Combinatorial Methods Used to Identify New Compositions of Ferromagnetic Shape-Memory Alloys
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RESEARCH/RESEARCHERS Combinatorial Methods Used to Identify New Compositions of Ferromagnetic Shape-Memory Alloys Combinatorial materials science is used to systematically study materials properties as a function of composition using a number of small samples differing slightly in composition. In a study published in the March issue of Nature Materials, I. Takeuchi and colleagues from the University of Maryland, Rowan University, and Neocera, Inc. have used thin-film composition spreads of multiferroic NiMn-Ga alloys, of which the so-called Heusler composition, Ni2MnGa, is a wellknown shape-memory alloy, to study both the mechanical behavior and ferromagnetic properties of the alloys. Multiferroic alloys exhibit the ability to switch between two conditions, such as ferromagnetic and paramagnetic, in more than one combination of conditions. The Ni-Mn-Ga alloys are ferromagnetic and are also shape-memory alloys, so, in addition to exhibiting martensitic transitions due to temperature change, shape-memory effects can, in many cases, be induced by an applied field, leading to a number of potential applications. The objective of the study was to determine which compositions of the ternary Ni-Mn-Ga system other than the Heusler composition exhibit ferromagnetic shape-memory effects. This required the development of techniques to measure both mechanical and magnetic properties of a series of compositions of the alloy system. For measuring magnetization, the researchers used a spread of patterned combinatorial array of 2 mm × 2 mm square grids and a scanning superconducting quantum interference device (SQUID) microscope. For detecting structural martensitic phase transition, a spread of compositions was deposited as thin films on a series of silicon cantilevers. When the cantilevers are heated causing a phase transformation, they bend, which is detected during visual inspection of the image of the cantilever arrays as a change in reflection resulting from a changing radius of curvature. Results showed that the strongest magnetization and the highest phase-transition temperatures were observed in regions away from the traditional Heusler composition. However, higher magnetization was coupled with a lower transition temperature. Thus there is a tradeoff between the two properties in the system. It was found that in a ternary system, the Heusler composition does not necessarily yield optimized functionality. 332
More importantly, this study demonstrates the use of combinatorial methods in conjunction with unique characterization techniques to systematically study an alloy system. GOPAL RAO
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