Metamagnetic transitions and magnetoelectricity in the spin-1 honeycomb antiferromagnet Ni2Mo3O8

Earlier neutron scattering studies suggested the coexistence of complex stripelike and zigzaglike antiferromagnetic orders in polar honeycomb lattice ${\mathrm{Ni}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$, while its magnetoelectric (ME) behavior as an emergent effect is thus of high interest. Here we report our observations of two metamagnetic transitions and novel ME responses of ${\mathrm{Ni}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ single crystals against high magnetic field $H$ up to \ensuremath{\sim}60 T. The $c$-axis (polar axis) spontaneous electric polarization ${P}_{\mathrm{spin}}$, emerging at the magnetic N\'eel temperature ${T}_{N}\ensuremath{\sim}5.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and its remarkable response to $H$ applied along the $c$ axis ($H//c$) and $a$ axis ($H//a$), respectively, provide the clear evidence for the magnetism-driven ferroelectricity. While the magnetism exhibits the in-plane anisotropy to some extent, the magnetic field dependencies of magnetization and electric polarization in the low-field region and high-field region are distinctly different. In the low-field region where a weak spin-flop type metamagnetic transition occurs, the electric polarization response shows the parabolic dependence of magnetic field applied along both the $c$ axis and in-plane $a$ axis. The second metamagnetic transition happens when the magnetic field extends up to the high-field region where the magnetization and electric polarization response at low temperature are characterized by an extraordinarily broad plateau for the magnetic field along the $c$ axis but roughly linear dependence for field along the $a$ axis. These unusual phenomena are discussed, based on the symmetry-related local ME tensor analysis, and it is suggested that both the spin current and p-d hybridization mechanisms may contribute to the spontaneous electric polarization and ME responses. The present work demonstrates ${\mathrm{Ni}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ as a unique multiferroic and promising platform for exploring the rich spin-1 physics and ME phenomena in honeycomb lattice.