Proof of the elusive high-temperature incommensurate phase in CuO by spherical neutron polarimetry.

CuO is the only known binary multiferroic compound, and due to its high transition temperature into the multiferroic state, it has been extensively studied. In comparison to other prototype multiferroics, the nature and even the existence of the high-temperature incommensurate paraelectric phase (AF3) were strongly debated-both experimentally and theoretically-since it is stable for only a few tenths of a kelvin just below the Néel temperature. Until now, there is no proof by neutron diffraction techniques owing to its very small ordered Cu magnetic moment. Here, we demonstrate the potential of spherical neutron polarimetry, first, in detecting magnetic structure changes, which are not or weakly manifest in the peak intensity and, second, in deducing the spin arrangement of the so far hypothetic AF3 phase. Our findings suggest two coexisting spin density waves emerging from an accidental degeneracy of the respective states implying a delicate energy balance in the spin Hamiltonian.

A comparative Raman study between PrMnO 3, NdMnO 3, TbMnO 3 and DyMnO 3.

In this paper, we present a detailed Raman study of the non-multiferroic compounds PrMnO 3 and NdMnO 3 and the multiferroic compounds TbMnO 3 and DyMnO 3 as a function of temperature and magnetic field. All studied systems show anomalous phonon shifts close to the Néel transition T N . In PrMnO 3 and NdMnO 3, the frequency softenings are partly attributed to an orbital-spin-phonon coupling whereas in TbMnO 3 and DyMnO 3, the relatively weak frequency shifts are rather attributed to an expansion of the Mn-O bond lengths. On the other hand, the frequencies of TbMnO 3 phonons are shifted as a function of magnetic field, while those of PrMnO 3 remain unaffected. These frequency shifts are interpreted in terms of local oxygen rearrangements under magnetic field that could play an important role in the multiferroicity of TbMnO 3 and DyMnO 3.

Magnetoelectric effect and phase transitions in CuO in external magnetic fields.

Apart from being so far the only known binary multiferroic compound, CuO has a much higher transition temperature into the multiferroic state, 230 K, than any other known material in which the electric polarization is induced by spontaneous magnetic order, typically lower than 100 K. Although the magnetically induced ferroelectricity of CuO is firmly established, no magnetoelectric effect has been observed so far as direct crosstalk between bulk magnetization and electric polarization counterparts. Here we demonstrate that high magnetic fields of ≈ 50 T are able to suppress the helical modulation of the spins in the multiferroic phase and dramatically affect the electric polarization. Furthermore, just below the spontaneous transition from commensurate (paraelectric) to incommensurate (ferroelectric) structures at 213 K, even modest magnetic fields induce a transition into the incommensurate structure and then suppress it at higher field. Thus, remarkable hidden magnetoelectric features are uncovered, establishing CuO as prototype multiferroic with abundance of competitive magnetic interactions.