Pressure-induced commensurate order in TbMn2O5 and DyMn2O5 : Influence of rare-earth anisotropy and 3d−4f exchange

The magnetic structure of ${\mathrm{TbMn}}₂{\mathrm{O}}₅$ and ${\mathrm{DyMn}}₂{\mathrm{O}}₅$ multiferroics has been studied by high-pressure neutron diffraction in a large pressure range up to 6.6 GPa. In both cases, we observe a pressure-induced commensurate magnetic phase with propagation vector $\left(\frac{1}{2}\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}\frac{1}{2}\right)$, growing with pressure at the expense of the ambient pressure phases. Being previously observed in ${\mathrm{YMn}}₂{\mathrm{O}}₅$ and ${\mathrm{PrMn}}₂{\mathrm{O}}₅$, this phase is most likely a generic feature of the $R{\mathrm{Mn}}₂{\mathrm{O}}₅$ multiferroic family. A simple model is proposed to explain qualitatively the emergence of this pressure-induced phase. Differences between ${\mathrm{TbMn}}₂{\mathrm{O}}₅$ and ${\mathrm{DyMn}}₂{\mathrm{O}}₅$ behaviors at ambient and low pressures provide clues on the interaction scheme.