This class of multiferroics is of the materials which show ferroelectricity in their magnetically ordered state and that too of a particular type. Moreover, very strong coupling between ferroelectric and magnetic order parameters has also been observed. In 2003, Kimura et al. reported²⁶ presence of spontaneous polarization in the magnetized state of the TbMnO₃. TbMnO₃ has various magnetic structures: it is an incommensurate antiferromagnet between 27 and 42 K and is commensurate antiferromagnet between 7 and 27 K. It is in the commensurate state between 7 and 27 K, the material show ferroelectricity. This discovery was followed by observation of similar effect in TbMn₂O₅ by Hur et al.²⁷ Subsequently variety of other materials have also been investigated such as Ni₃V₂O₈, MnWO₆ showing this effect. Magnetic spin structure can be either a spiraling cycloid type or a collinear type.