Magnetically induced ferroelectricity in multiferroic compounds of RMn ₂O₅

A review is given on the studies of microscopic magnetic properties and macroscopic dielectric properties as functions of temperature, magnetic field, and the variety of R³⁺ (R = rare-earth, Bi, and Y) ions in the multiferroic RMn₂O₅ compounds by neutron diffraction and dielectric measurements. We have observed successive dielectric and magnetic phase transitions, which concomitantly takes place and a so-called multiferroics is realized. In this system, the ferroelectricity is magnetically induced and the commensurate magnetism is indispensable to produce a spontaneous electric polarization. The microscopic origin of the magnetically induced ferroelectricity and the role of rare-earth magnetic moment for a colossal magnetoelectric effect in RMn₂O₅ will be discussed in the viewpoint of the magnetic structure.