Successive magnetic phase transitions and multiferroicity in Spin-1 triangular lattice antiferromagnet Ba${}_3$NiNb${}_2$O${}_9$

We report the magnetic and electric properties of Ba${}_3$NiNb${}_2$O${}_9$, which is a quasi-two-dimensional spin-1 triangular lattice antiferromagnet (TLAF) with trigonal structure. At low $T$ and with increasing magnetic field, the system evolves from a 120 degree magnetic ordering phase (A phase) to an up-up-down ($uud$) phase (B phase) with a change of slope at 1/3 of the saturation magnetization, and then to an “oblique” phase (C phase). Accordingly, the ferroelectricity switches on at each phase boundary with appearance of spontaneous polarization. Therefore, Ba${}_3$NiNb${}_2$O${}_9$ is a unique TLAF exhibiting both $uud$ phase and multiferroicity.