SPring-8, the large synchrotron radiation facility

Scientists at the National Institute for Materials Science (Sukekatsu Ushioda, President), including Xiaobing Ren, leader of the Ferroic Physics Group at the Sensor Materials Center, Sen Yang, a special research scientist, and Keisuke Kobayashi, leader of Beamline Station, discovered a morphotropic phase boundary,^*1 where both the magnetization direction and the crystal structure change, in a ferromagnetic material. They found giant magnetostriction that was 100-fold greater than that of iron with a phase boundary composition.

From these findings, the mechanism of giant magnetostriction, which had puzzled scientists for 30 years, was clarified, and the established knowledge in magnetics that the crystal structure is independent of the magnetic state was overturned. We now understand the origins of giant magnetostriction and the strong piezoelectric effect^*2 of ferroelectric materials in terms of similar principles.

All ferromagnetic materials, including iron, exhibit magnetostriction, meaning that they extend or shrink upon the application of a magnetic field. If a material has large magnetostriction, it can be used in various sensors and actuators. However, almost all ferromagnetic materials have only small magnetostriction with a deformation of about 0.0001 to 0.001%, and their practical application is difficult.

In contrast, giant magnetostriction 100-fold greater than the usual magnetostriction was observed in Terfenol-D,^*3 a magnetostrictive material discovered 30 years ago; however, its mechanism has not been clarified. Hence, no strategy has been reported for achieving such giant magnetostriction, and the search for giant magnetostrictive materials has relied solely on the experience of scientists.

Dr. Ren and his colleagues found, for the first time in the world, that the magnetic phase boundary of ferromagnetic materials also serves as the structural phase boundary, i.e., the morphotropic phase boundary, using the high-angular-resolution X-ray powder diffraction system at SPring-8. This finding overturned the established knowledge in magnetics that the crystal structure is independent of the magnetic state, and enabled us to understand the ferromagnetic morphotropic phase boundary and the ferroelectric morphotropic phase boundary of ferroelectric materials in terms of similar principles.

In addition, they also discovered giant magnetostriction 100-fold greater than that of iron in the rare-earth ferromagnetic alloy TbCo₂-DyCo₂^*4 with a phase boundary composition. This giant magnetostriction was verified to be essentially the same as the maximal piezoelectric effect of ferroelectric materials with phase boundary compositions. As a result of this research, the instabilities of the magnetization direction and crystal lattices at the morphotropic phase boundary were considered to be the origin of giant magnetostriction, which was understood in a similar manner to the large piezoelectric effect of ferroelectric and piezoelectric (PZT)^*5 materials.

These new findings in this research will provide strategy for exploring novel giant magnetostrictive materials (in particular, low-cost giant magnetostrictive materials) and contribute to the development and practical application of these materials.

The results of this research were published in the journal of the American Physical Society, Physical Review Letters.

Publication:
"Large Magnetostriction from Morphotropic Phase Boundary in Ferromagnets"
Sen Yang, Huixin Bao, Chao Zhou, Yu Wang, Xiaobing Ren, Yoshitaka Matsushita, Yoshio Katsuya, Masahiko Tanaka, Keisuke Kobayashi, Xiaoping Song, and Jianrong Gao
Physical Review Letters 104, 197201 (2010), published online 11 May 2010