Magnetoelectric phase diagrams of orthorhombic RMnO₃ (R=Gd, Tb, and Dy)

Magnetoelectric phase diagrams have been investigated for rare-earth manganites with orthorhombically distorted perovskite structure, RMnO₃ (R=Gd, Tb, and Dy). A variety of magnetic and electric phases emerge with varying R-site ion, temperature, and magnetic field in these systems. The magnetoelectric phase diagram varies sensitively with the direction of a magnetic field relative to the crystallographic axes. Although the ground state of GdMnO₃ with the largest ionic radius of R(r_R) is not ferroelectric in zero magnetic fields (H=0), a ferroelectric phase with electric polarization (P) along the a axis appears by applying H(>∼1 T) along the b axis. Both TbMnO₃ and DyMnO₃ show a ferroelectric order with P along the c axis even at H=0 below a lock-in transition temperature where nonzero wave vectors for magnetic and lattice modulations become nearly constant. These systems also exhibit a flop of the ferroelectric polarization (P‖c to P‖a) when H is applied along the a or b axis. By contrast, the application of H above ∼10 T along the c axis completely suppresses the ferroelectricity in TbMnO₃. Possible origins of the observed evolution of magnetoelectric phases are discussed in consideration of magnetism and lattice distortion in the perovskite rare-earth manganites.