Page 243 - Mechanics of Microelectromechanical Systems

230 Chapter 4
transduction purposes will be studied later in this chapter. The piezoelectric
layers (ZnO is a material frequently used in MEMS) can be deposited to the
substrate through sol-gel spin coating, which enables deposition of
thicknesses of up to

6 PIEZOMAGNETIC TRANSDUCTION

Ferromagnetic materials such as alloys containing iron, cobalt or nickel
are piezomagnetic, a property which is the magnetic counterpart of
piezoelectricity. Piezomagnetic materials produce therefore both the direct
effect, which consists of generation of a magnetic field under adequate
mechanical load and the reversed effect (generally known as
magnetostriction), which implies mechanical deformation as a result of
magnetization. The dimensional change under the action of an external
magnetic field in piezomagnetic materials is produced through alignment of
the material magnetic domains in accordance to the external field, which
creates internal motion and rearrangement with the macroscopic result of
dimensional alteration. Figure 4.43 depicts such a situation, whereby an iron-
based piezomagnetic alloy, such as Permalloy, elongates through application
of a magnetic field.

Figure 4.43 Elongation of an iron-based alloy under the action of the magnetic field

Other ferromagnetic compounds, such as those containing nickel, display the
reversed response and contract under the action of an external magnetic field.
Materials that expand are also called positive magnetostrictive, whereas the
ones that do contract are alternatively named negative magnetostrictive, as
shown by Jakubovics [5] for instance. The anisotropy in magnetized piezo-
materials is reflected in the sensitivity to the direction of an external
magnetic field. In a positive magnetostrictive material, application of an
external magnetic field about a direction parallel to the polarization direction
will lengthen the dimension parallel to that direction and will shorten the
other two dimensions, as sketched in Fig. 4.44 (a), which is the top-view of a
piezomagnetic plate. On the contrary, when the magnetic field is applied
perpendicularly to the polarization direction, the material will contract about
the polarization direction and will extend about the external field’s direction,