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

TL;DR

This study reports on the synthesis and detailed magnetic characterization of Na$_2$BaNi(PO$_4$)$_2$, revealing multiple quantum spin state transitions and anisotropic magnetic behaviors in a spin-1 triangular lattice antiferromagnet.

Contribution

It provides the first detailed experimental investigation of quantum spin state transitions in a spin-1 ETLAF with single crystals, highlighting anisotropy and complex magnetic phases.

Findings

Magnetic ordering at 430 mK with weak ferromagnetism and canted 120° structure.

Observation of a 1/3 magnetization plateau indicating a UUD phase.

Field-induced transitions to V, V', umbrella, and other phases.

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.