Control of martensite variants by magnetic field and giant magnetic field-induced strain in Fe-Pd ferromagnetic shape memory alloys

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Recently, ferromagnetic shape memory alloys have attracted considerable attention because some of them show a giant magnetic field-induced strain of several percent in association with rearrangement of martensite variants. Until now, we have reported that Fe-31.2(at%)Pd alloy exhibits a giant strain of about 3% by the application of magnetic field. Since the giant field-induced strain can be obtained only in the FCT martensite state, it is very important to raise the martensitic transformation temperature. In this study, we first investigated the effect of third elements on the FCT martensitic transformation temperature. As a result, we found that when the FCT transformation temperature is increased, the BCT transformation temperature also increases. The BCT martensite is not suitable for the magnetic field-induced strain and the transformation should be avoided for obtaining good properties. Then, we investigated the most suitable composition at which the FCT transformation temperatur … More e is highest in the Fe-Pd binary system. As a result we found that when Pd content is lower than 30at%, BCT martensite appear in addition to FCT martensite, and the single FCT phase condition is obtained for alloys with Pd content of 30at% and more. We tried to grow a single crystal of Fe-30at% Pd alloy, and found that the composition of specimen varies largely because of a large difference between solidus and liquidus lines at this composition. In order to reduce the segregation introduced during the growing process, we kept the specimen at a temperature just below the melting point for several days, and we succeeded to obtain a homogeneous single crystal whose FCT transformation temperature is close to the room temperature. By using this single crystal, we confirmed that the rearrangement of martensite variants occurs by the application of magnetic field. Furthermore, we measured magnetocrystalline anisotropy constant and twinning stress of an Fe-31.2Pd alloy in a wide temperature range. Using these results, we explained the mechanism of magnetic field-induced strain in ferromagnetic shape memory alloys. Less