Magnetic-ferroelectric interactions in RMnO₃
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Hexagonal RMnO₃ is multiferroic material as a combination of frustrated antiferromagnet (TN =72K) and ferroelectric (TC=875K) which consists of multiple temperature and field-dependent phases. Strong coupling between the ferroelectric and magnetic orders has been observed at a spin reorientation transition between a P6'₃cm' and a P6'₃c'm magnetic phases in HoMnO₃, but the origin of this coupling is not well understood. Most RMnO₃ order in a single magnetic phase with no spin reorientation transition, and so do not show any such strong magnetic-ferroelectric coupling. This work focuses on investigating what distinguishes HoMnO₃ from other RMnO₃ in order to better understand the magnetic-ferroelectric coupling in RMnO₃. In order to study the transition between P6'₃cm' and a P6'₃c'm magnetic phases in RMnO₃, single-crystal samples of the hexagonal multiferroics Ho1-xYxMnO₃, Er1-xYxMnO₃ and Dy1-xYxMnO₃ were grown at different compositions. YMnO₃ orders in a P6'₃cm', whereas ErMnO₃ and ErMnO3 order in a P6'₃c'm phase. HoMnO₃ is in the P6'₃c'm phase at high temperatures and the P6'₃cm' phase at low temperature. Neutron diffraction measurements were used to establish the magnetic phase diagram as a function of temperature and composition. For Er1-xYxMnO₃ the transition from P6'₃cm' to P6'₃c'm happens over a very narrow composition range on the Y-rich side of the phase diagram. For Dy1-xYxMnO₃ a spin reorientation transition occurs for samples with up to 40% Dy, the highest concentration sample. Unlike Ho doping, both Er- and Dy- doping initially produces the P6₃c'm phase as a low-temperature phase as Y concentration decreases. Such differences suggest significant complexity in the magnetic phase competition in RMnO₃. Our recent research is using inelastic neutron scattering to measure dynamics in HoMnO₃. Previous spin wave dispersion measurements have pr
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