These materials are magnetic analogues of piezoelectric materials. As such when they become magnetised they change dimensions. They are intrinsically ferromagnetic.

In a magneto-strictive material, intense magnetic fields (the saturation flux density is typically around 1T) will distort the shape of the electron orbitals to the extent that the material itself will undergo a dimensional change. This distortion can be directly related to the intensity of the magnetic field, but is not dependent on the field polarity.

Materials are said to have positive magneto-striction when the material expands in the direction of the magnetic field. This results in a transverse constriction. In these materials, the magnetisation can be increased by applying a tensile stress. The converse applies to negatively magneto-strictive materials.

The induced strain has relatively low hysteresis (typically 2-3%) and magneto-strictive elements can exert high forces (e.g. a 10mm rod operated at 0.1% strain requires a clamping force of around 4kN to produce zero displacement). A 0.1% strain is typically about the maximum strain that can be achieved. Response times of about 1ms can also be achieved.

This behaviour can also be used to convert electrical energy into sound energy and vice versa.

The disadvantage to these materials is the high cost of production. Raw materials such as terbium are amongst the largest contributors to this. Furthermore, the method of delivering the magnetic field for actuation is not readily compatible with embedding as in the case of composite materials. These materials are also brittle and difficult to shape.

Magneto-striction is also responsible for the audible hum heard around transformer cores.
Magneto-Strictive Materials

The best known magneto-strictive material is Terfenol which is a compound consisting of Terbium (Te) and iron (Fe), and ‘NOL’ denotes the Naval Ordinance Laboratory, where it was researched. Nickel, cobalt and iron are also known to exhibit magneto-striction as do some recently discovered rare earth elements. Nickel and cobalt are negatively magneto-strictive, whilst iron is positively magneto-strictive in the presence of a weak magnetic field and negatively magneto-strictive when subjected to stronger magnetic fields.

Some examples of electro-strictive ceramics include, lead magnesium niobate, lead titanate and lead lanthanum zirconate titanate. These all exhibit strain deformation in the presence of an electric field and hysteresis in the order of 2-3%. They can also achieve positional feedback of around 10nm and work at frequencies as high as 40kHz. The only disadvantage to these materials is they are strongly temperature dependent. For example, a material designed to operate at room temperature may experience a deterioration of up to 50% at 50°C.