Development of Giant Magnetostrictive Materials
Ferromagnetic shape memory alloys have been attracting much attention since the discovery of large magnetic-field-induced strain of several percent in Ni-Mn-Ga alloy. This strain was attributed to the motion of the martensitic twins, and is different from the conventional (magnetoelastic) magnetostriction. Thus the material is an attractive candidate for a new class of magnetic actuator materials. Apart from the practical interests, these materials also offer an excellent opportunity to investigate the various aspects of phase transformations and microstructural formation because magnetic and structural phase transformations can be realized in a single system. In Ni-Mn-Ga alloys, which is one of the prototypical ferromagnetic shape memory alloys, a high temperature cubic phase of this alloy has the L21 ordered structure, also known as the Heusler structure. We have been investigating the various aspects of phase transformation behavior, microstructures and effect of fourth elements in Ni-Mn-Ga alloys. Another candidate of magnetic actuator materials is Fe-Ga alloy. This exhibit a large manetoelastic strain of several hundreds ppm. The strain is smaller than those observed in Ni-Mn-Ga but the superior mechanical property makes Fe-Ga attractive. The mechanism of large magnetostriction in Fe-Ga has not been clarified yet and under investigation.
Fig. Giant magnetostriction of Fe-18mol%Ga at room temperature.