Magnetostrictive materials allow interchange of mechanical and magnetic energies, that means for example they produce strains in the magnetic field. These strains are called magnetostriction and were observed first in iron 1842 by James Prescott Joule. They can be quantified by the magnetostrictive coefficient, λ, which is the fractional change in length as the magnetization of the material increases from zero to the saturation value. The first magnetostrictive materials were iron, nickel, and cobalt. The terminus "Magnetostriction" is derived from the greek "magnet" and the latin "strictus"  - compressed, pressured, tense. It is defined as the field of magnetism that is concerned with the effects of interaction between magnetic and mechanical quantities (stress and strain). The most popular definition of magnetostriction says that magnetostriction is a change of shape, dimensions and mechanical properties under the influence of a magnetic field, or in reverse it is a change of magnetic properties (i.e. magnetization and permability) under the influence of stress and strain. Several magneto-mechanical effects have been discovered, as follow:

• Joule effect (magnetostriction or linear magnetostriction)
• Villari effect (inverse magnetostriction or inverse Joule effect)
• Barrett effect (volume magnetostriction)
• Guillemin effect
• Wiedemann effect
• Matteucci effect (inverse Wiedmann effect)
• Barnett effect
• Einstein - de Haas effect
• Barkhausen effect
  

The most advanced magnetostrictive materials, called Giant Magnetostrictive Materials (GMM), exhibit λ _II = 1000 ppm in H = 80 kA/m. They are alloys composed of iron (Fe), dysprosium (Dy) and terbium (Tb). They were discovered at NOL (Naval Ordnance Lab) and Ames Laboratory in 1965.
In 1970 the most popular room temperature giant magnetostrictive material (called Terfenol-D or GMM -Tb_xDy_1-xFe_y) was developed at NOL (Naval Ordnance Laboratory). It is manufactured in the form close to uniform crystal (with the help of Bridgman or Czochralski method) or as powders.
Some properties of magnetostrictive materials are presented in the table below.