Quantum spin state transitions in spin-1 equilateral triangular lattice antiferromagnet Na${}_2$BaNi(PO${}_4$)${}_2$

Abstract

We have grown single crystals of Na${}_2$BaNi(PO${}_4$)${}_2$, a new spin-1 equilateral triangular lattice antiferromagnet (ETLAF), and performed magnetic susceptibility, specific heat and thermal conductivity measurements at ultralow temperatures. The main results are (i) at zero magnetic field, Na${}_2$BaNi(PO${}_4$)${}_2$ exhibits a magnetic ordering at 430 mK with a weak ferromagnetic moment along the $c$ axis. This suggests a canted 120${}^{\circ }$ 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 the canted 120${}^{\circ }$ spin structure is transformed to a up up down (UUD) spin structure. With even higher fields, the UUD phase further evolves to possible V and V' phases; (iii) with increasing field along the $a$ axis, the canted 120${}^{\circ }$ spin structure is possibly transformed to a umbrella phase and a V phase. Therefore, Na${}_2$BaNi(PO${}_4$)${}_2$ is a rare example of spin-1 ETLAF with single crystalline form to exhibit easy-axis spin anisotropy and series of quantum spin state transitions.

9 pages, 10 figures, accepted for publication in Phys. Rev. B