Martensitic transformation in Fe-based shape memory alloys under magnetic field

Effects of magnetic field on martensitic transformations have been examined in order to know a magnetic field-induced martensitic transformation by using Fe$_3$Pt and Fe-Ni-Co-Ti shape memory alloys and an Fe-Ni alloy polycrystals, and control of variants by magnetic field by using Fe-Pd, Ni$_2$MnGa and Fe$_3$Pt shape memory alloy single crystals. Following results were obtained: (i) The effects of magnetic field on the martensitic transformation start temperature is explained by the equation proposed by our group. (ii) Magnetoelastic martensitic transformation (maretensites are induced only while a magnetic field is applied and are transformed back to the parent phase when the magnetic field is removed) appears in an ausaged Fe-Ni-Co-Ti shape memory alloy. (iii) In Fe-31.2Pd(at.%), whose easy axis is the $a$-axis in martensite state, a large expansion of about 3% appears in [OOl]$_{\rm p}$ (P is the symbol of the parent phase) direction at 77 K by applying a magnetic field along [001]$_{\rm p}$ direction. In Ni$_2$MnGa, whose easy axis is the $c$-axis in martensite state, a large contraction of about 3.8% appears in [001]$_{\rm p}$ direction at 77 K by applying a magnetic field along [001]$_{\rm p}$ direction. These strains are nearly the same as those expected from perfect conversion to the preferred variants under magnetic field. In Fe$_3$Pt, whose easy axis is the $c$-axis in martensite state, a large contraction of about 2.3% appears in [001]$_{\rm p}$ direction at 4.2 K by applying a magnetic field along [001]$_{\rm p}$ direction, and a part of this contraction (0.6%) recovers as the field is removed. Considering results of (iii), the mechanism of conversion of variants by magnetic field is discussed