Temperature dependence of the propagation vector in Ni_3-xCo_xV₂O₈ with x=0.1 and 0.5

We present the susceptibility and neutron diffraction data on multiferroic Ni_3-xCo_xV₂O₈ with x=0.1 and 0.5. The temperature dependence of the susceptibility indicates that the magnetic order-disorder transition occurs at T_HTI=8.5 K in both samples. Below T_HTI, the high-temperature incommensurate magnetic structure is realized, which undergoes a transition to the low-temperature incommensurate phase when the sample is cooled. The temperature evolution of the propagation vector k for x=0.1 is very weak, which confirms previous studies showing that substitution of 3.5% Ni ions by Co ions suppresses the explicit temperature dependence of k that is observed in the parent compound. On the other hand, we found that the vector k for x=0.5 exhibits a definite temperature dependence, which differs from the case of the undoped sample (x=0).