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Magnetostriction and Magnetostrictive Materials
| Magnetostriction is the changing of a material's physical
dimensions in response to changing its magnetization. In other words, a magnetostrictive
material will change shape when it is subjected to a magnetic field. Most
ferromagnetic materials exhibit some measurable magnetostriction. The highest room
temperature magnetostriction of a pure element is that of Co which saturates at 60
microstrain. Fortunately, by alloying elements one can achieve "giant"
magnetostriction under relatively small fields. The highest known magnetostriction are
those of cubic laves phase iron alloys containing the rare earth elements Dysprosium, Dy,
or Terbium, Tb; DyFe2, and TbFe2. However, these materials have
tremendous magnetic anisotropy which necessitates a very large magnetic field to drive the
magnetostriction. Noting that these materials have anisotropies in opposite directions,
Clark(1) and his co-workers at NSWC-Carderock, prepared alloys containing Fe,
Dy, and Tb. These alloys are generally stochiometric, of the form TbxDy1-xFe2
and have been coined Terfenol-D. Terfenol-D, operated under a mechanical-bias, strains to
about 2000 microstrain in a field of 2 kOe at room temperatures. For typical transducer
and actuator applications, Terfenol-D is the most commonly used engineering
magnetostrictive material. |
Terfenol-D response around room temperature, from Clark1 |
| The mechanism of magnetostriction at an atomic level is
relatively complex subject matter but on a macroscopic level may be segregated into two
distinct processes. The first process is dominated by the migration of domain walls within
the material in response to external magnetic fields. Second, is the rotation of the
domains. These two mechanisms allow the material to change the domain orientation which in
turn causes a dimensional change. Since the deformation is isochoric there is an opposite
dimensional change in the orthogonal direction. Although there may be many mechanism to
the reorientation of the domains, the basic idea, represented in the figure, remains that
the rotation and movement of magnetic domains causes a physical length change in the
material. |
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| Magnetostrictive materials are typically mechanically biased
in normal operation. A compressive load is applied to the material, which, due to the
magneto-elastic coupling, forces the domain structure to orient perpendicular to the
applied force. Then, as a magnetic field is introduced, the domain structure rotates
producing the maximum possible strain in the material. A tensile preload should orient the
domain structure parallel to the applied force though this has not yet been observed due
to the brittleness of the material in tension. |
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- Clark, A. E. Ferromagnetic Materials, vol 1, ed Wolfhart, E.P. (Amsterdam:
North-Holland) pp. 531
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