Conical antiferromagnetic order in the ferroelectric phase of Mn <inf>0.8</inf>Co <inf>0.2</inf>WO <inf>4</inf> resulting from the competition between collinear and cycloidal structures

Evolution of competing commensurate collinear (AF4) and incommensurate cycloidal (AF2) magnetic structures in Mn 0.8Co 0.2WO 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 [k 1=(12,0,0)] inherent to the pure CoWO 4 was observed below Néel temperature T N 20 K in Mn 0.8Co 0.2WO 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 FE 8.5 K [k 2=(-0.211,12,0.452)]. Ferroelectric polarization along b axis was revealed below T 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 (∼142 - 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 FE decreases compared to the pure MnWO 4. © 2012 American Physical Society.