The giant magnetostrictive material (hereafter refer to as GMM) is a kind of grain-oriented crystal us ually in the shape of rod. When exerting a compressive stress in the direction of the rod axes, its magnetostriction will be greatly increased especially in the low field section. This is so called "jump effect". So ordinarily, we exert compressive stress to get optimized performance.
    The magnetostriction expressed as ¦Ë_¡Î is the ratio of the length change ¦¤L in the field to the original length L of the rod without magnetic field.
    That is: ¦Ë_¡Î =¦¤L/L
    In designing of high performance devices by using the material, one should consider the characteristics of the material in order to make full use of it. And the following points should be taken into account:
    1. To exert prestress: it can be seen from Figure on the next page that ¦Ë_¡Î is only about 800 ¡Á10^-6 in the magnetic field of 240KA/m without any prestress, while it increases to above 1600 ¡Á10^-6 if a constant prestress is applied along the direction of the rod axes. That is why the prestress is usually applied.
    2. To exert bias magnetic field: the exertion of the bias magnetic field can reduced the driving magnetic field by 50%, namely only ¡À1/2 of the actuating field will be needed in comparison with that without a bias field, thereby greatly simplify the design. For instance, if a magnetic field of 60KA/m is needed to actuate the magnetostrictive rod to get a strain of 1000¡Á10^-6 in the absence of a bias magnetic field, it will need only ¡À30kA/m of actuating field now, if a bias field of 30kA/m is applied.
    3. To avoid stress from other directions than the rod axes direction: because the GMM rod is oriented along the rod axes, it can withstand exceptionally high compressive stress in this direction. The compressive stress of the material is about 700MPa. However, owning to the brittleness nature of this material, it cannot bear high shearing stress or tensile stress. Care must be taken in the desinging to ensure the rod working only under compressive stress to avoid possible damages to the material.

Design of the Regarding Devices

    The figure shows an example of transducer design with GMM. Parts 4 and 9 are the permanent magnets which are used to generate bias magnetic field. The front lid 7 and the back lid 8 make the spring 5 partly compressed to exert a stable compressive prestress to the magnetostrictive rod 3 in its axes direction. The driving magnetic field is created by circulating electric current in the solenoid composed of copper wire 2 winding aroud a insulating frame 10. And the driving current is supplied by outside electric system.