Ni–Mn–Ga multifunctional compounds

Ni–Mn–Ga alloys have been an exponentially growing source of interesting results for scientific research—the slow start has extended to hundreds of publications. There are already several review works dealing with the magnetic behavior of these alloys. Ever since the martensitic transformation was discovered in stoichiometric Ni2MnGa, different martensitic structures observed in various Ni–Mn–Ga alloys have made a wide range of different properties available. The crystal structure-dependent magnetic-field-induced strain may reach 6% in the five-layered martensite (10M) and 10% in the seven-layered martensite (14M) at a rather moderate magnetic field (below 1 T). In addition to the magnetic shape memory (MSM) phenomenon, Ni–Mn–Ga alloys have shown conventional shape memory effect, traditional and magnetic-field-assisted superelasticity, magnetocaloric and special transport properties; although, the existence of all these properties in the same alloy is not very likely. The multifunctionality can be obtained, for example, already in alloys showing the MSM effect: they can be used as actuators, sensors and perhaps also for energy harvesting. The application development has concentrated so far in actuators, such as proportional fluid valves, linear motors and optical applications, but the thin-film MSM materials open up totally new possibilities.