We present macroscopic and neutron diffraction data on multiferroic lightly Co-doped Ni3V2O8. Doping Co into the parent compound suppresses the sequence of four magnetic phase transitions, and only two magnetically ordered phases - the paraelectric high-temperature incommensurate (HTI) phase and the ferroelectric low-temperature incommensurate (LTI) phase - can be observed. Interestingly, the LTI multiferroic phase with a spiral (cycloidal) magnetic structure is stabilized down to at least 1.8 K, which could be revealed by measurements of the electric polarization and confirmed by neutron diffraction on single-crystal samples. The extracted magnetic moments of the LTI phase contain, besides the main exchange, also fine components of the cycloid allowed by symmetry, which result in a small amplitude variation of the magnetic moments along the cycloid propagation due to the site-dependent symmetry properties of the mixed representations. In the HTI phase, a finite imaginary part of the spine magnetic moment could be deduced yielding a spin cycloid instead of a purely sinusoidal structure with an opposite spin chirality for different spine spin chains. The magnetic ordering of the cross-tie sites in both phases is different in comparison to the respective ones in the pure Ni compound. A wider temperature stability range of the HTI phase has been observed in comparison to Ni3V 2O8, which can be explained by an additional single-ion easy-axis anisotropy due to Co doping. The larger incommensurability of the Co-doped compounds yields a larger ratio between the competing next-nearest-neighbor and nearest-neighbor interaction, which is J 2/J1=0.43 (0.47) for a doping level of 7% (10%) Co compared to 0.39 in the parent compound. © 2013 American Physical Society.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - 18 Nov 2013|