Magnetic and Magnetomechanical Properties of Ni-Mn-Ga Alloys with easy axis and easy plane of magnetization

Magnetically controlled shape memory (MSM) materials are considered now as a potential candidate for a new class of actuators and sensors. Magnetic and mechanical properties of two NiMnGa alloys with different thermally induced martensitic phases have been studied. Five-layered tetragonal martensite (c/a=0.94) in the first alloy is ferromagnetic with easy axis of magnetization. This martensite has low twinning stresses (approximately 2 MPa) and shows a giant magnetic field-induced strain. Non-modulated tetragonal martensite (c/a=1.20) in the second alloy is ferromagnetic with easy plain of magnetization. The absolute value of the magnetic anisotropy constant is approximately twice time higher in the second alloy compare to the first one. After mechanical training of the second alloy, stress at most 15 MPa is enough to produce approximately 19 % strain realized by twin boundary movement. Experimental data and theoretical considerations show, however, that in order to observe a giant magnetic field-induced strain in the second alloy the twinning stresses should be lower.