Magnetoelectric effect in multiferroic NdMn_2O_5

We have measured the dielectric constant for NdMn_2O_5 in an external magnetic field to map out the magnetoelectric phase diagram. The phase diagram corresponds well with the previously reported data of neutron diffraction and magnetic susceptibility. Our main finding is the observation of a dielectric anomaly in the low temperature phase with a strong magnetoelectric effect, which is attributed to the independent Nd~(3+) ordering. Moreover, the absence of the dielectric anomaly in the paramagnetic phase is discussed, keeping in view the exchange interaction and its dependence on the rareearth R~(3+) ionic radius.