Stabilization of multiferroic spin cycloid in Ni3V2O8by light Co doping

Abstract

We present macroscopic and neutron diffraction data on multiferroic lightly Co-doped Ni${}_{3}$V${}_{2}$O${}_{8}$. 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 Ni${}_{3}$V${}_{2}$O${}_{8}$, 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 ${J}_{2}/{J}_{1}=0.43$ (0.47) for a doping level of 7% (10%) Co compared to 0.39 in the parent compound.