Magnetostrictive Material
Magnetostriction is mostly found in the magnetic transition materials like iron, cobalt and nickel and also in the rare earth materials like lanthanum and terbium. The grains of these materials consist of numerous small randomly oriented magnetic domains, which can rotate and align under the influence of an external magnetic field. The magnetic orientation or alignment brings forth internal strain in the material, which is known as magnetostriction.
Similar effect is also found in the dielectrics under high electric field, which is known as electrostriction. These electro-mechanical phenomena are quite different from the piezoelectricity as these are essentially non-linear in nature and under unbiased field, the response is always unidirectional. In other words, the materials can only expand irrespective of the direction of the magnetic field applied to it.
The phenomenon of magnetostriction was discovered in nickel by James Joule in 1840. It was also observed later in other Ferromagnets and their alloys, although the maximum achievable strain was limited to 150 μ -strain only. Soon after, the discovery of low-temperature magneto-elasticity in rare earth elements, likes Tb (terbium), Dy (dysprosium) and Sm (samarium), has given a fresh impetus for continuing the search of magnetostrictive materials suitable for developing transducers.
Clark has obtained room-temperature magnetostriction in the alloy of Tb and Fe, which also has higher Curie temperature (around 7000K). Subsequently, it is found that by adding another rare earth material called dysprosium with Tb-Fe alloy, the magnetic anisotropy in the alloy can be reduced, thus generating even larger strains. The commercially available and well-known magnetostrictive material Terfenol-D is an example of the aforementioned alloy of terbium, iron and dysprosium. However, the proportion of Tb, Fe and Dy varies depending on specific requirement of magneto-elasticity and temperature characteristics.
It is shown experimentally that the ‘Terfenol' compound made using the composition
Dy0.73Tb 0.27Fe1.95 produces less free strain than the same compound made of
Dy0.7Tb0.3 Fe1.95 composition. However, in the former, the strain varies more linearly with the magnetic field as compared to the latter. This makes the first alloy more suitable for actuation purpose. Also, substituting dysprosium from Tb-Fe alloy by other rare earth materials like holmium or samarium, elastic characteristics can be significantly changed.
Fig 32.1: Magnetostrictive material with magnetic dipoles. Note
the strip expands due to the application of magnetic field
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