A new class of multiferroics such as TbMnO₃ exhibit gigantic magnetoelectric (ME) effects accompanied by a magnetic phase transition into a spiral magnetic ordered phase. Thus, the spiral magnetism is the key to understanding the ferroelectric and ME properties in these systems. In spiral magnets, inversion symmetry is broken owing to magnetic order, and some spiral-ordered structures such as a cycloidal one make the system polar. This means that a magnetic order can induce ferroelectricity. The ferroelectricity in the new class of multiferroics can be explained in terms of this scheme. Because spiral order often arises from the competition between nearest-neighbor and further-neighbor magnetic interactions, systems containing competing magnetic interactions are promising candidates for multiferroics. On the basis of this strategy several new multiferroics related to spiral magnetic orders have been discovered in the past few years. In this presentation, I show recent progress on the study of crystal growth and magnetoelectric properties in several multiferroics especially in which non-collinear spiral spin arrangements play crucial roles. (For example, low-field ME effect at room temperature in hexaferrite compounds, ferromagnetic and ferroelectric nature in an olivine compound, and antisymmetric off-diagonal ME effects in a spin-glass system.)

This work has been done in collaboration with Y. Hiraoka, Y. Yamaguchi, T. Honda, T. Ishikura, K. Okumura, Y. Kitagawa, H. Nakamura, Y. Wakabayashi, M. Soda, T. Asaka, T. Nakano, Y. Nozue, Y. Tanaka, S. Shin, J. S. White, and M. Kenzelmann.

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