Magnetic field-induced strain in iron-based ferromagnetic shape memory alloys

Magnetic field-induced strain (MFIS) along the [001](P) (the symbol "P" represents parent phase) direction due to the conversion of martensite variants has been investigated for a disordered Fe-31.2Pd(at. %) single crystal and an ordered Fe3Pt single crystal exhibiting martensitic transformations from cubic phases to tetragonal phases at 85 and 230 K, respectively. The tetragonality c/a of Fe-31.2Pd at 77 K is 0.940 and that of Fe3Pt at 14 K is 0.945. When magnetic field is applied to the martensite phase along the [001](P) direction, the specimen expands along the field direction for Fe-31.2Pd and contracts for Fe3Pt, suggesting that the c axis is the hard axis of magnetizaion for Fe-31.2Pd and is the easy axis for Fe3Pt. The conversion of variants by magnetic field is almost perfect for Fe-31.2Pd and is not perfect for Fe3Pt. The recoverable strain in the field removing process is small for Fe-31.2Pd and is about 0.6% for Fe3Pt. In the cooling process under magnetic field of 3.2 MA/m, the fraction of preferable variants reaches 100% in a narrow temperature range for Fe-31.2Pd, but it does not reach 100% for Fe3Pt (the maximum fraction is about 80%), although the fraction increases with decreasing temperature. (C) 2003 American Institute of Physics.