Effect of rapid solidification on giant magnetostriction in Fe-Pd alloy

Fe-29.6 at%Pd ferromagnetic shape memory alloy (FSMA) ribbons formed by rapidly solidified, melt-spinning methods, is expected to be useful as a new type of material which shows giant magnetostriction as well as a quick response. The giant magnetostriction in the rolling direction, which is caused by movement and re-arrangement of martensite twin boundaries, depends strongly on applied field direction and has a maximum value of 8 × 10^-4 when applied magnetic field is normal to the surface. Such a directional dependence is probably caused by fine columnar microstructure formed by rapid solidification methods. To confirm this hypothesis, we analyze magnetostriction, magnetic property and crystal structure of Fe-29.6 at%Pd bulk sample before rapid solidification and compare these properties with those of the ribbon sample. The results show that (1) strength of bulk magnetostriction is 1/10 of that of the ribbon sample, (2) the coercive force of ribbon sample has strong directional dependency, while the coercive force of bulk sample is isotropic indicating isotropic distribution of twin boundaries, (3) X-ray diffraction of bulk sample shows that the diffraction peaks due to martensite phase with fct structure are weak and those due to bcc structure are strong. From these results, it can be concluded that remarkable anisotropy of giant magnetostriction of ribbon sample is caused by the fine structure formed by the melt-spinning method. It may be possible to apply this method successfully to other FSMA and Ni₂MnGa, which is difficult to manufacture owing to its brittleness.