New features in the phase diagram of TbMnO3

The H, T phase diagram of the multiferroic perovskite TbMnO3 was studied by high resolution thermal expansion alpha T and magnetostriction Delta L H L measurements. Below T N similar or equal to 42 K, TbMnO3 shows antiferromagnetic order, which changes at T FE similar or equal to 28K where simultaneously a spontaneous polarization P parallel to c develops. Sufficiently high magnetic fields applied along a or b induce a polarization flop to P parallel to a. We find that all of these transitions are strongly coupled to the lattice parameters. Thus, our data allow for a precise determination of the phase boundaries and also yield information about their uniaxial pressure dependencies. The strongly hysteretic phase boundary to the ferroelectric phase with P parallel to a is derived in detail. Contrary to previous reports, we find that even in high magnetic fields there are no direct transitions from this phase to the paraelectric phase. We also determine the various phase boundaries in the low temperature region related to complex reordering transitions of the Tb moment