Y. Tokura 1,2,3
1. Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
2. ERATO Multiferroics Project, JST, Tokyo 113-8656, Japan
3. Correlated Electron Research Center (CERC), AIST, Tsukuba 305-8562, Japan
Multiferroics, the materials in which both (anti)ferromagnetism and ferroelectricity can coexist, are the prospective candidate which can potentially host the gigantic magnetoelectric (ME) effect. A useful hint to designing of the strong magnetoelectric coupling has been gained by the recent discovery of the ferroelectricity in the transverse-spiral magnets, such as perovskite manganites. The direction of the polarization can be totally determined by the clock-wise or counter-clock-wise rotation of the spin in proceeding along the spiral propagation axis, that is called the spin helicity. In those compounds, the spontaneous P can be easily controlled by an external magnetic field of specific direction, such as the generation and/or flopping of the spontaneous polarization, which may be viewed as the gigantic ME effect. Conversely, the spin helicity can be controlled by an external electric field, as demonstrated by recent polarized neutron scattering experiments. The multiferroics based on this mechanism has recently been realized also in the conical spin state of chromite spinels where the transverse spiral component coexists with the uniform magnetization component along the cone axis. In those compounds, the clamping between the magnetic and ferroelectric domains can show up, perhaps enabling the magnetic (electric) control of the ferroelectric (ferromagnetic) domains.
Multiferroics with the strong ME correlation may also provide a unique arena to test new optical effects, say the magnetoelectic optical effects. This includes the so-called magnetization-induced second harmonic generation (MSHG) and the nonreciprocal dichroism dependent on the light propagation direction (but not on the light polarization), termed the optical magnetoelectric (OME) effect. The former can be applied to probe the interface magnetism as well, while the latter may have potential of producing new optical devices with nonreciprocal functions.
Strategy for exploring such multiferroics as showing strong ME coupling and novel optical functions is argued in terms of the designed spin superstructure and tailor-made materials.
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