Ferroelectric phase transitions of3d-spin origin inEu1−xYxMnO3

Abstract

Perovskite-type rare-earth manganites $R\mathrm{Mn}{\mathrm{O}}_{3}$ ($R=\mathrm{Tb}$ and Dy) undergo the ferroelectric transitions concomitantly with the collinear to noncollinear magnetic transitions of Mn $3d$ spins, in which the electric polarization direction can be controlled by an external magnetic field. To clarify how the magnetic transition of Mn $3d$ spins is correlated with the ferroelectric transition, we have investigated a mixed-crystal system of perovskite-type ${\mathrm{Eu}}_{1\ensuremath{-}x}{\mathrm{Y}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.4)$ without the $4f$ magnetic moments of the rare earth ions. The choice of the present system has enabled us to systematically investigate the correlation between dielectric, magnetic (of Mn spin origin), and superlattice properties with varying the perovskite lattice distortion or equivalently the competing Mn spin superexchange interactions. The compounds with $x=0.3$ and 0.4 undergo the ferroelectric transitions with the spontaneous polarization along the $a$ axis, which is associated with the formation of a $ab$-plane cycloidal spin structure with the modulation vector along the $b$ axis. Among them, the $x=0.4$ compound was found to undergo the polarization flop from along the $a$ axis to the $c$ axis as induced not only thermally but also by a magnetic field applied along the $a$ axis. The magnetic-field induced polarization flop and its mechanism for $x=0.4$ can be fully accounted for in terms of the rotation of the cycloidal spin plane from $ab$ to $bc$ stemming from the Mn spin flops. This is contrasted by the case of $\mathrm{Tb}\mathrm{Mn}{\mathrm{O}}_{3}$ and $\mathrm{Dy}\mathrm{Mn}{\mathrm{O}}_{3}$ with the rare-earth $4f$ Ising moments coupled to the Mn $3d$ spins, in which magnetic fields along the $b$ axis and the $a$ axis, but not along the $c$ axis (up to $14\phantom{\rule{0.3em}{0ex}}\mathrm{T}$), can flop the polarization from along the $c$ axis to the $a$ axis.