Magnetic shape memory: Magnetoelastic sponges.

news & views Nevertheless, a few hurdles must be overcome before SBA dielectrics will make a significant impact on large-scale transistor manufacturing. Perhaps the biggest problem at the moment is the slow response of the sodium ions to an external electric field. Consequently, the capacitance of the SBA films is reduced at higher frequencies. Solid-polymer electrolytes typically suffer from a similar problem, and in this case the frequency response can be substantially improved by increasing the ions' concentration and mobility 4. Another limitation of the SBA process reported by Katz et al. is that the temperature required to anneal the sol–gel-deposited SBA films (400 °C or more) is at present too high for large-area transistor applications, such as flat-panel displays. If this temperature can be lowered to near 250 °C, SBA may again become as popular as it once was. t he discovery of giant magnetic-field-induced strains of about 10% in single crystals of the magnetic shape-memory alloy Ni–Mn–Ga under 2-T magnetic fields (attainable by permanent magnets) brought a new dimension to the concept and design of field-triggered actuators 1,2. However, because Ni–Mn–Ga single crystals are difficult to grow, costly and brittle, investments in widespread applications of such actuators have been very limited. The good news is that many of these problems can be overcome by using foams of these materials, as reported by Chmielus and coauthors on page 863 of this issue 3. The authors demonstrate that polycrystalline foams of Ni–Mn–Ga prepared under certain conditions create a network of single crystals that can collectively show magnetic-field-induced strains up to an impressive 4% — surpassing by far any field-induced strain in polycrystalline materials (in particular in Terfanol D with 0.15% strain). Magnetic-field-induced strain in Ni–Mn–Ga relies on the presence of a diffusionless, solid–solid phase change, otherwise known as the martensitic transformation. The transformation temperature separates the high-temperature, high-symmetry cubic austenite phase (with low magnetic anisotropy) from the low-temperature, lower-symmetry-modulated orthorhombic martensite phase (with high magnetic anisotropy). The martensite phase has a twin structure and strong magnetocrystalline anisotropy. The twin boundaries are highly mobile and are set into motion when an external magnetic field is applied. Crystallographically, several twin variants are possible. If a single crystal of Ni–Mn–Ga is prepared so that it has only a single variant, giant magnetic-field-induced strains can occur. The origin of magnetic-field-induced strains is shown schematically in Fig. 1. For simplicity, we show a single-variant single …