Conical antiferromagnetic order in the ferroelectric phase of Mn${}_{0.8}$Co${}_{0.2}$WO${}_4$ resulting from the competition between collinear and cycloidal structures

Evolution of competing commensurate collinear (AF4) and incommensurate cycloidal (AF2) magnetic structures in Mn${}_{0.8}$Co${}_{0.2}$WO${}_4$ multiferroic was studied by neutron diffraction, magnetic, and pyroelectric characterization measurements. In contrast to pure and slightly Co doped MnWO${}_4$, the antiferromagnetic AF4 collinear phase [${\mathbf{k}}_1=(\frac{1}{2},0,0)$] inherent to the pure CoWO${}_4$ was observed below Néel temperature $T_N\approx 20$ K in Mn${}_{0.8}$Co${}_{0.2}$WO${}_4$. This collinear order survives down to the lowest temperature reached in the experiments (2 K) even after the appearance of the second (cycloidal AF2) spin order below $T_{\text{FE}}\approx 8.5$ K [${\mathbf{k}}_2=(-0.211,\frac{1}{2},0.452)$]. Ferroelectric polarization along $b$ axis was revealed below $T_{\text{FE}}$ in the low temperature conical phase resulting from the superposition of the AF4 and AF2 spin structures. The arrangement of the spins after the two successive magnetic transitions are thoroughly described. In particular, we found that spins in the AF4 phase are aligned along the easy direction in the $ac$ plane ($\sim {142}^{\circ }$ with respect to the $c^{*}$ axis), while the cycloidal AF2 spin order is developed in the magnetically hard directions, perpendicular to the easy one, and consequently the $T_{\text{FE}}$ decreases compared to the pure MnWO${}_4$.

Figure 1

(a) Temperature dependence of susceptibility along the $b$ axis and along two distinctive directions within the $ac$ plane, namely for the field applied at an angle $\alpha$ of about 37${}^{\circ }$ with respect to the $c$ axis and for the field perpendicular to it; (b) angular dependencies of susceptibility in $H=1$ kOe within the $ac$ plane, at 25 and 5 K; (c) temperature dependence of the inverse susceptibility.

Figure 2

(a) Crystal structure of Mn${}_{0.8}$Co${}_{0.2}$WO${}_4$. Circles in the octahedra (green) correspond to Mn/Co, big (blue) circles between the octahedra are tungsten atoms, and smallest circles (red) are oxygen atoms. (b) Agreement plot of the nuclear structure refinement at 25 K.

Figure 3

Integrated intensities of two magnetic reflection as a function of the temperature. The error bars are inside the symbols.

Figure 4

Magnetic structures of Mn${}_{0.8}$Co${}_{0.2}$WO${}_4$ at 11 K (a) and 2 K (b). Circles in the octahedra (green) correspond to Mn/Co, big (blue) circles between the octahedra are tungsten atoms, and smallest circles (red) are oxygen atoms. The corresponding agreement plots of the refinements are plotted on (c) and (d).

Figure 5

Temperature dependence of electric polarization along $b$ axis in $E=0$. Data was collected while heating, after cooling the sample from the paraelectric state down to 1.9 K in poling field $E_p=1.8$ V/cm.