Magnetic and magnetoelectric dynamics in RMn2O5 (R = Gd and Eu)

The dynamics of ferroelectromagnetic (multiferroic) crystals RMn2O5 (R = Eu and Gd), showing magnetic and ferroelectric phase transitions with close transition temperatures (40 and 30 K for R = Eu and Gd, respectively), was studied in the frequency and temperature ranges 20-300 GHz and 5-50 K, respectively. It was found that magnetic resonance spectra of GdMn2O5 are characteristic for the homogeneous long-range magnetic order both of the antiferomagnetic Mn-subsystem and Gd magnetic subsystem, possessing a large ferromagnetic moment. In other RMn2O5 crystals (with nonmagnetic or weakly magnetic rare-earth ions) incommensurate or space-modulated magnetic structures in Mn-subsystem are present. The strong Gd-Mn and Gd-Mn-Gd exchange interactions are responsible for the homogeneous magnetic state. A strong effect of the magnetic state on the ferroelectric state was discovered in GdMn2O5 crystal. The temperature of the ferroelectric phase transition in GdMn2O5 is shifted to the lower temperature as compared with other RMn2O5 crystals. Mixed magnetoelectric excitations in GdMn2O5 were observed for the first time.