Magnetostrictive Materials
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etostrictive Materials
KristI B. Hathaway and Arthur E. Clark Introduction Smart materials combine sensors, intelligence, and actuators to allow a material to respond to its environment. Magnetostrictive materials can be used as both the sensors and actuators in such materials. High-power magnetostrictive actuators can deliver forces greater than 50 MPa with strains of up to 0.6%, while other magnetostrictive sensor materials can provide hundreds of times the sensitivity of semiconductor strain gages. Magnetoelastic materials also have adaptable elastic moduli which may be varied by external magnetic fields. Background Magnetostriction is the change in any dimension of a magnetic material caused by a change in its magnetic state. In this article we concentrate on ferromagnetic materials exhibiting Joule magnetostriction, which is a change in linear dimension parallel to an applied magnetic field (see Figure 1), and the reciprocal effect in which the material changes its magnetic state under the influence of applied stress. The phenomenon of magnetostriction has been known for well over a century, since Joule discovered in 1847 the change in length of an iron rod when magnetized. The modern era of magnetostrictive materials began in 1963 with the measurement of nearly 1% magnetostrictive strains at low temperatures in the basal planes of Dy and Tb. A search for magnetostrictive materials with high magnetostriction at room temperature led to the alloying of rare earths with transition metals, culminating in the discovery in 1971 of giant room-temperature magnetostriction in the Laves phase compound TbFe2. The magnetostrictive strain is that strain which arises from a reorientation of the atomic magnetic moments. The most fundamental measure of a magnetostrictive material is the saturation strain, that is, the maximum magnetostrictive strain that is produced when the material reaches magnetic saturation under the application of a magnetic field. In ferromagnetic ma-
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Magnetic field
X-Al/l Figure 1. Joule magnetostriction of a magnetic rod.
Magnetostriction in Various Materials Table I shows the room-temperature magnetostrictions of a variety of polycrystalline ferro- and ferrimagnets. Magnetostriction was first discovered and exploited in the magnetic transition metals Fe, Co, and Ni. In these materials, the magnetic 3d electron shell is relatively extended in space and
MRS BULLETIN/APRIL 1993
Magnetostrictive Materials
the 3d electrons hybridize with 3d electrons on neighboring atoms and with the 4sp conduction electrons. Thus their magnetic properties arise partially from itinerant electrons and their magnetostrictions (and magnetic anisotropies) depend sensitively on atomic structure. The cubic elements Fe and Ni have two independent single-crystal magnetostriction constants, A100 and Am, which at room temperature
-150 -100
.I. o
2
are (24 X HT6, - 2 3 X 10 ~6) for Fe and (-63 X 1(T6, -48 X 10"6) for Ni, respectively.2 The hexagonal element Co has four independent magnetostriction constants which are (
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