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

Abstract

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.