Quantum Spin State Transitions in the Spin-1 Equilateral Triangular Lattice Antiferromagnet Na2BaNi(PO4)2

We have grown single crystals of Na2BaNi(PO4)(2), a spin-1 equilateral triangular lattice antiferromagnet (ETLAF), and performed magnetic susceptibility, specific heat, and thermal conductivity measurements at ultralow temperatures. The main results are as follows: (i) At zero magnetic field, Na2BaNi(PO4)(2) exhibits a magnetic ordering at 430 mK with a weak ferromagnetic moment along the c axis. This suggests a canted 120 degrees spin structure, which is in a plane including the crystallographic c axis due to the existence of an easy-axis anisotropy and ferromagnetically stacked along the c axis. (ii) With increasing field along the c axis, a 1/3 magnetization plateau is observed, which means that the canted 120 degrees spin structure is transformed into an up-up-down (UUD) spin structure. With even higher fields, the UUD phase further evolves into possible V and V' phases. (iii) With increasing field along the a axis, the canted 120 degrees spin structure is possibly transformed into an umbrella phase and a V phase. Therefore Na2BaNi(PO4)(2) is a rare example of a spin-1 ETLAF with single-crystalline form that exhibits easy-axis spin anisotropy and a series of quantum spin state transitions.