External and internal magnetic-field effects on ferroelectricity in orthorhombic rare-earth manganites

We report the dielectric and magnetic properties of the perovskite (Eu,Y)MnO$_3$ crystal {\it without} the presence of the $4f$ magnetic moments of the rare earth ions. The subject compound, (Eu,Y)MnO$_3$, was controlled the average ionic radius of the $A$ site so that it was the same as that of TbMnO$_3$ in which the intriguing magnetoelectric effect has been recently discovered. The (Eu,Y)MnO$_3$ crystal was found to have two distinct ferroelectric phases with polarization along the $a$ ($P_a$, $T$$\le$23K) and $c$ ($P_c$, 23K$\le$$T$$\le$25K) axes in the orthorhombic $Pbnm$ setting in a zero magnetic field. In addition, we have demonstrated a magnetic-field-induced switching between these ferroelectric phases: $P_a$ changed to $P_c$ by the application of magnetic fields parallel to the $a$ axis ($H_a$). In analogy to the case of $P_c$ in TbMnO$_3$, this result is possibly interpreted as follows. In the case of (Eu,Y)MnO$_3$, Mn $3d$ spins rotate in the $ab$ plane and $P_a$ would emerge in a zero field. In the $H_a$, the field will force the spins to rotate in the $bc$ plane, in which $P_c$ would be stabilized. Magnetization measurements supported this interpretation: We confirmed the change of the spin rotation axis of the helix from the $c$ axis to the $a$ axis induced by application of the $H_a$ because there is no $4f$ moments acting as an internal magnetic field and interacting with the $3d$ spins. Results obtained with other rare-earth manganites such as (Gd,Tb)MnO$_3$ and (Eu,Ho)MnO$_3$ will be presented.