The complex multiferroic phase diagram of Mn1-xCoxWO4

The complete magnetic and multiferroic phase diagram of Mn1-xCoxWO4 single crystals is investigated by means of magnetic, heat capacity and polarization experiments. We show that the ferroelectric (FE) polarization P-bar in the multiferroic state abruptly changes its direction twice upon increasing Co content, x. At xc1 = 0.075, P-bar rotates from the b-axis into the a-c-plane and at xc2 = 0.15 it flips back to the b-axis. The origin of the multiple polarization flops is identified as an effect of the Co anisotropy on the orientation and shape of the spin helix leading to thermodynamic instabilities caused by the decrease of the magnitude of the polarization in the corresponding phases. A qualitative description of the FE polarization is derived by taking into account the intrachain (c-axis) as well as the interchain (a-axis) exchange pathways connecting the magnetic ions. In a narrow Co concentration range (0.1 ⩽ x ⩽ 0.15), an intermediate phase, sandwiched between the collinear high-temperature and the helical low-temperature phases, is discovered. The new phase exhibits a collinear and commensurate spin modulation similar to the low-temperature magnetic structure of MnWO4. (paper)