Member of Technical Staff, Lucent Technologies, Bell Laboratories (USA)
Ferromagnets develop a spontaneous magnetization that is sensitive to an external magnetic field; in a similar fashion, the spontaneous polarization in ferroelectric materials depends on an external electric field. The study of these systems encompasses a considerable portion of condensed matter physics. These materials also figure prominently in recent electronic devices. However, these two collective properties (ferromagnetism and ferroelectricity) have been investigated mainly as distinct and independent phenomena. One of the coupling phenomena between magnetism and ferroelectricity is the “magnetoelectric effect”, that is, the induction of magnetization by means of an electric field and that of an electric polarization by means of a magnetic field. This phenomenon attracted some interest in the 1960s and 70s, as the effect can provide an additional degree of freedom in device design. However, there have been no applications using the magnetoelectric effect, mainly due to material limitations. Given large magnetic (electric) responses to an external magnetic (electric) field in ferromagnetic (ferroelectric) materials, one approach to achieving a large magnetoelectric response is the use of materials in which ferromagnetic and ferroelectric orders exist simultaneously. Such materials are called “multiferroics.”
Figure) Spiral magnetic structure which breaks inversion symmetry and results in ferroelectricity.
We have explored novel multiferroics showing gigantic magnetoelectric responses. Consequently, perovskite-type rare-earth manganese oxides including TbMnO3 were found to exhibit ferroelectricity accompanied by an unexpected spiral magnetic order . By using these compounds, we observed a gigantic magnetoelectric effect, that is, a control of the magnitude and direction of an electric polarization vector by applying a magnetic field. Furthermore, we proposed that frustrated magnets possessing complex magnetic structures, such as a spiral magnetic structure (Figure), can be promising candidates for multiferroics with gigantic magnetoelectric effects. Based on these materials’ design strategy, we found that some other transition-metal oxides with spiral magnetic orders exhibit remarkable magnetoelectric couplings . Our discovery has provided a new route to design materials showing ferroelectricity with magnetic origin as well as gigantic magnetoelectric responses.
 T. Kimura et al. Nature 426, 55 (2003).
 T. Kimura et al. Phys. Rev. Lett. 94, 137201 (2005).