Large and sensitive magnetostriction in ferromagnetic composites with nanodispersive precipitates

Large and sensitive magnetostriction (large strain induced by small magnetic fields) is highly desired for applications of magnetostrictive materials. However, it is difficult to simultaneously improve magnetostriction and reduce the switching field because magnetostriction and the switching field are both proportional to the magnetocrystalline anisotropy. To solve this fundamental challenge, we report that introducing tetragonal nanoprecipitates into a cubic matrix can facilitate large and sensitive magnetostriction even in random polycrystals. As exhibited in a proof-of-principle reference, Fe–Ga alloys, the figure of merit—defined by the saturation magnetostriction over the magnetocrystalline anisotropy constant—can be enhanced by over 5-fold through optimum aging of the solution-treated precursor. On the one hand, the aging-induced nanodispersive face-centered tetragonal (FCT) precipitates create local tetragonal distortion of the body-centered cubic (BCC) matrix, substantially enhancing the saturation magnetostriction to be comparable to that of single crystal materials. On the other hand, these precipitates randomly couple with the matrix at the nanoscale, resulting in the collapse of net magnetocrystalline anisotropy. Our findings not only provide a simple and feasible approach to enhance the magnetostriction performance of random polycrystalline ferromagnets but also provide important insights toward understanding the mechanism of heterogeneous magnetostriction. The performance of materials that convert magnetic to mechanical energy and vice versa has been improved by introducing nanoprecipitates into their structure. Magnetostrictive materials can expand or contract under the influence of an external magnetic field. This property makes them useful for low-power sensors and energy harvesting. Simultaneously optimizing both the size of the magnetostrictive effect and the material’s sensitivity to small magnetic fields is difficult because increasing one effect tends to decrease the other. Tianyu Ma, Xi’an Jiaotong University, China, and Xiaobing Ren, National Institute for Materials Science, Tsukuba, Japan, and co-workers developed an approach to solve this problem by introducing tiny imperfections into a ferromagnetic alloy. By changing the atomic structure of small regions within the alloy iron−gallium, the researchers were able to achieve a five-fold improvement in the material’s magnetostrictive performance. a Aging-time dependent saturation magnetostriction for Fe73Ga27 random polycrystals. Bright-field images for the b 1373 K-quenched, c 1 h-aged and d 12 h-aged Fe73Ga27 random polycrystals. e Figure of merit as a function of aging time for Fe81Ga19 random polycrystals. f Comparison of saturation magnetostriction among our optimally aged Fe–Ga random polycrystals, the quenched random polycrystals doped with a third element and the single crystals.