Magnetic and mechanical properties of FeNiCoTi and NiMnGa magnetic shape memory alloys

Large magnetic field induced strains have been reported in ferromagnetic shape memory alloys. Two such alloys, Ni-Mn-Ga and Fe-Ni-Co-Ti are explored. A single crystal of Ni-Mn-Ga is shown to deform by bending approximately six degrees under the influence of an applied field. This deformation is caused by the motion of a single twin boundary with stable variants of martensite on either side. This effect was demonstrated using either divergent or homogeneous fields. Fe-Ni-Co-Ti is a shape memory steel with high saturation magnetization being developed as a magnetic shape memory material. Material properties in this alloy can be controlled by composition and heat treatment and the effects of both are explored. Variation of the Ni to Co ratio has been found to have a strong effect on the martensite transition temperatures. Aging treatments cause Ni,Ti precipitates to form, which affect the martensite transition and subsequently the magnetization. The structure of most of the Fe-Ni-Co-Ti alloys tested showed lenticular martensite at room temperature with a single sample showing retained thin plate martensite in austenite after cooling to 77K.