Thin film magnetostrictive materials can be extremely useful as the active material in microactuators. In this article, some results on a novel type of multilayer structure which combines exchange coupled giant magnetostrictive materials and materials with large magnetic polarizations are presented for the first time. Giant magnetostrictions are achieved at low fields, due to the polarization enhancement in such multilayers. Therefore such composite materials should be much more appropriate for applications than the simple homogeneous alloy films studied up to now.
The magnetization and magnetostriction have been studied in amorphous (Tb1−xDyx)(Fe0.45Co0.55)y films. A well-defined easy axis is created by magnetic field annealing and the sperimagnetic cone structure, characteristic of these amorphous alloys, is reduced. The anisotropy has a minimum at x=0.73 as in the R Fe2 Laves phase. This points to the similarity between the local environments in the amorphous and crystalline states. A giant magnetoelastic coupling coefficient b of 60 MPa is developed at 300 K in low applied field for x=0, optimally annealed films. Assuming a Young’s modulus and a Poisson ratio of 80 GPa and 0.31, respectively, the magnetostriction is evaluated at 1020×10−6. This is much larger than previously reported values in other amorphous films.
The deflection of bimorphs consisting of thin films of nickel on silicon substrates has been accurately measured as a function of applied in-plane magnetic field, using an optical method. Striking differences in the magnetoelastic behavior are observed for these systems compared with bulk nickel, mainly due to the anisotropic behavior of silicon substrates and to large in plane tensile stresses in nickel films, which favor an easy magnetization axis perpendicular to the film plane: Deflections measured with the magnetic field parallel and perpendicular to the length of the bimorph exhibit the same sign, and the ‘‘parallel deflection’’ is enhanced by a factor 1.3. These are the first experiments demonstrating the validity of a formula recently derived for predicting the deflection of magnetostrictive bimorphs.
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