Magnetic and structural properties of 5M polycrystalline NiMnGa

Magnetic shape memory alloys have been gained a large research interest due to there magnetic and structural properties. Plastic deformation in the martensitic phase is caused by twin boundary motion, which may be caused by an applied magnetic field or by stress. The Ni₅₀Mn₂₉Ga₂₁ was prepared as cubic samples which were cut erosively from a cast ingot prepared by directional solidification. The behaviour of the twin boundaries in an increasing magnetic field could be analysed by an optical microscope and a chromatic white light sensor. The latter can reveal the topography of the surface which is modulated by the twin boundaries. The transformation kinetics from austenite to martensite is measured at constant temperature and field. The fraction of the transformed phase could be described by the Johnson-Mehl-Avrami-Kolmogorow equation in dependency of time. Hysteresis loops were measured at different temperatures around the martensitic point (T_M) in the first quadrant. Due to the transformation from martensite to austenite the hysteresis loop is different for increasing and decreasing field. Below and above T_M the loops are the same for both directions. Thus the observed hysteresis is due to a phase change from martensite to austenite or vice versa. Within one phase the hysteresis is negligible.

Financial support by the DFG within SPP 1239 is gratefully acknowledged.