Magnetic and mechanical properties of NiMnGa single crystals

Summary form only given. In Ni-Mn-Ga shape memory ferromagnetic alloys a giant strain may be obtained by applying an external magnetic field to single crystals in the martensitic phase. It has been shown that the mechanism producing a giant strain is due to the motion of the martensite twin boundaries. In this work we discuss the equivalence between magnetic and mechanical energy through the comparison of mechanical stress-strain curves with magnetic field-strain curves obtained at a temperature T below the martensite transition temperature, where the single crystals may exhibit a super-plastic mechanical behavior. It is shown that a repeatable strain behavior (due to mechanical or magnetic excitation) can be obtained on samples which have been stabilized by a series of mechanical cycles. For this purpose N/sub 47.34/Mn/sub 32.32/Ga/sub 20.34/ and Ni/sub 50/Mn/sub 29.5/Ga/sub 20.5/ single crystals are prepared by the Bridgman method and cut after orientation by Laue diffraction in a final rectangular geometry with [110] and [100] faces. To obtain the mechanical characteristics the samples are subjected to compressive stress along the [100] direction in an automatic testing machine measuring stress and strain under controlled temperature conditions. Once a stable mechanical behavior is obtained, a combination of mechanical stress and magnetic field is applied to obtain a very large deformations by motion of the martensite twin boundaries. Stress, strain and magnetization are measured in the longitudinal sample direction, corresponding to the [100] crystallographic axis. It is found that during the super-plastic behavior a strain of about 2% can be obtained with an increase of the applied compressive stress of the order of 2 MPa and this mechanical excitation can be substituted by an applied magnetic field of the order of 0.5 T.