Materials Science and Engineering A 481–482 (2008) 80–85 Ni–Mn–Ga multifunctional compounds O. S ¨ oderberg ∗ , I. Aaltio, Y. Ge, O. Heczko, S.-P. Hannula Helsinki University of Technology, Laboratory of Materials Science, P.O. Box 6200, FI-02015 TKK, Finland Received 31 May 2006; received in revised form 27 November 2006; accepted 10 December 2006 Abstract 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 Ni 2 MnGa, 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. © 2007 Elsevier B.V. All rights reserved. Keywords: Ni–Mn–Ga; Magnetic shape memory effect; Magnetoelastic properties; Magnetocaloric properties; Transport properties 1. Introduction Several review works dealing with Ni–Mn–Ga alloys have been published [1–10]. Most of these deal with the magnetic behavior of the Ni–Mn–Ga alloys that, so far, are the best working magnetic shape memory (MSM) alloys. They can produce actuation with rather high frequency and large shape changes at ambient temperature in a moderate magnetic field, which is a combination that other ferromagnetic shape memory alloys cannot fulfill. However, several martensitic phases and phase transformations observed in Ni–Mn–Ga alloys (for e.g., [11–13]) provide even a wider range of different properties avail- able. In addition to the magnetic shape memory effect (MSME), they show conventional thermally induced shape memory effect (SME), traditional and magnetic-field-assisted superelasticity (SE and MAFS) and interesting magnetocaloric and transport properties [14–22]. Even though all phenomena rarely occur in the same alloy, the multifunctionality can be obtained when a MSM alloy is utilized for actuating, sensing and energy har- ∗ Corresponding author. Tel.: +358 94512681; fax: +358 94512677. E-mail address: outi.soderberg@tkk.fi (O. S ¨ oderberg). vesting [23]. So far, the application development based on the MSM elements has mainly concentrated in actuators, such as proportional fluid valves, linear motors and optical applica- tions, but the thin-film MSM materials and composites introduce totally new possibilities. In this article, selected parts of this wide field of alloys properties and following applicability are reviewed. 2. Alloy tuning and structures The structure and properties of Ni–Mn–Ga alloys are strongly dependent on their chemical composition. The properties of Ni–Mn–Ga alloys can be tuned further by the whole prepara- tion route: solidification rate, annealing with different cooling speeds, cutting and crystal orientation, grinding, polishing, deformations, etc. The structural and magnetic transitions and their temperatures, the crystal lattice type and internal or sur- face stresses, all can be influenced. So far, there is not enough published data to perform this tuning precisely, but the basic information is available. There are several articles summarizing the effect of the alloy composition on the crystal structure, e.g. [9,12,24] and the phase transformation temperatures, e.g. [9,12,24–28]. 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.12.191