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 moments.