Quantum spin state transitions in the spin-1 equilateral triangular lattice antiferromagnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Na</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi>BaNi</mml:mi><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:msub><mml:mi>PO</mml:mi><mml:mn>4</mml:mn></mml:msub><mml:mo>)</mml:mo></mml:mrow><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>
N. Li, Qing Huang, A. Brassington, Xiaoyu Yue, Wangsheng Chu, Shuangkui Guang, X. H. Zhou, Pengfei Gao, Erxi Feng, Huibo Cao, Eun Sang Choi, Y. Sun, Q. J. Li, Xinguo Zhao, Haidong Zhou, X. F. Sun
Abstract
We have grown single crystals of ${\mathrm{Na}}_{2}\mathrm{BaNi}{({\mathrm{PO}}_{4})}_{2}$, 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, ${\mathrm{Na}}_{2}\mathrm{BaNi}{({\mathrm{PO}}_{4})}_{2}$ exhibits a magnetic ordering at 430 mK with a weak ferromagnetic moment along the $c$ axis. This suggests a canted ${120}^{\ensuremath{\circ}}$ 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 that the canted ${120}^{\ensuremath{\circ}}$ 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 ${\mathrm{V}}^{\ensuremath{'}}$ phases. (iii) With increasing field along the $a$ axis, the canted ${120}^{\ensuremath{\circ}}$ spin structure is possibly transformed into an umbrella phase and a V phase. Therefore ${\mathrm{Na}}_{2}\mathrm{BaNi}{({\mathrm{PO}}_{4})}_{2}$ 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.