with oppositely signed magnetocrystalline anisotropy, such that effective anisotropy of the compound was near zero at room temperature.  Since this time, Terfenol-D has become the preeminent magnetostrictive material, although research continues into new materials constantly.

The operation of magnetostrictive materials like Terfenol-D can be understood using a simple ellipse model, pictured at right.  The material may be thought of as an ellipse where the magnetization runs along the longest axis.  Applying a field to the ellipse has the result of rotating the magnetization in the direction of the field and subsequently observing a change in shape. 

The magnetostriction may be increased over the previous case by first applying a “preload” to the material and then applying a magnetic field.  This process is pictured at right.  By rotating the ellipses perpendicular to the applied field before applying the field, the total magnetostriction, e, is increased over the non-preloaded case.  In practice most magnetostrictive actuation devices incorporate a preloading mechanism to benefit from this effect.

A new class of magnetostrictive materials has recently been intensively investigated.  These materials have been termed Ferromagnetic Shape Memory Alloy Materials (FSMA).  They exhibit a twinning mechanism similar to that observed in traditional SMA materials such as NiTi and CuZn.  However, in a FSMA the shape change may be initiated using an applied magnetic field.  The translation of a twin boundary due to an applied magnetic field is shown schematically at right.  This class of materials is potentially more important for actuation than traditional SMA’s due to the possible of relatively high frequency operation.  These materials, principally NiMnGa alloys, have exhibited up to 9% magnetic field controlled strain in single crystal specimens.

Another material recently investigated is an iron/gallium alloy termed Galfenol by its inventors at the Naval Surface Warfare Center (Clark et al.).  Building upon studies performed with iron/aluminum alloys in the 70’s, researchers discovered that magnetostriction peaked in iron/gallium alloys at a volume fraction of 17% gallium.  Single crystal Galfenol exhibits magnetostriction on the order of 400 ppm at low applied fields.  In addition, unlike Terfenol-D, Galfenol is tough and may be machined and used in devices where Terfenol-D may fracture.  While the magnetostriction mechanism in Galfenol is still under investigation, the magnetostriction seems to be more similar to the classic magnetostriction of iron and nickel than the newer giant magnetostrictive alloys. 

Translation of a twin boundary in a FSMA material due to the application of a magnetic field resulting in deformation. (O’Handley et al. 1998)

While Terfenol-D and other magnetostrictive materials are potentially important as actuator materials, some limitations have limited their adoption over more traditional piezoelectric and electrostrictive materials.  The two most important issues  are eddy current losses due to high frequency operation, and poor durability.  Due to the time changing magnetic field used to actuate Terfenol-D, eddy currents are developed within the actuator material itself and hamper its use at frequencies above 2 kHz.  Polymer matrix composites, using a particulate form of the magnetostrictive material, essentially eliminate eddy current losses to 100 kHz and beyond.  Furthermore, the polymer matrix used to bind the particulate produces a relatively tough material that can better accommodate tensile and shear loading states.  Hoping to take advantage of these benefits, research into magnetostrictive composite materials has been performed at UCLA since 1994. 

• Hathaway, K. and Clark, A. E., "Magnetostrictive Materials," MRS Bulletin, 34-41, April, 1993.
• Clark, A.E., “Magnetostrictive rare earth-Fe₂ Compounds,” in Ferromagnetic Materials: A Handbook on the Properties of Magnetically Ordered Substances Vol. 1, Wolfarth, E.P., ed., 531-589, 1980.
• McKnight, G.P., See Ch. 2 - Introduction in [112] Oriented Magnetostrictive Composites, PhD Thesis, UCLA, 2002.