Discovery of polarization flop transition and simultaneous measurements of magnetic ordering and dielectric properties of TmMn₂O₅

We have performed simultaneous measurements of magnetic neutron diffraction, ferroelectric D-E hysteresis loops and permittivity along the b- and a-axes of multiferroic TmMn₂O₅ down to 2.5 K. It was confirmed that the permittivity peak coincides with an increase in both the polarization along the b-axis and a magnetic commensurate phase, which agrees with a previous similar experiment on ErMn₂O₅. We also discovered a polarization flop from the b-axis to the a-axis below 5K coincident with a magnetic phase transition from one incommensurate phase to another incommensurate phase, which was confirmed by the simultaneous measurements. Although both phases are almost equivalent in the magnetic propagation wave vector, different magnetic structures induce a completely different ferroelectricity. Ferroelectric hysteresis loops along both the b- and a-axes were clearly observed by the double-wave method.